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  • 09 October 2019

The structure of DNA

  • Georgina Ferry 0

Georgina Ferry is a science writer based in Oxford, UK. A revised edition of her biography Dorothy Crowfoot Hodgkin has just been published by Bloomsbury Reader.

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On 25 April 1953, James Watson and Francis Crick announced 1 in Nature that they “wish to suggest” a structure for DNA . In an article of just over a page, with one diagram (Fig. 1), they transformed the future of biology and gave the world an icon — the double helix. Recognizing at once that their structure suggested a “possible copying mechanism for the genetic material”, they kick-started a process that, over the following decade, would lead to the cracking of the genetic code and, 50 years later, to the complete sequence of the human genome.

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Nature 575 , 35-36 (2019)

doi: https://doi.org/10.1038/d41586-019-02554-z

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Nature PastCast: The other DNA papers

Host: Kerri Smith

This is the Nature PastCast , each month raiding Nature ’s archive and looking at key moments in science. In this show, we’re going back to the 1950s.

Music: I’ve Got the World on a String by Ella Fitzgerald

Voice of Nature: John Howe

From the Editorial and Publishing Offices of Nature , Macmillan and Co., St Martin’s Street, London. Nature , April 25 th 1953.

Page 734, Microsomal particles of normal cow’s milk . Page 737, Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid, J. D. Watson and F. H. C. Crick.

Raymond Gosling

Walking into the lab and seeing this double helix, of course, it looked familiar because all of the stator of the dimensions was the stuff that we got from our X-ray diffraction patterns. So, it looked right and it was sheer elegance.

I’m Raymond Gosling, co-author of one of the papers in Nature , 1953, April, on the structure of DNA.

Melinda Baldwin

My name is Melinda Baldwin. I’m a historian of science at the American Academy of Arts and Sciences in Cambridge, Massachusetts. I think a lot of people don’t necessarily know that there were three DNA papers instead of just the one, and I think the big reason that the Watson and Crick paper became the one that we do remember is because that’s the one where the structure of DNA was published, and I think as a consequence the second two papers have really fallen out a bit of consciousness. The Franklin and Gosling paper was primarily about crystallographic work.

Page 740, Rosalind E. Franklin and R. G. Gosling, King’s College London, Molecular Configuration in Sodium Thymonucleate .

Georgina Ferry

I’m Georgina Ferry. I’m a science writer and author. At the time, X-ray crystallography of large molecules – the sort of molecules that you get in living bodies – was still a very, very small field. It had really started in the 1930s. Everybody was interested in the structure of proteins back in the 30s because nobody thought that DNA could possibly be complicated enough to be the molecule of life. That wasn’t really discovered until the mid-40s and then, obviously, it became very important to study its structure.

The only time I could get at the X-ray set in King’s, the only one that existed, was in the basement of the chemistry department, and that was below the level of the Thames and I was only allowed to play with it in the evenings.

What you need is an X-ray source, which in those days would have been an X-ray tube. I mean it was a form of technology that was available from the 19 th century but it’s a tube full of gas that you run an electric current through and it emits X-rays, and then in order to study your molecule, the thing you’re interested in, you have to crystallise it. You surround that, in the early days, with photographic film so that when the X-rays come in, they hit the atoms in the crystal and they’re diffracted out and they make spots on the photographic film.

I needed lots of fibres. One would produce the diffraction pattern so weak that you’d never see it, so I wound 35 fibres round a paperclip and then pushed the clip open a bit to make the fibres taught.

Sodium thymonucleate fibres give two distinct types of X-ray diagram. The first corresponds to a crystalline form, structure A. At higher humidities, a different structure, structure B, appears.

And the best structure B pattern we ever got is photo 51, which I took and was called 51 because that was the 51 st photograph that we’d taken, Rosalind and I, in our efforts to sort out this A and B difference.

It’s a really beautiful photo. It’s very crisp, it’s very clean, it’s got this really neat ‘X’ shape, and apparently if you know something about crystallography, this photo just screams helix.

What is puzzling, I think is still puzzling, is why they didn’t pursue that photograph once they had it.

Now, Rosalind was absolutely determined that there was so much information in structure A’s diffraction pattern that was what she wanted to do and therefore put this photo 51 on one side and said we’ll come back to that. I only wish I’d been able to plug the value of looking at structure B as well as Structure A.

Ella Fitzgerald – I’ve Got the World on a String

So, Rosalind Franklin was working with Maurice Wilkins but the two of them had a pretty bad working relationship. Apparently, Franklin thought that she was being brought to King’s College London as an independent investigator who would be in charge of her own research. Wilkins thought that she was being brought in as an assistant, and eventually the relationship grew so fraught that Franklin stopped showing him her data, and she was planning on moving to Birkbeck College. Somehow, Wilkins got a copy of photo 51.

I took it down the corridor and gave it to him because it had reached the stage now when Rosalind was going to leave, so she suggested that I go down the corridor and give this beautiful structure B pattern, this photo 51, to Maurice. Maurice couldn’t believe it when I offered it to him. He couldn’t believe that I hadn’t stolen it from her desk. He didn’t think that she could ever offer him something as interesting as this. He’d only had it for two or three days when Watson chipped up.

He showed it to James Watson when James came down to visit him and to chat a little bit about DNA.

Who of course knew what a helical diffraction pattern would look like because Crick had two years previously published a theoretical paper of what the diffraction pattern of a helix would look like.

Watson’s got this great passage in The Double Helix where he said my pulse sped up and my heart began to race because he looked at this photo and realised immediately that DNA was helical and that he knew what size the turns had to be. So, this photo contained all of the information that he needed to build the model that he and Crick ended up being famous for.

We wish to suggest a structure for the salt of deoxyribose nucleic acid (D. N. A). This structure has two helical chains, each coiled round the same axis.

So, it was pretty out of order for Watson and Crick to start working on DNA because they knew full well that Maurice Wilkins was working on it at King’s and subsequently Rosalind Franklin joined him there and she was also working on it. But it was King’s’ problem, and there was very much a sort of unspoken gentleman’s agreement – it would be understood that a particular group or lab was working on one problem and you wouldn’t then go and do that one.

You didn’t go to work on another man’s problem, especially if he’d got a whole team working on it.

In the Watson and Crick paper, it’s not credited. Watson and Crick say they were stimulated by a general knowledge of the unpublished results of Wilkins and Franklin.

We have been stimulated by a knowledge of the general nature of the unpublished experimental results and ideas of Dr Wilkins, Dr Franklin and…

But they don’t cite photo 51 specifically and then Franklin and Gosling, in their paper, say this photo clearly supports the model that Watson and Crick had put forth.

Rosalind’s reaction was, I think, typical of Rosalind. She wasn’t furious or didn’t use the word ‘scooped’. What she actually said was we all stand on each other’s shoulders. We had this second-, third-prize feeling that we were within a millimetre or two of the right answer ourselves.

So, Watson and Crick had their paper ready to go. They had the structure solved. They wanted to publish it in Nature . Apparently, John Randall, the uber-head of the Kings College London Laboratory, was a member of The Athenaeum, the British social club in London, and so was L. J. F. Brimble, then one of the co-editors of Nature . So, apparently, Brimble approached Randall to say well, we’ve got this paper under consideration, don’t you want the King’s work represented as well? And I think Watson and Crick and Wilkins had already agreed that they would publish two papers side-by-side. Wilkins sort of knew that his work was going to be outshone by Watson and Crick, but he certainly wanted it published. And then apparently after the two of them had agreed to publish the two papers together, Rosalind Franklin said, well, I want a paper on the crystallographic work that Ray Gosling and I did in there as well, and so it was really by conversation by the editors and the heads of the laboratories that the editors agreed to print these paper as quickly as possible. So, famously, the three DNA papers were not peer-reviewed. I think that was quite typically for the Brimble-and-Gale editorship, that they placed a lot of trust in particular laboratory heads and particular friends in the British scientific community and so if Laurence Bragg said that something was good and important, they were going to print it.

There wasn’t a huge fuss made, even within science, about the DNA structure until probably the early 60s when the code began to be cracked because obviously – as Watson and Crick famously said –

Voice of Nature : John Howe

It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.

But the actual code wasn’t cracked until the early 60s, and that was when the power of this discovery really started to make a big difference.

Elsewhere in Nature , Page 757, Appointments vacant. Physicists wanted for fundamental research on felt and applied research of the felt-making industry, The British Hat and Allied Felt-makers Research Association, Manchester.

Page 716, Department of Scientific and Industrial Research UK, The gross expenditure of the department was £5.5 million as against £5 million in the previous year.

The climbing of Mount Everest and the coronation of the Queen and all these things came together so that ’53 in that lab was seen as an almost miraculous time.

Everywhere you looked you could see that it fitted a double helix. It was uncanny. It just screamed at you. I’ve often asked how long would it have been before we as a group saw that and I really don’t know the answer to that. It was a stroke of genius on his part.

Nature . Annual subscription £6. Payable in advance. Postage paid to any part of the world.

Kerri Smith

The Nature PastCast was produced by me, Kerri Smith, with contributions from Raymond Gosling, writer Georgina Ferry and historian Melinda Baldwin. In episode two of this twelve-part series on the history of science, we’re heading back to the 1980s.

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Locating an Essay’s DNA

bonomo small

I’ve long wondered what it is about abandoned buildings that intrigues me so . Wherever I am, I search them out; were I feeling melodramatic, I might say that they search me out. I always seem to be driving past or near them, in large cities and small towns, though this may have as much to say about a depressed economic climate than about any sixth sense I may claim. At any rate, I find abandoned buildings, and photograph them; the ones I haven’t photographed have imprinted themselves inside me. Were I to explore abandoned buildings in an essay, that essay’s narrow subject would involve the buildings themselves and my narrative relationship to them: where they are; where I am; how I found them; their physical condition and descriptive details, etc.

An essay that deliberates on its narrow subject exclusively can be a pleasing essay, lingering affectionately on sensory description, on evocation of time and place, on regional history, maybe on relevant stories. But that essay’s larger subject is what will draw its readers toward something more valuable and memorable. Here, if I was writing the abandoned-buildings essay, I’d ask the important question, not What? , but Why? And here’s where the essay would turn. Why am I writing about abandoned buildings? I might explore what resonance they have for me on an emotional level, try and chase the sensations that resist language; that’s a tough place to get to, but almost always worthwhile for an essayist. (What I don’t know and what I can’t name.) I might explore less what I see in front of me than why I am here: that is, what brings me? Decrepitude slumming? Ruin romanticizing? Neither of those impulses reflects very well on me, and conflict is usually a good place to linger in an essay. I probably sentimentalize abandoned buildings and their devastating music of loss. Don’t I argue strenuously against sentimentality, resist it? Why do I give in here? What do I gain, or lose? If I sentimentalize ruins, I conveniently ignore, or elide, the genuine suffering such ruins might have brought to the buildings’ owners (whom I might interview) or neighbors (ditto). My essay might open outward to the culture in which the buildings exist in their slow, stubborn demise, the people who once lived inside, or those who live nearby. In moving from the narrow subject (what) to the larger subject (why) the essay gains dimensionality; the heat that’s generated from the friction of the two subjects warms the essay, produces something more mature, round, maybe even valuable.

Here’s the secret: the larger subject is always contained in the DNA of the urge to write; we use the narrow subject to write toward and hopefully discover the larger subject. So, what’s your narrow subject? Where does it take you? Careful: you might not come back.

Joe Bonomo ’s most recent books are  This Must Be Where My Obsession with Infinity Began , a collection of essays, and  Conversations with Greil Marcus . The music columnist at The Normal School literary magazine, he teaches at Northern Illinois University and appears online at  No Such Thing As Was .

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9.1 The Structure of DNA

Learning objectives.

  • Describe the structure of DNA
  • Describe how eukaryotic and prokaryotic DNA is arranged in the cell

In the 1950s, Francis Crick and James Watson worked together at the University of Cambridge, England, to determine the structure of DNA. Other scientists, such as Linus Pauling and Maurice Wilkins, were also actively exploring this field. Pauling had discovered the secondary structure of proteins using X-ray crystallography. X-ray crystallography is a method for investigating molecular structure by observing the patterns formed by X-rays shot through a crystal of the substance. The patterns give important information about the structure of the molecule of interest. In Wilkins’ lab, researcher Rosalind Franklin was using X-ray crystallography to understand the structure of DNA. Watson and Crick were able to piece together the puzzle of the DNA molecule using Franklin's data ( Figure 9.2 ). Watson and Crick also had key pieces of information available from other researchers such as Chargaff’s rules. Chargaff had shown that of the four kinds of monomers (nucleotides) present in a DNA molecule, two types were always present in equal amounts and the remaining two types were also always present in equal amounts. This meant they were always paired in some way. In 1962, James Watson, Francis Crick, and Maurice Wilkins were awarded the Nobel Prize in Medicine for their work in determining the structure of DNA.

Now let’s consider the structure of the two types of nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The building blocks of DNA are nucleotides, which are made up of three parts: a deoxyribose (5-carbon sugar), a phosphate group , and a nitrogenous base ( Figure 9.3 ). There are four types of nitrogenous bases in DNA. Adenine (A) and guanine (G) are double-ringed purines, and cytosine (C) and thymine (T) are smaller, single-ringed pyrimidines. The nucleotide is named according to the nitrogenous base it contains.

The phosphate group of one nucleotide bonds covalently with the sugar molecule of the next nucleotide, and so on, forming a long polymer of nucleotide monomers. The sugar–phosphate groups line up in a “backbone” for each single strand of DNA, and the nucleotide bases stick out from this backbone. The carbon atoms of the five-carbon sugar are numbered clockwise from the oxygen as 1', 2', 3', 4', and 5' (1' is read as “one prime”). The phosphate group is attached to the 5' carbon of one nucleotide and the 3' carbon of the next nucleotide. In its natural state, each DNA molecule is actually composed of two single strands held together along their length with hydrogen bonds between the bases.

Watson and Crick proposed that the DNA is made up of two strands that are twisted around each other to form a right-handed helix, called a double helix . Base-pairing takes place between a purine and pyrimidine: namely, A pairs with T, and G pairs with C. In other words, adenine and thymine are complementary base pairs, and cytosine and guanine are also complementary base pairs. This is the basis for Chargaff’s rule; because of their complementarity, there is as much adenine as thymine in a DNA molecule and as much guanine as cytosine. Adenine and thymine are connected by two hydrogen bonds, and cytosine and guanine are connected by three hydrogen bonds. The two strands are anti-parallel in nature; that is, one strand will have the 3' carbon of the sugar in the “upward” position, whereas the other strand will have the 5' carbon in the upward position. The diameter of the DNA double helix is uniform throughout because a purine (two rings) always pairs with a pyrimidine (one ring) and their combined lengths are always equal. ( Figure 9.4 ).

The Structure of RNA

There is a second nucleic acid in all cells called ribonucleic acid, or RNA. Like DNA, RNA is a polymer of nucleotides. Each of the nucleotides in RNA is made up of a nitrogenous base, a five-carbon sugar, and a phosphate group. In the case of RNA, the five-carbon sugar is ribose, not deoxyribose. Ribose has a hydroxyl group at the 2' carbon, unlike deoxyribose, which has only a hydrogen atom ( Figure 9.5 ).

RNA nucleotides contain the nitrogenous bases adenine, cytosine, and guanine. However, they do not contain thymine, which is instead replaced by uracil, symbolized by a “U.” RNA exists as a single-stranded molecule rather than a double-stranded helix. Molecular biologists have named several kinds of RNA on the basis of their function. These include messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA)—molecules that are involved in the production of proteins from the DNA code.

How DNA Is Arranged in the Cell

DNA is a working molecule; it must be replicated when a cell is ready to divide, and it must be “read” to produce the molecules, such as proteins, to carry out the functions of the cell. For this reason, the DNA is protected and packaged in very specific ways. In addition, DNA molecules can be very long. Stretched end-to-end, the DNA molecules in a single human cell would come to a length of about 2 meters. Thus, the DNA for a cell must be packaged in a very ordered way to fit and function within a structure (the cell) that is not visible to the naked eye. The chromosomes of prokaryotes are much simpler than those of eukaryotes in many of their features ( Figure 9.6 ). Most prokaryotes contain a single, circular chromosome that is found in an area in the cytoplasm called the nucleoid.

The size of the genome in one of the most well-studied prokaryotes, Escherichia coli, is 4.6 million base pairs, which would extend a distance of about 1.6 mm if stretched out. So how does this fit inside a small bacterial cell? The DNA is twisted beyond the double helix in what is known as supercoiling. Some proteins are known to be involved in the supercoiling; other proteins and enzymes help in maintaining the supercoiled structure.

Eukaryotes, whose chromosomes each consist of a linear DNA molecule, employ a different type of packing strategy to fit their DNA inside the nucleus ( Figure 9.7 ). Before the structure of DNA was even uncovered, Marie Maynard Daly and Arthur E. Mirsky conducted extensive research in the 1940s and 1950s to understand the molecules and structures in involved. At the most basic level, DNA is wrapped around proteins known as histones to form structures called nucleosomes. The DNA is wrapped tightly around the histone core. This nucleosome is linked to the next one by a short strand of DNA that is free of histones. This is also known as the “beads on a string” structure; the nucleosomes are the “beads” and the short lengths of DNA between them are the “string.” The nucleosomes, with their DNA coiled around them, stack compactly onto each other to form a 30-nm–wide fiber. This fiber is further coiled into a thicker and more compact structure. At the metaphase stage of mitosis, when the chromosomes are lined up in the center of the cell, the chromosomes are at their most compacted. They are approximately 700 nm in width, and are found in association with scaffold proteins.

In interphase, the phase of the cell cycle between mitoses at which the chromosomes are decondensed, eukaryotic chromosomes have two distinct regions that can be distinguished by staining. There is a tightly packaged region that stains darkly, and a less dense region. The darkly staining regions usually contain genes that are not active, and are found in the regions of the centromere and telomeres. The lightly staining regions usually contain genes that are active, with DNA packaged around nucleosomes but not further compacted.

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The structure and function of dna.

Biologists in the 1940s had difficulty in accepting DNA as the genetic material because of the apparent simplicity of its chemistry. DNA was known to be a long polymer composed of only four types of subunits, which resemble one another chemically. Early in the 1950s, DNA was first examined by x-ray diffraction analysis, a technique for determining the three-dimensional atomic structure of a molecule (discussed in Chapter 8). The early x-ray diffraction results indicated that DNA was composed of two strands of the polymer wound into a helix. The observation that DNA was double-stranded was of crucial significance and provided one of the major clues that led to the Watson-Crick structure of DNA. Only when this model was proposed did DNA's potential for replication and information encoding become apparent. In this section we examine the structure of the DNA molecule and explain in general terms how it is able to store hereditary information.

  • A DNA Molecule Consists of Two Complementary Chains of Nucleotides

A DNA molecule consists of two long polynucleotide chains composed of four types of nucleotide subunits. Each of these chains is known as a DNA chain , or a DNA strand . Hydrogen bonds between the base portions of the nucleotides hold the two chains together ( Figure 4-3 ). As we saw in Chapter 2 ( Panel 2-6 , pp. 120-121), nucleotides are composed of a five-carbon sugar to which are attached one or more phosphate groups and a nitrogen-containing base. In the case of the nucleotides in DNA, the sugar is deoxyribose attached to a single phosphate group (hence the name deoxyribonucleic acid ), and the base may be either adenine (A), cytosine (C), guanine ( G ), or thymine (T) . The nucleotides are covalently linked together in a chain through the sugars and phosphates, which thus form a “backbone” of alternating sugar-phosphate-sugar-phosphate (see Figure 4-3 ). Because only the base differs in each of the four types of subunits, each polynucleotide chain in DNA is analogous to a necklace (the backbone) strung with four types of beads (the four bases A, C, G, and T). These same symbols (A, C, G, and T) are also commonly used to denote the four different nucleotides—that is, the bases with their attached sugar and phosphate groups.

DNA and its building blocks. DNA is made of four types of nucleotides, which are linked covalently into a polynucleotide chain (a DNA strand) with a sugar-phosphate backbone from which the bases (A, C, G, and T) extend. A DNA molecule is composed of two (more...)

The way in which the nucleotide subunits are lined together gives a DNA strand a chemical polarity. If we think of each sugar as a block with a protruding knob (the 5′ phosphate) on one side and a hole (the 3′ hydroxyl ) on the other (see Figure 4-3 ), each completed chain, formed by interlocking knobs with holes, will have all of its subunits lined up in the same orientation. Moreover, the two ends of the chain will be easily distinguishable, as one has a hole (the 3′ hydroxyl) and the other a knob (the 5′ phosphate) at its terminus. This polarity in a DNA chain is indicated by referring to one end as the 3 ′ end and the other as the 5 ′ end .

The three-dimensional structure of DNA — the double helix —arises from the chemical and structural features of its two polynucleotide chains. Because these two chains are held together by hydrogen bonding between the bases on the different strands, all the bases are on the inside of the double helix, and the sugar -phosphate backbones are on the outside (see Figure 4-3 ). In each case, a bulkier two-ring base (a purine ; see Panel 2-6 , pp. 120–121) is paired with a single-ring base (a pyrimidine ); A always pairs with T, and G with C ( Figure 4-4 ). This complementary base-pairing enables the base pairs to be packed in the energetically most favorable arrangement in the interior of the double helix. In this arrangement, each base pair is of similar width, thus holding the sugar-phosphate backbones an equal distance apart along the DNA molecule . To maximize the efficiency of base-pair packing, the two sugar-phosphate backbones wind around each other to form a double helix, with one complete turn every ten base pairs ( Figure 4-5 ).

Complementary base pairs in the DNA double helix. The shapes and chemical structure of the bases allow hydrogen bonds to form efficiently only between A and T and between G and C, where atoms that are able to form hydrogen bonds (see Panel 2-3, pp. 114–115) (more...)

The DNA double helix. (A) A space-filling model of 1.5 turns of the DNA double helix. Each turn of DNA is made up of 10.4 nucleotide pairs and the center-to-center distance between adjacent nucleotide pairs is 3.4 nm. The coiling of the two strands around (more...)

The members of each base pair can fit together within the double helix only if the two strands of the helix are antiparallel —that is, only if the polarity of one strand is oriented opposite to that of the other strand (see Figures 4-3 and 4-4 ). A consequence of these base-pairing requirements is that each strand of a DNA molecule contains a sequence of nucleotides that is exactly complementary to the nucleotide sequence of its partner strand.

  • The Structure of DNA Provides a Mechanism for Heredity

Genes carry biological information that must be copied accurately for transmission to the next generation each time a cell divides to form two daughter cells. Two central biological questions arise from these requirements: how can the information for specifying an organism be carried in chemical form, and how is it accurately copied? The discovery of the structure of the DNA double helix was a landmark in twentieth-century biology because it immediately suggested answers to both questions, thereby resolving at the molecular level the problem of heredity. We discuss briefly the answers to these questions in this section , and we shall examine them in more detail in subsequent chapters.

DNA encodes information through the order, or sequence, of the nucleotides along each strand. Each base —A, C, T, or G —can be considered as a letter in a four-letter alphabet that spells out biological messages in the chemical structure of the DNA. As we saw in Chapter 1, organisms differ from one another because their respective DNA molecules have different nucleotide sequences and, consequently, carry different biological messages. But how is the nucleotide alphabet used to make messages, and what do they spell out?

As discussed above, it was known well before the structure of DNA was determined that genes contain the instructions for producing proteins. The DNA messages must therefore somehow encode proteins ( Figure 4-6 ). This relationship immediately makes the problem easier to understand, because of the chemical character of proteins. As discussed in Chapter 3, the properties of a protein , which are responsible for its biological function, are determined by its three-dimensional structure, and its structure is determined in turn by the linear sequence of the amino acids of which it is composed. The linear sequence of nucleotides in a gene must therefore somehow spell out the linear sequence of amino acids in a protein. The exact correspondence between the four-letter nucleotide alphabet of DNA and the twenty-letter amino acid alphabet of proteins—the genetic code —is not obvious from the DNA structure, and it took over a decade after the discovery of the double helix before it was worked out. In Chapter 6 we describe this code in detail in the course of elaborating the process, known as gene expression , through which a cell translates the nucleotide sequence of a gene into the amino acid sequence of a protein.

The relationship between genetic information carried in DNA and proteins.

The complete set of information in an organism's DNA is called its genome , and it carries the information for all the proteins the organism will ever synthesize. (The term genome is also used to describe the DNA that carries this information.) The amount of information contained in genomes is staggering: for example, a typical human cell contains 2 meters of DNA. Written out in the four-letter nucleotide alphabet, the nucleotide sequence of a very small human gene occupies a quarter of a page of text ( Figure 4-7 ), while the complete sequence of nucleotides in the human genome would fill more than a thousand books the size of this one. In addition to other critical information, it carries the instructions for about 30,000 distinct proteins.

The nucleotide sequence of the human β-globin gene. This gene carries the information for the amino acid sequence of one of the two types of subunits of the hemoglobin molecule, which carries oxygen in the blood. A different gene, the α-globin (more...)

At each cell division , the cell must copy its genome to pass it to both daughter cells. The discovery of the structure of DNA also revealed the principle that makes this copying possible: because each strand of DNA contains a sequence of nucleotides that is exactly complementary to the nucleotide sequence of its partner strand, each strand can act as a template , or mold, for the synthesis of a new complementary strand. In other words, if we designate the two DNA strands as S and S′, strand S can serve as a template for making a new strand S′, while strand S′ can serve as a template for making a new strand S ( Figure 4-8 ). Thus, the genetic information in DNA can be accurately copied by the beautifully simple process in which strand S separates from strand S′, and each separated strand then serves as a template for the production of a new complementary partner strand that is identical to its former partner.

DNA as a template for its own duplication. As the nucleotide A successfully pairs only with T, and G with C, each strand of DNA can specify the sequence of nucleotides in its complementary strand. In this way, double-helical DNA can be copied precisely. (more...)

The ability of each strand of a DNA molecule to act as a template for producing a complementary strand enables a cell to copy, or replicate , its genes before passing them on to its descendants. In the next chapter we describe the elegant machinery the cell uses to perform this enormous task.

  • In Eucaryotes, DNA Is Enclosed in a Cell Nucleus

Nearly all the DNA in a eucaryotic cell is sequestered in a nucleus , which occupies about 10% of the total cell volume. This compartment is delimited by a nuclear envelope formed by two concentric lipid bilayer membranes that are punctured at intervals by large nuclear pores, which transport molecules between the nucleus and the cytosol . The nuclear envelope is directly connected to the extensive membranes of the endoplasmic reticulum . It is mechanically supported by two networks of intermediate filaments: one, called the nuclear lamina , forms a thin sheetlike meshwork inside the nucleus, just beneath the inner nuclear membrane ; the other surrounds the outer nuclear membrane and is less regularly organized ( Figure 4-9 ).

A cross-sectional view of a typical cell nucleus. The nuclear envelope consists of two membranes, the outer one being continuous with the endoplasmic reticulum membrane (see also Figure 12-9). The space inside the endoplasmic reticulum (the ER lumen) (more...)

The nuclear envelope allows the many proteins that act on DNA to be concentrated where they are needed in the cell, and, as we see in subsequent chapters, it also keeps nuclear and cytosolic enzymes separate, a feature that is crucial for the proper functioning of eucaryotic cells. Compartmentalization, of which the nucleus is an example, is an important principle of biology; it serves to establish an environment in which biochemical reactions are facilitated by the high concentration of both substrates and the enzymes that act on them.

Genetic information is carried in the linear sequence of nucleotides in DNA . Each molecule of DNA is a double helix formed from two complementary strands of nucleotides held together by hydrogen bonds between G -C and A-T base pairs. Duplication of the genetic information occurs by the use of one DNA strand as a template for formation of a complementary strand. The genetic information stored in an organism's DNA contains the instructions for all the proteins the organism will ever synthesize. In eucaryotes, DNA is contained in the cell nucleus .

  • Cite this Page Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. The Structure and Function of DNA.
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12.2: DNA organization inside a cell

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DNA Organization in Prokaryotes

A cell’s DNA, packaged as a double-stranded DNA molecule, is called its genome. In prokaryotes, the genome is composed of a single, double-stranded DNA molecule in the form of a loop or circle (Figure \(\PageIndex{1}\)). The region in the cell containing this genetic material is called a nucleoid (remember that prokaryotes do not have a separate membrane-bound nucleus). Some prokaryotes also have smaller loops of DNA called plasmids that are not essential for normal growth. Bacteria can exchange these plasmids with other bacteria, sometimes receiving beneficial new genes that the recipient can add to their chromosomal DNA. Antibiotic resistance is one trait that often spreads through a bacterial colony through plasmid exchange.

oval bacteria containing loop of bacterial DNA and smaller circles of plasmid DNA.

The size of the genome in one of the most well-studied prokaryotes, E.coli, is 4.6 million base pairs (which would be approximately 1.1 mm in length, if cut and stretched out). So how does this fit inside a small bacterial cell? The DNA is twisted by what is known as supercoiling. Supercoiled DNA is coiled more tightly than would be typically be found in a cell (more than 10 nucleotides per twist of the helix). If you visualize twisting a rope until it twists back on itself, you have a pretty good visual of supercoiled DNA. This process allows the DNA to be compacted into the small space inside a bacteria.

DNA Organization in Eukaryotes

Eukaryotes have much more DNA than prokaryotes. For example, an E. coli bacteria contains roughly 3 million base pairs of DNA, while a human contains roughly 3 billion. In eukaryotes such as humans and other animals, the genome consists of several double-stranded linear DNA molecules (Figure \(\PageIndex{2}\)), which are located inside a membrane-bound nucleus. Each species of eukaryotes has a characteristic number of chromosomes in the nuclei (plural of nucleus) of its cells.  A normal human gamete  (sperm or egg) contains 23 chromosomes. A normal human body cell, or somatic cell, contains 46 chromosomes (one set of 23 from the egg and one set of 23 from the sperm; Figure \(\PageIndex{2}\)). The letter n is used to represent a single set of chromosomes; therefore, a gamete (sperm or egg) is designated 1 n , and is called a  haploid cell.   Somatic cells (body cells) are designated 2 n and are called diploid cells.

The 23 chromosomes from a human female are each dyed a different color so they can be distinguished. During most of the cell cycle, each chromosome is elongated into a thin strand that folds over on itself, like a piece of spaghetti. The chromosomes fill the entire spherical nucleus, but each one is contained in a different part, resulting in a multi-colored sphere. During mitosis, the chromosomes condense into thick, compact bars, each a different color. These bars can be arranged in numerical order to form a karyotype. There are two copies of each chromosome in the karyotype.

Matched pairs of chromosomes in a diploid organism are called homologous (“same knowledge”) chromosomes. Of a pair of homologous chromosomes, one came from the egg and the second came from the sperm. Homologous chromosomes are the same length and have specific nucleotide segments called genes in exactly the same location, or locus. Genes, the functional units of chromosomes, determine specific characteristics by coding for specific proteins. Traits are the variations of those characteristics. For example, hair color is a characteristic with traits that are blonde, brown, or black.

Each copy of a homologous pair of chromosomes originates from a different parent; therefore, the sequence of DNA present in the two genes on a pair of homologous chromosomes is not necessarily identical, despite the same genes being present in the same locations on the chromosome. These different versions of a gene that contain different sequences of DNA are called alleles.

If you look at Figure \(\PageIndex{3}\), you can see a pair of homologous chromosomes. The chromosome shown in the figure is chromosome 15. The HERC2 gene is located on this chromosome. This gene is one of at least three genes that helps determine eye color. Each person inherits two copies of the HERC2 gene: one from the egg and one from the sperm. However, the alleles of the HERC2 gene that they inherit can be different. In the figure, the cell containing this homologous pair of chromosomes contains one blue allele and one brown allele.

pair of chromosome 15s showing blue and brown line at position of herc2 gene

The variation of individuals within a species is due to the specific combination of the genes inherited from both parents. Even a slightly altered sequence of nucleotides within a gene can result in an alternative trait. For example, there are three possible gene sequences (alleles) on the human chromosome that code for blood type: sequence A, sequence B, and sequence O. Because all diploid human cells have two copies of the chromosome that determines blood type, the blood type (the trait) is determined by which two versions of the marker gene are inherited. It is possible to have two copies of the same allele on both homologous chromosomes, with one on each (for example, AA, BB, or OO), or two different alleles, such as AB.

Minor variations of traits, such as blood type, eye color, and handedness, contribute to the natural variation found within a species. However, if the entire DNA sequence from any pair of human homologous chromosomes is compared, the difference is less than one percent. The sex chromosomes, X and Y, are the single exception to the rule of homologous chromosome uniformity: Other than a small amount of homology that is necessary to accurately produce gametes, the genes found on the X and Y chromosomes are different.

Eukaryotic Chromosomal Structure and Compaction

If the DNA from all 46 chromosomes in a human cell nucleus was laid out end to end, it would measure approximately two meters; however, its diameter would be only 2 nm. Considering that the size of a typical human cell is about 10 µm (100,000 cells lined up to equal one meter), DNA must be tightly packaged to fit in the cell’s nucleus. At the same time, it must also be readily accessible for the genes to be expressed. During some stages of the cell cycle, the long strands of DNA are condensed into compact chromosomes.

Eukaryotes, whose chromosomes each consist of a linear DNA molecule, employ a complex type of packing strategy to fit their DNA inside the nucleus (Figure \(\PageIndex{4}\)). At the most basic level, DNA is wrapped around proteins known as histones to form structures called nucleosomes . The histones are evolutionarily conserved proteins that form an octamer of eight histone proteins attached together. DNA, which is negatively charged because of the phosphate groups, is wrapped tightly around the histone core, which has an overall positive charge. This nucleosome is linked to the next one with the help of a linker DNA. This is also known as the “beads on a string” structure. This is further compacted into a 30 nm fiber, which is the diameter of the structure. At the metaphase stage, the chromosomes are at their most compact, are approximately 700 nm in width, and are found in association with scaffold proteins.

There are five levels of chromosome organization. From top to bottom: The top panel shows a DNA double helix. The second panel shows the double helix wrapped around proteins called histones. The middle panel shows the entire DNA molecule wrapping around many histones, creating the appearance of beads on a string. The fourth panel shows that the chromatin fiber further condenses into the chromosome shown in the bottom panel.

DNA replicates in the S phase of interphase. After replication, the chromosomes are composed of two linked sister chromatids (Figure \(\PageIndex{5}\)) . This means that the only time chromosomes look like an “X” is after DNA replication has taken place and the chromosomes have condensed. During the majority of the cell’s life, chromosomes are composed of only one copy and they are not tightly compacted into chromosomes. When fully compact, the pairs of identically packed chromosomes are bound to each other by cohesin proteins. The connection between the sister chromatids is closest in a region called the centromere. The conjoined sister chromatids, with a diameter of about 1 µm, are visible under a light microscope. The centromeric region is highly condensed and thus will appear as a constricted area. In Figure \(\PageIndex{4}\), it is shown as an oval because it is easier to draw that way.

linear chromosomes to sister chromatids

Unless otherwise noted, images on this page are licensed under CC-BY 4.0 by OpenStax.

OpenStax, Biology. OpenStax CNX. December 21, 2017 https://cnx.org/contents/[email protected]

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Dna and evolution.

WHAT IS DNA? DNA, or deoxyribonucleic acid, is a molecule found in the nuclei of cells. DNA contains genes, the building blocks of all organisms.

THE STRUCTURE OF DNA The most important function of DNA is its ability to replicate itself repeatedly. DNA must be copied when new cells are formed, when genetic material is passed from parents to offspring, and when coding for RNA (ribonucleic acid) to make proteins. The structure of DNA – a double helix – allows DNA to be copied successfully many times over with very few errors.

DNA’s double helix (which looks like a twisted ladder) is made of units called nucleotides. Each nucleotide consists of a phosphate, sugar, and base. The phosphates and sugars form the sides of the ladder, while the bases form the rung. A base from a nucleotide on one side of the ladder will chemically bond with a nucleotide from the other side, forming the rung. Certain bases always pair together; adenine always pairs with thymine and guanine always pairs with cytosine. These four bases have different chemical structures, causing them to pair in this specific manner.

When DNA replicates, the bonds between bases break and the DNA “unzips” itself. New nucleotides are joined to either side of the broken ladder by the work of DNA polymerase, an enzyme. Enzymes are proteins that mediate and initiate chemical reactions. When the polymerase has traveled the entire length of the DNA, it will have formed two new ladders from the original single ladder. Now the DNA has been perfectly copied from one strand into two. Some enzymes will even “proofread” the DNA to try to catch any errors!

When errors do occur during copying, mutations arise. Some mutations are beneficial, and some are not. If the mutations occur in sex cells, they can also be passed from parents to offspring. The existence of random mutations is essential for evolution theory. Populations will naturally vary; some individuals may have certain mutations while others do not. Those with beneficial mutations may be more likely to survive and produce offspring, passing their mutation to some of their offspring. Those with detrimental mutations may not be less likely to survive and produce offspring. (IMAGE from http://evolution.berkeley.edu/evosite/ evo101/images/dna-mutation.gif demonstrating a mutation)

Several scientists were responsible for the eventual discovery of DNA’s structure. Erwin Chargaff and his colleagues noticed in the mid-20th century that the amount of adenine always equaled the amount of thymine and that the same was true of guanine and cytosine. Rosalind Franklind and Maurice Wilkins performed X-ray crystallography of DNA, and the resulting image suggesting a helical shape. By putting these pieces of information together, Francis Crick and James Watson developed the double-helix model of DNA.

HOW DOES DNA PRODUCE PROTEINS? The other major feature of DNA is its ability to make proteins. Proteins provide structure for our bones and other tissues, transport materials like iron throughout our bodies, help materials move from one cell to another, function as hormones that regulate our body’s functions, act as enzymes in chemical reactions, and fight diseases in the form of antibodies. In short, proteins are among the most important cells in the body.

A protein, at its most basic level, is a chain of amino acids. Amino acids are made from codons, sequences of three nucleotides in the DNA. There are 20 amino acids found in humans. While some amino acids can be made from more than one codon, each codon can only produce one amino acid. This feature of amino acids is called redundancy.

The process of making proteins is complicated, but to summarize here, the making of proteins begins with the unzipping of DNA, as if it were going to copy itself into two DNA strands. Instead, the nucleotides join the now open side of the ladder to form mRNA, or messenger RNA. This molecular has a slightly different chemical structure than DNA, allowing it to take the genetic code from the nucleus to the cytoplasm. In the cytoplasm, mRNA will bind with a structure called a ribosome. In the ribosome, each codon of mRNA is matched with the amino acid for which the codon codes. As the codons are read in sequence, the amino acids are also assembled in the same order, forming a protein. In this way, different sections of DNA can eventually make different kinds of proteins.

An older theory of genetics maintains the principle of “one gene, one protein.” However, modern genetics has discovered that oftentimes, proteins are determined by the coordinated activities of several genes.

WHAT IS A GENE? A gene is a section of DNA responsible for a certain trait. Often, these traits are physical like the color of our hair or the length of our toes. However, genes can also produce subtler traits, like whether we have a propensity to develop cancer or what our blood type is. We inherit our genes from our biological parents.

Gregor Mendel, a monk living in the 19th century, was the first scientist to describe our modern understanding of genes. He noticed that different pea plants had different characteristics, and he rigorously bred them to see how offspring inherited the traits of their parents.

Many earlier scientists thought that offspring were a “blending” of their parents; you can see in yourself that you may look sort of like your mom and sort of like your dad. However, if you look closer, you may notice that you are not just a blending of their features. Instead, some parts of you (like your eye color) may be more similar to one parent than the other. Mendel noticed this in pea plants, too.

He realized that genes came in versions, producing different traits. For example, one gene codes for color in pea plant flowers. It may produce a purple or white color; these different versions are called alleles. It might help to think of them as “flavors” of ice cream – whether strawberry or vanilla, it’s still ice cream. However, unlike when you mix strawberry and vanilla ice cream, when Mendel bred a purple and white pea plant together, he did not get light-purple offspring. Instead, he got all purple plants. He realized that the purple allele was dominant over the recessive white allele. If a plant inherited a purple allele from one of its parents, it would be purple. It could only be white if it inherited two white alleles, one from each of its parents.

In this way, Mendel discovered several important principles of inheritance. First, an offspring inherited exactly half of its genetic material from each parent. Second, genes came in alleles and some alleles could be dominant over other alleles. Finally, he also noticed that traits independently assorted – just because a plant was purple did not also mean it had yellow seeds. These traits were inherited from different genes. You can already see how important these principles are to evolution. Traits can be inherited from parent to offspring, and the natural occurrence of different alleles creates variation within a population.

A Punnett Square is a model used by scientists to demonstrate this kind of inheritance. The genotypes – the genetic codes – of the parents are on the sides of the square. They each have two alleles – one from each of their own parents. One allele is selected from each parent and the resulting genotypes are then combined (like a multiplication table). These show the possible genotypes of a single offspring. Since genes independently assort each time parents procreate, each offspring has a possibility of being one of the four genotypes produced. A dominant allele is always written in capital letters, and a recessive allele is always written in lowercase. To determine the phenotype, or physical trait, of the possible offspring, just look at the genotype. If there is a capital letter, even just one, the offspring will have a dominant phenotype. If it has two recessive alleles, it will bear the recessive phenotype.

This was the earliest form of genetics, which is still called Mendelian (or Classical) genetics. In reality, scientists have discovered that genes are much more complicated. Some traits require the combined action of multiple genes, like hair color. Others have more than two alleles, like blood type, which has three alleles – A, B, and O. To make matters more complicated, the A and B alleles of blood are codominant. An individual who inherits an A from one parent and a B from another has AB blood type. Some genes are regulated by other genes, and some genes will not function if a mutation is present.

DO MY GENES DETERMINE WHO I AM? The short answer is, yes, our genes determine our bodies. They provide the biological information that makes us who we are. Although future developments in science and medicine may allow us to change parts of ourselves, right now we cannot change our genetic code. For example, we cannot change the genes that give us our natural hair color. Instead, if we want to change our hair color, we would have to dye it. The same is true for many disorders and diseases that have a genetic origin; we cannot change them once we inherit them from our parents.

Genes may also determine certain parts of our personalities. Research has demonstrated that genes may relate to our sexuality, the development of addictions, how our moods change, and many other elements of human psychology. However, if you know identical twins, you may already realize how difficult these studies are. Even with the same genetic code, identical twins often form distinct personalities. A lot remains to be learned in this field.

Finally, although earlier theories of genetic determination maintained that all human features were determined by genes, modern scientists understand that environment also plays a role in forming many of our physical traits, personality characteristics, and illnesses. Additionally, epigenetic effects may cause genes to turn off or on, downregulate, or upregulate. Changing how a gene is expressed will change the trait produced, even if it does not change the basic DNA sequence of the gene.

BIBLIOGRAPHY

Nash JM. 1998. The Personality Genes. Time. Monday April 27.

Stanford C, Allen JS, and Anton SC. 2009. Biological Anthropology: The Natural History of Humankind. Upper Saddle River, New Jersey: Pearson Prentice Hall.

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  • Frequently Asked Questions
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  • Reconstructions of Early Humans
  • Chesterfield County Public Library
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  • Human Origins Program Team
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  • Members Thoughts on Science, Religion & Human Origins (video)
  • Science, Religion, Evolution and Creationism: Primer
  • The Evolution of Religious Belief: Seeking Deep Evolutionary Roots
  • Laboring for Science, Laboring for Souls:  Obstacles and Approaches to Teaching and Learning Evolution in the Southeastern United States
  • Public Event : Religious Audiences and the Topic of Evolution: Lessons from the Classroom (video)
  • Evolution and the Anthropocene: Science, Religion, and the Human Future
  • Imagining the Human Future: Ethics for the Anthropocene
  • Human Evolution and Religion: Questions and Conversations from the Hall of Human Origins
  • I Came from Where? Approaching the Science of Human Origins from Religious Perspectives
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  • "Shaping Humanity: How Science, Art, and Imagination Help Us Understand Our Origins" (book by John Gurche)
  • What Does It Mean To Be Human? (book by Richard Potts and Chris Sloan)
  • Bronze Statues
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163 DNA Essay Topic Ideas & Examples

🏆 best dna topic ideas & essay examples, 💡 most interesting dna argumentative essay topics, 📑 good research topics about dna, 📌 simple & easy dna essay titles, 👍 good essay topics on dna, ❓ questions about dna.

  • The Use of DNA Technology in the O. J. Simpson’s Murder Trial The tests revealed that the blood samples taken from the crime scene, the victims’ blood and the blood at the gate matched Simpson’s blood.
  • Benefits and Challenges of DNA Profiling The simplest option is to take a sample from the suspect and compare it with the DNA found at the crime scene.
  • Forensic DNA Analysis: A Technique to Achieve a Conclusion of Identity Thus, a DNA match corroborates the fact that the suspect was at the scene of the crime and this evidence can help in establishing a case against the suspects.
  • Rosalind Franklin: The Discovery of the DNA Structure The discovery of the spatial structure of DNA undoubtedly made a decisive contribution to the development of modern biological science and related fields.
  • Ethics of Informed Consent in DNA Research The ethical issue that is the focus of the current study is the use of patient DNA for research by a company without their knowledge and consent.
  • Biochemical Metabolism: Foreign DNA Molecule The virtual gel should show the band pattern that would result from incubating the plasmid with restriction enzymes as indicated below.
  • Sex and Biology of Gender, From DNA to the Brain The video helped me actualize my prior knowledge on sex and gender as well as enriched my understanding of what biological processes make people transgender. In conclusion, the video under analysis helped me improve my […]
  • DNA Cloning and Sequencing: The Experiment The plasmid vector pTTQ18 and the GFP PCR product will be digested with restriction enzymes and the desired DNA fragments obtained thereof will be purified by Polyacrylamide gel electrophoresis and ligated with DNA ligase resulting […]
  • DNA Barcoding Sequence Analysis of Unknown Plant The efficiency of this instrumental method is built on the idea of close similarity in the structure of DNA molecules to be more precise, the arrangement of nucleotides in it between closely related species: the […]
  • The DNA Extraction Procedure: Scientific Experiment It touches on plant cell DNA extraction, animal cell DNA extraction, sequence used in DNA extraction and composition of the sample.
  • DNA Analysis: A Crime-Fighting Tool or Invasion of Privacy? This paper set out to demonstrate that DNA analysis offers a versatile tool for fighting crime and therefore ensuring the success of our civilization.
  • The Amplification of DNA Samples The isothermal amplification of nucleic acids represents a simplified process that allows for the quick and efficient accumulation of nucleic acid sequences in an environment of constant temperatures.
  • Application of DNA in Criminal Forensics In phylogenetic studies, the analysis of DNA from fossil remains allows one to determine the taxonomic identity of a species, while in forensics, one can find the connection between traces and the perpetrator or the […]
  • Neanderthal DNA in the Genomes The article shares the reasons behind the presence of Denisovans; genetic fingerprints are present in many parts of the world today.
  • The Nature of DNA Structure Discovery Thus, scientists should expand the idea about the nature of discovery without relying only on insight or results, acknowledging Franklin as a discoverer of DNA structure. It is time to reconsider the nature of discovery […]
  • The Discovery of the Deoxyribonucleic Acid (DNA) Structure Watson and Crick are independent; they come up with the idea of building a DNA structure on their own. Chadarevien argues that the image of Crick, Watson, and the double-helical DNA model has a great […]
  • Transfer of Beta-Carotene via DNA Techniques Adding yeast as a vector may significantly alleviate the incorporation of the new genes into any species because it includes protein which is vital for the species’ growth and rapid gene manipulation.
  • DNA Sequencing with Polymerase Chain Reaction Sixteen possible combinations of the four nucleotide bases of the DNA would give rise to the 16 amino acids. This explains the high melting point of a high G + C content DNA.
  • Mitochondria DNA (mtDNA) in Genealogy It is the development of mtDNA that enabled Sykes to trace and guess about the lives of the clan mothers since through it he was able to assess the genetic makeup of modern Europeans.
  • Deoxyribonucleic Acid (DNA): Structure and Function This is true of the current article, “The Structure of DNA,” which describes the function, structure, and biological significance of the most important molecule in nature.
  • DNA Evidence: The Case of the Golden State Killer Thus, DNA evidence should be used to narrow the circle of suspects before the technology is improved and other people could safely submit their DNA samples.
  • DNA Analysis in Criminal Cases The murderer, Bradley Robert Edwards, was recognized to be guilty after committing two rapes in 2016 his DNA samples were taken from under the nails of the victim as she was fighting the rapist in […]
  • DNA Profiling and Required Genetic Testing The reliable tests for conducting genetic testing should be more than one in order to remove the element of doubt on matching DNA bands.
  • DNA Microarray Technology and Applications These DNA microarrays are used by scientists in order to determine the appearance levels of a big number of genes, and also to the manifold region of a genome.
  • Covalent Modification of Deoxyribonucleic Acid Regulates Memory Formation The article by Miller and Sweatt examines the possible role of DNA methylation as an epigenetic mechanism in the regulation of memory in the adult central nervous system.
  • Short Tandem Repeat (STR) DNA Analysis and the CODIS Database The core STR loci developed provides a foundation for global databases of DNA and has future implications in the field of forensic science.
  • DNA Analysis in Criminal Investigations DNA analysis is a method aimed at the identification of a person according to his or her characteristics of DNA. In the earlier stages of an investigation, when the mentioned technique serves as a powerful […]
  • Ethical Issues on DNA Testing On some occasions, parents and clinicians have used such knowledge to manipulate the fetus’s genetic structure, hindering natural reproduction and messing with God’s creation.
  • Importance of Deoxyribonucleic Acid The history of the discovery of DNA dates back to 1865 when Gregory Mendel used theories of heredity in analyzing the genetic profiles of pea plants.
  • Knowing One’s DNA Genetic Makeup: Pros and Cons In addition, the knowledge that one might not get a job or insurance because of their genetic makeup is stressful and depressive.
  • The Concept of DNA Barcoding The first step towards safeguarding and gaining from biodiversity involves sampling, identifying, and studying the biological specimens to identify the extent of the diversity and use that knowledge for the benefit of the country.
  • Forensic Analysis of DNA and Biological Material This was the first stage when carrying out the DNA test on a biological material. Notably, the forensic analyst was not allowed to touch the collection pad of the swab as a precaution measure.
  • Developmental Biology: DNA and MicroRNA This is augmented by the strengthening of patterns and the increase in the number of lateral cells that are crucial for the process to be successful.
  • Deoxyribonucleic Acid: Review The goals of this experiment are: to enable us to become well acquainted with the physical characteristics of DNA by separating it from living tissue, and the use of each stage in the isolation process […]
  • Interesting and Relevant Applications of DNA Technology Week One Activities Learning Outcomes DNA Technology in Laboratory Medicine Diagnostic Relevance and future prospects. Interesting and Relevant Applications of DNA Technology Areas Most striking and need further review in my career – modernized to detect pathogens from the clinical samples in the diagnostic hospitals. Preferred method of identifying organisms based on genomic make up. […]
  • DNA Retention and National Security The experiences of Kuwait and the UAE are yet to demonstrate the consequences of the extreme expansion of DNA retention system, but another country has also provided some information for the consideration in the worldwide […]
  • Deoxyribonucleic Acid Profiling in Forensics The last part of the analysis includes discussion of the potential for error in DNA profiling. It has to do with the fact that DNA is a material that fulfills most of the criteria making […]
  • DNA Tests in the O.J. Simpson’s Case The fact that John’s DNA results match the crime blood DNA results does not prove beyond reasonable doubt that he is responsible for Sally’s murder.
  • The E.Z.N.A Commercial Kit: Soil DNA Extraction Optimisation In this paper, the researcher sought to investigate the effectiveness of using the kit for the purposes of optimising the extraction of DNA from marine soils.
  • The Helical Structure of DNA: Watson and Crick’s Opinion In addition, the author of this paper makes a comparison between the structure proposed by the two biologists and the information provided in recent textbooks.
  • Restriction of Lambda DNA in the Laboratory The DNA in the head of the virus has a unique structure. The restriction site is used for the purposes of recognizing a particular DNA molecule.
  • DNA Vaccines: Optimization Methods The three optimization methods scientists have been using to optimize DNA vaccines are the use of regulatory elements, optimization of the codons, and addition of the kozak sequences.
  • Comparative Sequence Study in Human and Primate DNA Samples In general, the differences between DNA samples are qualitative and quantitative, and this is explained by the fact that these are responsible for the key biological differences between humans and primates.
  • Molecular Components of the DNA Molecule The DNA serves as storage of the genetic information in the form of codes. The DNA polymerase is the enzyme that is responsible for the combination of the phosphate and the nucleotide.
  • FRET Detection or DNA Molecules It is for this reason that the method is possibly applicable in the DNA sequencing methods that are composed of single molecules and these are viewed as belonging to the “next-generation”.
  • The Concept of DNA Cloning In the approach based on cells both the replicating molecule or the biological vehicle known as the vector and the foreign DNA fragment are cut using the same restriction enzyme to produce compatible cohesive or […]
  • Biotechnology, Nanotechnology Its a Science for Brighter Future, DNA This means that there should be efforts that are aimed at the promotion of this field so that we can be in a position of solving most of these problems.
  • Post Conviction DNA Testing The DNA was first presented as evidence in court in the year 1986 in the USA, and in the subsequent years it presented serious challenges in the court rooms, presently it is been accepted in […]
  • Deoxyribonucleic Acid (DNA) Explained to Students In the chromosomes, DNA is organized and compacted by chromatin proteins. The interaction of DNA and other proteins is guided by compact structures.
  • DNA Fingerprinting as Biotechnology Application DNA fingerprinting, also known as genetic fingerprinting or DNA profiling is a method used to identify a specific individual. DNA fingerprinting is used to determine the parents of a person i.e.establish paternity.
  • Deoxyribonucleic Acid (DNA) Nanotechnology: Chemical and Physical Structure and Properties The essence of DNA in every living organism and certain viruses is that it forms the basis of the genetic instructions that are essential in the development and functioning of these organisms.
  • Use of the Information Technology to Solve Crimes: DNA Tests and Biometrics The modus operandi of the IAFIS is as follows: The fingerprints are taken after arrest, processed locally, and then electronically transmitted to state or federal agencies for processing.”The fingerprints are then electronically forwarded through the […]
  • Criminal Justice and DNA: “Genetic Fingerprinting” DNA is one of the popular methods used by criminologists today, DNA technique is also known as “genetic fingerprinting”.the name given the procedure by Cellmark Diagnostics, a Maryland company that certified the technique used in […]
  • Structure of Deoxyribonucleic Acid The nucleotides join to one another by covalent bonds between the sugar of one nucleotide and the phosphate of the next. The sequence of nucleotides in the DNA strand can be different and vary in […]
  • The Innocence Project, Habib Wahir’s Case: DNA Testing During the appeal, the court found that the semen left in the lady by the culprit did not match Habib’s DNA.
  • DNA and Genealogy Solving Cold Case Murders: The Modern Technology The issues above are essential, and they make people ask questions of whether it is reasonable to use modern technology in DNA and genealogy.
  • Modern Technology in DNA and Genealogy Solving Cold Case Murders The purpose of the study lays in establishing the relationship between ethical, legal, and privacy challenges of using genomics during the investigation.
  • DNA and Evolution – What’s Similar Transformation, in molecular genetics, is a change in the hereditary properties of cells as a result of the penetration of foreign DNA into them.
  • The Main Objective of DNA Fingerprinting in Agriculture Therefore, the main objective of DNA fingerprinting in agriculture is to overcome the limitation of insufficient dissimilarity among prior genotypes and come up with the best ideas to discover new molecular markers and collect data […]
  • DNA Diagnostic Technologies Description This has made it possible to understand the aspects concerning the development of human life as well as genetic causes of abnormalities that are seen in the human body. In the treatment of genetic diseases, […]
  • Importance of Expanding FBI’s Forensic DNA Laboratory In addition, acceptance of DNA analysis results as evidence in the Court of law has entrenched DNA analysis in forensic investigations. These have increased the number of samples for DNA analysis in FBI forensic laboratories.
  • Meiosis and Splitting of the Dna Into Gametes Meiosis is the basic process happening in the cells carrying the genetic information about the organism into two cells, while the number of chromosomes in the resulting cells is divided into two equal parts, thus […]
  • Biotechnology: Copying DNA (Deoxyribonucleic Acid) It refers to a new but identical collection of cells acquired from an original cell by the process of fission, wherein a cell divides itself forming two cells, or by the process of mitosis, wherein […]
  • DNA as the Secret of Life Deoxyribonucleic Acid which is commonly referred to as DNA is the nucleic acid that is used in the study of the genetics of the development and the functioning of almost all living organisms with an […]
  • DNA in Criminal Investigations In fact, it is possible to speak about the advent of a new field of criminalistics, DNA profiling. RFLP analysis is very discriminative, though, it is worth mentioning, that the samples have to be undamaged, […]
  • Deoxyribonucleic Acid (DNA): Structure & Function The significant factor was that the two strands run in the reverse directions and the molecule had a definite base pairing.
  • Oswald T. Avery and the Discovery of the DNA Oswald Avery was a man driven with the desire to contribute to humanity but when he finally discovered something of utmost importance the world of science was not quick enough to give recognition to his […]
  • Biomedical Discovery of DNA Structure The first parts of the book comprised of the opening of Sir Lawrence Bragg, who gave an overview of the entire book and talked about the significance of Francis Crick and James Watson’s discovery with […]
  • Artificial Manipulation of DNA Technology There is microinjection of genes in the zygote pronuclear and the other technique is by injecting the stem cells of the embryo into blastocoels.
  • Infectious Bacterial Identification From DNA Sequencing The first is the preparation of the DNA sequence and matching it with the database of known DNA sequences. Given below is a screenshot of the process PCR Amplification: To prepare the polymerase chain reaction, […]
  • Mattew Warren: Four New DNA Letters Double Life’s Alphabet In this article, the author describes the work of Steven Benner and other scientists who contributed to the improvement in understanding the nature of synthetic DNA bases.
  • DNA Testing Techniques and Challenges Therefore, even though the major part of the evidence can be inaccessible, the sample can be amplified due to the development of technology. The final stage is the evaluation of the accuracy of the analysis […]
  • DNA Profiling and Analysis Interpretation Regarding the case of the robbery and murder of a man and a woman, different types of physical evidence can be collected. However, this method can be less effective in case of the contamination of […]
  • Species Identification Reveals Mislabeling of Important Fish Products in Iran The article under discussion is devoted to the method of DNA barcoding in fish species identification and its ability to shed light on the situation with product identity in Iran.
  • Sleep Helps to Repair Damaged DNA in Neurons The researchers found that the chromosomes in the fish’s neurons would often change shape while their owners slept, enabling the repair of the damage accumulated in periods of activity.
  • DNA Replication as a Semiconservative Process The process of DNA replication has been studied extensively as the pathway to understanding the processes of inheritance and the possible platform for addressing a range of health issues occurring as a result of DNA […]
  • Obtaining a DNA Sample Legally Furthermore, it is impossible to search not only the suspect’s house but also the curtilage, which is also protected according to the Fourth Amendment because it is a private territory.
  • Dr. Michio Kaku’s Predictions of the DNA Screening In the documentary, the city planners warn the public that the insufficient growth and the development of the suburban areas threaten both the economy of the country as well as its community.
  • Exponentials and Logarithms: the Cell and DNA The result will be; log to the base of 2 of ‘x’ equals ‘y’.’y’ usually refers to the power to which one raises ‘2’ to get ‘x’ This can be simplified as follows; F =2x […]
  • Bird DNA Extraction: Sex Determination of Gallus Gallus DNA was obtained from blood, muscle tissue and feathers of the bird. The last step was to visualize the DNA extract through gel electrophoresis and making conclusions of the bird’s sex.
  • Recombinant DNA Technology and pGLO Plasmid Use Transformation of bacterial cells, which is one of the approaches used in genetic engineering, involves the transfer of genetic material from one bacterium to another using a plasmid vector.
  • DNA in Action: Sockeye Salmon Fisheries Management The researchers in the article carried out an analysis entailing a total sum of 9300 salmon fish species. The latter was followed by mixed stock samples in the lower region of Fraser River and test […]
  • DNA-Binding Specificities of Human Transcription Factors The main purpose of the experiment was to analyze and determine how human transcription factors are specifically bound by DNA. Most human transcriptional factors have been systematically analyzed in the methodology and result sections of […]
  • Innovator’s DNA: Entrepreneurial Assessment With time, I discovered that the questioning spirit was a reflection of what goes on in the mind of an entrepreneur.
  • DNA Evidence and Its Use in the US Criminal Law The concluding statement of the Supreme Court of the United States defined the procedure of obtaining DNA samples as a procedure identical to taking fingerprints or taking pictures of the crime scene.
  • Wildlife Forensic DNA Laboratory and Its Risks The mission of the Wildlife Forensic DNA Laboratory is to provide evidence to governmental and non-governmental organizations to ensure the protection of the wildlife in the country.
  • Genes, Deoxyribonucleic Acid (DNA), and Heredity Others said RNA and DNA are the same and that they are responsible for making proteins. The statement “you are your genes” is virtually right because DNA is the basis of heredity and it is […]
  • How Has DNA Changed the Field of Physical Anthropology? It is indeed correct to argue that contemporary DNA research has not only changed the field of physical anthropology in major ways, but it continues to alter and broaden our understanding and perceptions in a […]
  • Organizational DNA Analysis Moreover, due to the spontaneous growth of the organization that took a snowball design, it experienced a challenge in supplying its products to the target consumers.
  • DNA as an Evidence From a Crime Scene The mitochondrial DNA is transferred directly from the mother to the offspring and in this case, there is no DNA of the father present here.
  • DNA Definition and Its Use by the US Police The location for most DNA is the nucleus though some may be found in the mitochondria and is called mitochondrial DNA.
  • The DNA of an Entrepreneur: Is There an Entrepreneur Gene
  • The Use and Importance of DNA Profiling in the Police Force in America
  • The Role of DNA Technology in Crime Investigation
  • The Future of Computers and DNA Computing
  • Will a National DNA Database Decrease Crime in the U.S
  • The Human Genome Project and Patenting DNA
  • The Essential Features of the Watson-Crick Model on the DNA
  • The Structure of the DNA and the Future of Genetic Engineering
  • The Effectiveness of DNA Evidence in Obtaining Criminal Convictions
  • The Evolution of DNA Silencing Technology over the Years
  • The History, Function and Advancement in DNA Technologies
  • The Different Uses and Importance of DNA Replication
  • The Advancement of DNA Testing in Criminal Trials and Its Benefits
  • The Idea of Cloning Animals and Humans since the Discovery of DNA
  • The Biochemical Description of the DNA and Its Importance in Cloning
  • Why Ageing Occurs Are All Under The Guideline Of DNA
  • Self Assembling Circuits Using DNA, The Next Computer Breakthrough
  • Significance of Discoveries in Genetics and DNA
  • Understanding Recombinant DNA Technology
  • The Contribution of DNA Profiling to Changing the Crime Solving System
  • Understanding How Genetic Engineering Works from the Perspective of the DNA
  • The Use of Recombinant DNA Technology
  • The Random Amplified Polymorphic DNA Polymerase Chain Reaction
  • The Genesis of DNA Profiling and Its Use in the Modern World
  • The Effects Of Gene Editing On Human DNA
  • Use Of DNA In Criminal Investigations
  • Tools and Techniques for DNA Manipulation
  • Timeline on Our Understanding of DNA and Heredity
  • Rosalind Franklin: Unsung Hero Of The DNA Revolution
  • The Impact of the Use of DNA Analysis for Forensic Analysis
  • Your DNA: Who Has Access To It And How It Should Be Used
  • Structure and Analysis of DNA and Implications for Society
  • The Light and Dark Side of DNA Technology
  • The Functions Of DNA And Protein Synthesis
  • Watson and Crick and the Discovery of DNA’s Structure
  • The Uses of DNA Technology in Forensic Science
  • The Discovery and Understanding of the Structure of DNA by James Watson and Francis Crick
  • The Work of James Watson and Francis Crick on the Exploration of DNA Structure
  • Uses Of DNA And Fingerprints In Crime Scene Investigation
  • The Structure of DNA and the Risks Inherent in Understanding It
  • The Concept and Role of DNA Fingerprinting in Solving Crimes
  • How Is DNA Deciphered?
  • Which Scientists Participated in the Discovery of DNA?
  • What Experiments Did Scientists Use to Discover DNA?
  • What Is the Structure of DNA?
  • What Are the Methods of Diagnosing Plant Diseases Based on DNA?
  • DNA: What Are the Potential Effects on Skeletal Muscle Aging in Humans?
  • How Many Crimes Are Solved by DNA?
  • What Does Vitamin Help With DNA Repair?
  • Why Do Researchers Study DNA?
  • Is It Possible to Control the Aids Virus With a DNA Vaccine?
  • How Does DNA Help Fight Crime?
  • What Are the Analytical Methods of DNA Extraction?
  • What Is a DNA Sequence?
  • How Would You Analyze the DNA Matches of Identical Twins?
  • When Was DNA Discovered?
  • How Different Is Human DNA From Animal DNA?
  • Is It Possible to Clone the DNA of Animals and Plants?
  • How Many DNA Molecules Are in a Chromosome?
  • Is It Possible to Artificially Create DNA?
  • Can Cancer Be Detected in DNA?
  • How Accurate Is DNA Evidence?
  • What Is the DNA Replication Process?
  • Can Siblings Have Different DNA?
  • Why Does ATM Deficiency Accelerate DNA Damage in HIV-Infected Individuals?
  • Can DNA Evidence Ever Be Wrong?
  • A Bioethical Question: Is DNA Fingerprinting Mandatory?
  • Which Part of DNA Carries Genetic Information?
  • How Is DNA Used in Research?
  • What Are the Types of DNA?
  • What Is the Basic Structure of DNA?
  • Determinism Research Topics
  • Epigenetics Essay Titles
  • Down Syndrome Topics
  • Gene Titles
  • Vaccination Research Topics
  • Genetic Engineering Topics
  • Innovation Titles
  • Genetics Research Ideas
  • Chicago (A-D)
  • Chicago (N-B)

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2022 DNA Day Essay Contest: Full Essays

1 st  Place : Man Tak Mindy Shie, Grade 12 Teacher:  Dr. Siew Hwey Alice Tan School:  Singapore International School (Hong Kong) Location:  Hong Kong, China

Many would say that the most significant stride in recent genetics has been the completion of the human genome, which laid the basis for studying genetic variation. However, let us not forget that this began with the understanding of heredity based on Gregor Mendel’s observations in 1857.

Observations from Mendel’s pea plant hybridization experiments led to two fundamental principles of inheritance (1). The first was the Law of Segregation, which states that reproductive gamete cells transmit only one allele to their offspring. This means that a diploid offspring will inherit one allele from each parent. We now understand genes to be the units of heredity that carry genetic information and alleles to be different variants of a gene (2). Mendel’s second principle, the Law of Independent Assortment, states that alleles are assorted independent of each other during gamete formation, leading to individual traits being inherited independently (1). Additionally, Mendel discovered that alleles could either be dominant or recessive. An allele that constituted a phenotypic trait over the other in a heterozygous genotype was labeled dominant, while the other phenotypically unexpressed allele was called recessive (3). A class of diseases was subsequently named after Mendel as they follow the same observations; Mendelian disorders are inherited monogenic diseases that result from mutation at a single gene locus (4). A notable example is phenylketonuria, where loss-of-function mutations in the PAH gene cause systemic excess phenylalanine, resulting in behavioral abnormalities (5).

Mendel’s Laws still provide important insight in understanding Mendelian traits. For example, the Law of Segregation created the basis of dominant and recessive phenotypic ratios (6). The phenotypic ratios in family pedigrees thus allow inference of dominant and recessive traits. This is additionally helpful when an unknown disorder is found to be a Mendelian trait. Since Mendelian traits have complete penetrance, i.e. individuals carrying the pathogenic variant always express the associated trait, it is possible to search for the gene-of-interest when parental genomes are also sequenced. In present-day analysis, Whole Exome Sequencing leverages the fact that most complete penetrance genes lie in the coding region of the genome; this reduces cost and search space for identifying novel diseases (7).

We now know that the Law of Independent Assortment is applicable only when traits are located on different chromosomes. Therefore, it is important in laying the assumption of the lack of linkage between different traits whose loci are genetically far apart. Traditionally, linkage analysis used this prerequisite to identify specific loci within the disease-causing organism, as genes in proximity are often in linkage and do not sort independently (7). Regardless, this stipulation could lead to the belief that the Law of Independent Assortment has less direct value in understanding Mendelian disorders.

In contrast to monogenic diseases, complex diseases arise from multiple genetic and/or environmental factors, displaying complicated inheritance and genetics (1,8). Asthma, for example, was shown to be associated with more than 100 genes with significant inter-population variation (9), and is clinically associated with environmental allergens. Researchers are still looking for contributing variants of many common complex diseases as, unlike Mendelian Disorders, the additive inheritance explained by the associated variants does not explain the genetic contribution to the disease determined by twin studies (8). This is known as the ‘missing heritability problem’, and has prompted scientists to look for other clues.

One way to unravel complex disease genetics lies in the functional characterization of gene variants. Mendelian Diseases thus became an important way to study the link between the genotype-phenotype relationship due to a clear causal relationship and complete penetrance. This puts us in a better position to understand why a variant results in a phenotypic trait (6). Moreover, Mendelian traits allow us to elucidate the functional perturbation due to the mutation itself, providing an excellent opportunity to understand how a change in RNA/protein function caused by mutations can contribute to pathogenesis (6). When variants within complex traits, whether rare or common, are involved within neighboring variants of Mendelian traits, molecular insight may be provided regarding the pathways involved in pathogenesis. Therefore, studying the molecular basis of Mendelian traits could provide essential clues to the bigger puzzle of complex disease.

In the late 2000s, Genome-Wide Associated Studies focused on complex traits and forced Mendelian Diseases to take a back seat; yet today we find that many genetic variants must first be understood through studying Mendelian Diseases. While most Mendelian Diseases are low in incidence, they nonetheless provide valuable lessons as we continue on our journey to understand human genetics.

Citations/References:

  • Kennedy, M.A. (2005). Mendelian Genetic Disorders. In eLS, (Ed.). https://doi.org/10.1038/npg.els.0003934
  • Cooper, G. M. (2000). The Cell: A Molecular Approach. 2nd edition. NCBI. Retrieved 2022, from https://www.ncbi.nlm.nih.gov/books/NBK9944/
  • Wanjin, X., & Morigen, M. (2015). Understanding the cellular and molecular mechanisms of dominant and recessive inheritance in genetics course. Yi chuan = Hereditas, 37(1), 98–108. https://doi.org/10.16288/j.yczz.2015.01.014
  • Prosen, T., & Hogge, W. (2008). Molecular and Mendelian Disorders. The Global Library of Women’s Medicine. https://www.glowm.com/section-view/heading/Molecular%20and%20Mendelian%20Disorders/item/223#.YhygEJNBzAN
  • MedlinePlus. (2021). Phenylketonuria. https://medlineplus.gov/genetics/condition/phenylketonuria/
  • Mendel, G., & Bateson, W. (2013). Mendel’s Principles of Heredity Dover Books on Biology. Courier Corporation.
  • Antonarakis, S. E., Chakravarti, A., Cohen, J. C., & Hardy, J. (2010). Mendelian disorders and multifactorial traits: the big divide or one for all?. Nature reviews. Genetics, 11(5), 380–384. https://doi.org/10.1038/nrg2793
  • What are complex or multifactorial disorders?: MedlinePlus Genetics. (2021). Medline Plus. https://medlineplus.gov/genetics/understanding/mutationsanddisorders/complexdisorders/
  • Allergic asthma: MedlinePlus Genetics. (2020). MedlinePlus. https://medlineplus.gov/genetics/condition/allergic-asthma/

2 nd  Place: Gillian Wells, Grade 11 Teacher:  Mrs. Rebecca Hodgson School:  Ulverston Victoria High School Location:  Ulverston, England, UK

Mendel is often referred to as the “Father of Modern Genetics” (1). Prior to his experiments in plant hybridization, it was believed inherited traits resulted from blending the traits of each parent (2). From his studies, Mendel derived three principles of inheritance: the laws of dominance (in a heterozygote, the dominant allele conceals the presence of the recessive allele), segregation (each individual possesses two alleles for a specific trait, one inherited from each parent, and segregated during meiosis) and independent assortment (alleles for separate traits are inherited independently) (3, 4).

These principles give a pattern of inheritance followed by Mendelian or monogenic disorders – disorders caused by variation in a single gene (5). Mendel’s law of dominance explains the pattern of inheritance for autosomal dominant monogenic disorders, which present in individuals with only one dominant mutated allele (2). The heredity of dominant disorders – for example, Huntington’s disease and myotonic dystrophy – therefore follow the same pattern as the dominant traits Mendel observed in pea plants (4, 6). Mendel’s law of dominance also explains the pattern of inheritance for autosomal recessive monogenic disorders, which are not expressed in heterozygous individuals (carriers) as the dominant allele ‘hides’ the mutated recessive allele. Therefore, in families with multiple affected generations, the disorder will appear to ‘skip’ generations, only presenting in individuals that inherit two recessive mutated alleles of the same gene, one from each parent, as explained by Mendel’s law of segregation (2). The heredity of recessive disorders – for example, phenylketonuria and cystic fibrosis – therefore follow the same pattern as the recessive traits Mendel observed in pea plants (4, 6).

This understanding of inheritance patterns establishes the causal relationship between genes and Mendelian disorders, between genotype and phenotype (7). From this, many Mendelian disorder gene identification approaches have been developed, from positional cloning and linkage mapping to whole exome and genome sequencing (8, 9). The results are compiled in Online Mendelian Inheritance in Man (OMIM), a comprehensive database of human genes and genetic disorders, with over 26,000 entries describing over 16,000 genes and 9,000 Mendelian phenotypes (10, 11). Identifying these causal genes improves understanding of specific Mendelian disorders, allowing for molecular diagnosis and carrier testing (9).

In contrast, complex or polygenic diseases are caused by variation in multiple genes interacting with environmental and lifestyle factors, and so do not follow Mendelian inheritance patterns (12). However, widespread comorbidity between Mendelian disorders and complex diseases has been identified, suggesting a genetic association (14). Recent studies have shown that nearly 20% of the identified genes underlying Mendelian disorders contain variants responsible for genome-wide association study (GWAS) signals that cause complex diseases. 15% of all genes underlie Mendelian disorders. Mendelian genes are therefore enriched in GWAS signals and so contribute to the etiology of corresponding complex diseases (13, 14).

Given that different variants of the same gene can give rise to several different phenotypes, some Mendelian genes carry variants that contribute to complex diseases as well as causal variants for Mendelian disorders (13, 15). For example, the gene ABCA4 causes the monogenic conditions retinitis pigmentosa and Stargardt disease, as well as the complex disease age-related macular degeneration (15). Therefore, selecting genes that cause Mendelian disorders for candidate gene association studies can reveal variants that contribute to the etiology of complex diseases, allowing their genetic basis to be understood (10).

Given this genetic association between Mendelian disorders and complex diseases, the identification of Mendelian genes and knowledge of their expression can be used to further understand the mechanisms of associated complex diseases. An example in cardiovascular disease (CVD) research is the identification of causal genes for the monogenic disorder severe hypercholesterolemia. This has provided invaluable insights into lipid transport, leading to an improved understanding of CVD. From this, successful therapies have been developed for CVD using knowledge of the relevant genes and pathways (16). Mutation mechanisms observed in Mendelian disorders that can provide insight into complex disease include anticipation, gene dosage effects, and uniparental disomy (10).

Overall, Mendel’s discoveries revolutionized genetics, creating a model of inheritance that led to advancements in the diagnosis, treatment, and genetic understanding of inherited Mendelian disorders. In turn, research of Mendelian disorders has provided an understanding of the causes and mechanisms of complex diseases through genetic association – up to 23% of genes known to cause Mendelian disorders have been associated with a complex disease (17). The study of Mendelian phenotypes has and will continue to provide breakthroughs in the development of treatments and therapies of all genetic disorders (10).

References/Citations:

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3 rd  Place:  Yiyang Zhang, Grade 11 Teacher:  Dr. Qiongyu Zeng School:  Shanghai High School International Division Location:  Shanghai, China

Natural populations are characterized by astonishing phenotypic diversity determined by genes and dynamic environmental factors. In 1865, Gregor Mendel showed how traits are passed between generations through his classical pea crosses, giving us the first insight into the heritable basis of phenotypic variation [1]. Mendel’s findings revolutionized the concept of genotype-phenotype relationships and laid the foundation for modern genetics. However, our understanding of the spectrum and continuum between Mendelian and non-Mendelian diseases remains incomplete, and more work is needed to fully unravel the mechanisms underlying human diseases [2].

Mendelian diseases such as sickle cell anemia are characterized by monogenic genetic defects that result in discrete phenotypic differences [3]. Such Mendelian mutations are thought to segregate in predictable patterns, similar to the simple traits Mendel demonstrated in his pea crosses. Indeed, genetic mapping in family-based studies has led to remarkable discoveries of rare chromosomal abnormalities in patients with Mendelian diseases such as Duchenne muscular dystrophy [4]. However, even monogenic diseases follow a Mendelian inheritance pattern only sporadically. For example, in cystic fibrosis (CF), which has nearly 2000 mutant alleles in the primary causative gene Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) and six other loci associated with but not causing the disease, patients exhibit considerable interindividual variability in symptom severity [5, 6]. Thus, there is no pure Mendelian inheritance [7] or, in other words, there are essentially no simple diseases [8].

In Mendelian diseases, mutations in critical genes are usually embryologically lethal, which explains the low prevalence of Mendelian disorders in natural populations [9]. In contrast, common forms of human disease such as diabetes, heart disease, and cancer occur in previously healthy individuals, and instead of dominant disease-causing alleles, many weak genetic factors exert miniscule and accumulative effects on phenotypic outcomes. This multifactorial nature of complex diseases, which are either oligogenic or polygenic [10], means that they do not strictly adhere to Mendelian inheritance patterns in conventional mapping analyses, as segregation of genetic variants in the recombinant offspring of genetically distinct parents can easily hide extreme phenotypes and mask association signals. Therefore, researchers have developed a threshold model that assumes that there is a distribution of susceptibility for a particular trait in the population and that the trait only occurs when a threshold is exceeded [11]. This model could explain ‘all or none’ phenotypes such as cleft palate and why relatives of affected individuals are at higher risk of multifactorial traits such as hypertension or diabetes than the general population [12].

With the advent of genome-wide association studies (GWAS), which use a large sample of unrelated individuals, significant progress has been made in reliably identifying genes that influence the risk of complex diseases [13]. However, even though many thousands of disease susceptibility loci have been characterized, challenges remain, such as the ‘dark matter of inheritance’ that cannot be assigned for most complex traits [14]. Several explanations have been proposed for this, including numerous low-influence variants, rare variants, poorly recognized structural variants, and inadequate estimation of gene-gene and gene-environment interactions [15].

Gene interaction was first demonstrated in retinitis pigmentosa (RP). Since the structural integrity of retinal photoreceptors depends on the functional complexes formed by Retinal Degeneration Slow (RDS) and Rod Outer segment Membrane protein 1 (ROM1), mutations at discrete loci disrupt digenic interactions and produce the same phenotype as alleles of the same locus [16, 17]. This is a perfect example of how pushing the boundaries of Mendelian genetics can help us unravel the true physiological and cellular nature of complex diseases.

In addition to gene-gene interactions, gene-environment interactions also contribute to quantitative traits and trigger the occurrence of complex diseases such as asthma, which are influenced by numerous genetic and nongenetic factors [18]. Environmental factors can also influence traits epigenetically. For example, the more methyl donors such as folic acid or vitamin B12 are present in the diet of young mice, the higher the frequency of methylation at the CpG site of the agouti gene and the darker the coat coloration in adulthood [19, 20].

Our understanding of the causes of disease has evolved from a simplified paradigm of the Mendelian model (one variant-one disease) to a more sophisticated polygenic model. Expanding Mendelian concepts and constructing theoretical models with higher complexity is the first step toward creating a conceptual continuum between Mendelian and non-Mendelian genetic traits. In the long term, genomics and phenomics will continue to be inexhaustible sources of information to elucidate the genetic architecture of both single gene anomalies and complex diseases and to enable more personalized diagnosis and treatment.

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  • Assimes, T.L. and P.S. de Vries, Making the Most out of Mendel’s Laws in Complex Coronary Artery Disease. J Am Coll Cardiol, 2018. 72(3): p. 311-313.
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Honorable Mentions

Lena Chae Glenbrook South High School Glenview, Illinois Teacher: Mrs. Marianne Gudmundsson

Angelina Jolie, a famous actress, underwent bilateral mastectomy and oophorectomy to prevent hereditary breast and ovarian cancer that is prevalent in her family [1]. This was only possible because she was able to predict her risk of developing these cancers in her lifetime, which was substantially high enough to warrant prevention surgery. We now know that germline mutations found in BRCA1/2 genes are responsible for hereditary breast and ovarian cancer syndrome transmitted in an autosomal dominant fashion [2]. This discovery was made possible through progress in genetics which began with Mendel’s experiments in the 1800s [3].

Mendel’s discovery helped us better understand Mendelian disorders that involve single-gene mutations. First, the principles of inheritance found in plants opened up opportunities for scientists to apply their observations to patterns they noticed across human generations. This progress towards human studies from plants, helped scientists dissect human diseases that are inherited in a systematic manner. Second, Mendel’s discoveries allowed us to discover and understand the genetic material known as DNA. Because of Mendel’s observations, Watson and Crick were able to demonstrate the structure of the DNA molecule through their discovery of the double helix [4]. The Human Genome Project led by Craig Venter and Francis Collins laid the foundation for us to locate genes responsible for pathogenesis [5]. Third, understanding both the inheritance pattern of specific human hereditary diseases, along with the knowledge of the sequences in the human genome, contributed to the specific discovery of the mutations in such hereditary diseases. For instance, mutations in the HTT gene can cause Huntington’s disease [6], while mutations in the CFTR gene can cause cystic fibrosis [7]. Due to Mendel’s original discovery and experiments, scientists have been able to link genetics to human pathology.

The study of Mendelian disorders aided in a better understanding of complex diseases in two different ways. First, pedigree studies, or family tree analysis, were used to study monogenic Mendelian disorders with high penetrance; this led to a realization that many human diseases cannot be explained by the Mendelian principle of inheritance. Except for a few hereditary diseases, most human diseases involve more than one gene abnormality when comparing the affected versus unaffected members within a family. This finding led to the concept of stepwise multigene abnormalities and environmental interaction with respect to pathogenesis. Second, Genome-Wide Association Studies (GWAS), which is the population-level study of genes and human diseases, could be understood as an aggregate of linkage analyses based on Mendelian principles [8]. It also extended the field of genetics. GWAS made it possible for scientists to define the role of single DNA mutations in complex diseases. Hundreds of thousands of single-nucleotide polymorphisms (SNPs) can be tested to explore the associations between these variants and disease in larger populations. For example, through the GWAS study, over 40 loci have been found to be associated with Type 2 Diabetes Mellitus (DM) [9]. Another highly heritable psychiatric disorder, schizophrenia, is linked with 108 genetic loci according to a GWAS consisting of more than 150,000 samples [10]. An improved understanding of comprehensive genomic mutations involved in such complex diseases led to the creation of a risk profile score (RPS), which is currently used to predict the risk of such disease development [11].

However, human diseases can sometimes be more than just changes in DNA. Both pedigree analysis and GWAS assume that hereditary diseases can fully be explained by genetic mutations. But epigenetic changes can be equally or more important [12]. Epigenetic processes such as DNA methylation or histone modifications, triggered by environmental and behavioral changes, may turn the target gene expressions “on” or “off”. Furthermore, protein modification may also play a role in pathogenesis. Therefore, to better understand complex diseases, it is critical to utilize both the study of genetics stemming from Mendel’s discoveries, and the non-genetic processes including epigenetics, transcriptomics, and proteomics [12].

In summary, Mendel’s discovery helped us better understand Mendelian disorders but also more complex diseases. Owing to Mendel’s principles of inheritance, scientists are now equipped with platforms and techniques to analyze both Mendelian disorders and complex diseases. Individualized treatments are now made possible through accurate diagnoses including identification of mutations leading to disease. Just as Angelina Jolie was able to prevent hereditary breast and ovarian cancer through germline DNA profiling, further in-depth DNA screening in a population can lead to a significant reduction in the risk of various hereditary and complex diseases.

  • Jolie, A. (2013, May 14). My Medical Choice. New York Times, pp. 25–25.
  • Rebbeck, T. R., Friebel, T., Lynch, H. T., Neuhausen, S. L., van ’t Veer, L., Garber, J. E., Evans, G. R., Narod, S. A., Isaacs, C., Matloff, E., Daly, M. B., Olopade, O. I., & Weber, B. L. (2004). Bilateral prophylactic mastectomy reduces breast cancer risk in BRCA1 and BRCA2 mutation carriers: THE PROSE Study Group. Journal of Clinical Oncology, 22(6), 1055–1062. https://doi.org/10.1200/jco.2004.04.188
  • B. (2021, May 21). Gregor Mendel. Biography. https://www.biography.com/scientist/gregor-mendel
  • Pray, L. (2008) Discovery of DNA structure and function: Watson and Crick. Nature Education 1(1):100
  • Adams, J. (2008) Sequencing human genome: the contributions of Francis Collins and Craig Venter. Nature Education 1(1):133
  • Conneally P. M. (1984). Huntington disease: genetics and epidemiology. American journal of human genetics, 36(3), 506–526.
  • Gallati S. (2003). Genetics of cystic fibrosis. Seminars in respiratory and critical care medicine, 24(6), 629–638. https://doi.org/10.1055/s-2004-815659
  • Manolio T. A. (2010). Genomewide association studies and assessment of the risk of disease. The New England journal of medicine, 363(2), 166–176. https://doi.org/10.1056/NEJMra0905980
  • Hakonarson, H., & Grant, S. F. (2011). Genome-wide association studies (GWAS): impact on elucidating the aetiology of diabetes. Diabetes/metabolism research and reviews, 27(7), 685–696. https://doi.org/10.1002/dmrr.1221
  • Schizophrenia Working Group of the Psychiatric Genomics Consortium (2014). Biological insights from 108 schizophrenia-associated genetic loci. Nature, 511(7510), 421–427. https://doi.org/10.1038/nature13595
  • Xiao, E., Chen, Q., Goldman, A. L., Tan, H. Y., Healy, K., Zoltick, B., Das, S., Kolachana, B., Callicott, J. H., Dickinson, D., Berman, K. F., Weinberger, D. R., & Mattay, V. S. (2017). Late-Onset Alzheimer’s Disease Polygenic Risk Profile Score Predicts Hippocampal Function. Biological Psychiatry: Cognitive Neuroscience and Neuroimaging, 2(8), 673–679. https://doi.org/10.1016/j.bpsc.2017.08.004
  • Centers for Disease Control and Prevention. (2020, August 3). What is epigenetics? Centers for Disease Control and Prevention. Retrieved March 1, 2022, from https://www.cdc.gov/genomics/disease/epigenetics.htm

  Aadit Jain International Academy Bloomfield Hills, Michigan Teacher: Mrs. Suzanne Monck

Nearly two centuries ago, Gregor Mendel launched the scientific community into the vast world of genetics and diseases with his experiments on the common pea plant (1,2). Specifically, his principles have been instrumental in the plethora of discoveries that have been made in Mendelian disorders. With around 400 million people worldwide suffering from one of the 7,000 Mendelian disorders, much research today centers on identifying the genetic causes of these diseases (3). While Mendel was unaware of genes and DNA when he conducted his study (2), his discoveries kickstarted the substantial research that scientists have undertaken on Mendelian disorders.

Mendel’s principles have directly allowed scientists to understand how Mendelian disorders are inherited. For example, his notable discovery that phenotypes of recessive traits can skip generations (2) applies to Mendelian disorders in the case of carriers (4). These are individuals who may not display the disorder phenotype but still carry and can pass on the altered gene (4). Therefore, it is essential to analyze pedigrees of affected families to determine whether the disease-causing gene has a dominant or recessive phenotype. Importantly, this knowledge helps genetics professionals understand the risk that individuals have of passing on a disorder (5). For example, a person who suffers from an autosomal dominant disorder bears a 50% chance of passing the affected gene to each offspring (5). In contrast, two heterozygous parents for an autosomal recessive disorder have a 25% chance of having an offspring affected with the disorder with each pregnancy (5).

Mendel’s principles of uniformity, segregation, and independent assortment demonstrate how genes and alleles are inherited (2). However, subsequent research revealed exceptions such as the sex-linked pattern of inheritance (2,6). Contrary to inheritance of autosomal single-gene diseases, males and females receive a different number of copies of the implicated gene for sex-linked disorders due to their respective pairs of sex chromosomes (1). As a result, sex-linked diseases tend to be prevalent in only one gender (1). For example, Hemophilia A, a blood clotting disorder, typically affects only males because it is an X chromosome-linked recessive disease (1). It is evident that although Mendel’s principles have laid a strong foundation of inheritance patterns, the scientific community’s understanding of Mendelian disorders is greatly enhanced through new research.

Mendel’s discoveries have been fundamental in developing effective methods to test for disorders. With the understanding that the same allele codes for a specific phenotype, researchers have individuals with the same phenotype disorder undergo sequencing in order to identify the defective gene (7). Such was the case in 2010, when scientists discovered that the MLL2 gene was responsible for Kabuki syndrome: 7 out of 10 individuals in the group suffered from a loss of function in that gene (7). Since then, with the Matchmaker Exchange (MME) and the Monarch Initiative, there has been an emphasis on sharing phenotype and genotype data in order to discover new Mendelian disorders (7).

Although complex diseases are influenced by several factors and do not fully follow the inheritance patterns (8), investigating Mendelian disorders can provide insight into the implicated genes and pathways in them. By analyzing data from established databases, genetic researchers found that in fact 54% of Mendelian disease genes play a notable role in complex diseases as well (9). Genes underlying both diseases tend to be associated with more phenotypes and protein interactions, so studying them can be quite useful in understanding Mendelian disorders and consequently complex diseases (9). In some cases, individuals diagnosed with complex diseases have an underlying monogenic condition that is the cause (10). This specifically highlights the significance of research techniques for single-gene disorders to investigations of complex diseases. In the case of hypercholesterolemia, for example, monogenic forms of the disease were used to determine the impact of lipid transport and to identify the involved pathways in the development of this complex disease (10).

Research on Mendelian disorders has helped scientists understand gene function and mechanisms overall. Studying single-gene disorders can further provide insight into the genetic pathways of complex diseases (9). In fact, with genome-wide association studies (GWAS) into single nucleotide polymorphisms (SNP), thousands of genes implicated in complex diseases have been identified (9). Although many details of complex diseases have been established, heritable aspects still remain uncertain (9). Overall, knowledge of Mendel’s principles and Mendelian disorders will be essential in this case and others as research delves further into disease processes.

  • Chial, Heidi. “Mendelian Genetics: Patterns of Inheritance and Single-Gene Disorders.” Edited by Terry McGuire. Nature Education, 2008, www.nature.com/scitable/topicpage/mendelian-genetics-patterns-of-inheritance-and-single-966/.
  • Miko, Ilona. “Gregor Mendel and the Principles of Inheritance.” Edited by Terry McGuire. Nature Education, 2008, www.nature.com/scitable/topicpage/gregor-mendel-and-the-principles-of-inheritance-593/.
  • Ganguly, Prabarna. “NIH funds new effort to discover genetic causes of single-gene disorders.” National Human Genome Research Institute, 15 July 2021, www.genome.gov/news/news-release/NIH-funds-new-effort-to-discover-genetic-causes-of-single-gene-disorders.
  • “Carrier.” National Human Genome Institute, www.genome.gov/genetics-glossary/Carrier.
  • “If a genetic disorder runs in my family, what are the chances that my children will have the condition?” MedlinePlus, medlineplus.gov/genetics/understanding/inheritance/riskassessment/.
  • Nickle, Todd, and Isabelle Barrette-Ng. “3.5: Sex-Linkage- An Exception to Mendel’s First Law.” Biology LibreTexts, 3 Jan. 2021, bio.libretexts.org/Bookshelves/Genetics/Book%3A_Online_Open_Genetics_(Nickle_and_Barrette-Ng)/03%3A_Genetic_Analysis_of_Single_Genes/3.05%3A__Sex-Linkage-_An_Exception_to_Mendels_First_Law.
  • Seaby, Eleanor G., et al. “Strategies to Uplift Novel Mendelian Gene Discovery for Improved Clinical Outcomes.” Frontiers in Genetics, 17 June 2021, www.frontiersin.org/articles/10.3389/fgene.2021.674295/full.
  • Craig, Johanna. “Complex Diseases: Research and Applications.” Edited by Alexandre Vieira. Nature Education, 2008, www.nature.com/scitable/topicpage/complex-diseases-research-and-applications-748/.
  • Jin, Wenfei, et al. “A systematic characterization of genes underlying both complex and Mendelian diseases.” Human Molecular Genetics, vol. 21, no. 7, 20 Dec. 2011, academic.oup.com/hmg/article/21/7/1611/2900796.
  • Chong, Jessica X., et al. “The Genetic Basis of Mendelian Phenotypes: Discoveries, Challenges, and Opportunities.” Science Direct, www.sciencedirect.com/science/article/pii/S0002929715002451.

  Sharanya Ravishanker Conestoga High School Berwyn, Pennsylvania Teacher: Mrs. Liz Gallo

Through his genetic experimentations with pea plants, Gregor Mendel established the following Laws of Inheritance that remain critical to our understanding of heredity: The Law of Segregation, The Law of Independent Assortment, and The Law of Dominance (1, 2). In summation, phenotypes—expressed characteristics—are correlated with the type of allele inherited from each parent during gamete formation when genes randomly separate. If an allele is dominant, it is expressed; if an allele is recessive, the associated characteristic will not be displayed unless a matching recessive allele is inherited from the other parent.

These laws and inheritance patterns form the basis of our understanding of Mendelian disorders, rare monogenic diseases caused by alterations—often single-nucleotide polymorphisms (SNPs) and corresponding amino-acid substitutions resulting in the production of unwanted or malfunctioning proteins—in just one of the 25,000 genes in a human genome (3, 4, 5). These mutations typically occur in germline cells, and are thus passed down through DNA to every cell of the offspring (6). Well known Mendelian diseases include cystic fibrosis, sickle cell anemia, and Huntington’s disease.

Through the application of Mendel’s Laws, geneticists have identified five modes of inheritance for Mendelian disorders: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, and mitochondrial (7), paving the way for geneticists to accurately diagnose Mendelian disorders, a step crucial in providing patients with the treatment and specific care they require, as well as revealing significant information vital to the family planning of individuals who carry recessive alleles for threatening disorders. Genealogical records and pedigree analyses have been utilized to trace inheritance through families, but next-generation sequencing technology has gained traction as a method to detect changes in nucleotide orders. Exome-sequencing, for example, focuses on identifying variants in the protein-coding region (exons), and is regarded as cost-effective due to its specificity, focusing on only 1% of the human genome (8, 9, 10). On the other hand, whole genome sequencing can be advocated for due to its capture of DNA variations outside of exons as well as within. Still, as benign polymorphisms are highly prevalent and frequent, entire genome sequencing can make it difficult to prioritize harmful mutations due to the sheer amount of variants shown (9, 11). RNA sequencing can provide support here by quantifying the effect to which a gene is expressed (11, 12 ).

Information gathered from these methods and Mendelian principles regarding dominance also enable geneticists to determine trait-associated gene loci, allowing for a better understanding of protein formation, modification, and function (13). In fact, as Rockefeller University president and accomplished biochemist Dr. Richard Lifton notes, understanding the connection between genes and expressed traits—SNP and product—has served as “starting points for understanding disease and human biology in general”. For example, analysis of a Mendelian form of hypertension resulted in the discovery of a pathway regulating salt reabsorption and potassium secretion in the kidney (14). Similar discoveries of pathways as a result of studies into Mendelian disorders can increase our understanding and ability to treat complex disorders such as cancer, even if these diseases disregard Mendelian principles of inheritance on account of being caused by numerous genetic and environmental factors interacting with one another.

In the same vein, understanding the results of SNP modification allows for research into the genetic susceptibility for various complex disorders and its correlation with environmental exposure. For example, it was determined that individuals whose genotype is homozygous recessive for xeroderma pigmentosum are highly susceptible to UV light related disorders due to mutations in DNA-repairing genes. Similarly, individuals with a mutation in the Alpha-1 gene are at a greater risk for emphysema, especially through smoking, though the mutation itself isn’t causative of the disease (15). The aforementioned linkages between genes and phenotypes would not be possible without the research into Mendelian disorders that revealed crucial information regarding the impacts of individual genes on expressed phenotypes.

Overall, studies into Mendelian diseases—in turn impacted by the understanding of Mendel’s Laws of Inheritance—have contributed significantly to our knowledge of more complex disorders. This knowledge will prove beneficial in developing more efficient medicinal drugs and therapies that effectively target detrimental proteins or alter gene expression to receive desired results (16). As Dr. James Luspki, Professor of Molecular and Human Genetics at Baylor College of Medicine says, “We’re on the threshold of new explanations of disease inheritance and development” (14). Resulting discoveries from studies into Mendelian principles and disorders will undoubtedly clear the way towards greater advancements in our ability to treat complex disorders.

References/Citations: Mendel’s Law of Segregation. 15 Aug. 2020, https://bio.libretexts.org/@go/page/13271. “Inheritance of Traits by Offspring Follows Predictable Rules.” Nature. Scitable by Nature Education, www.nature.com/scitable/topicpage/inheritance-of-traits-by-offspring-follows-predictable-6524925/#:~:text=One%20allele%20for%20every%20gene,same%22)%20for%20that%20allele. Accessed 22 Feb. 2022. Jackson, Maria et al. “The genetic basis of disease.” Essays in biochemistry vol. 62,5 643-723. 2 Dec. 2018, doi:10.1042/EBC20170053 Coding single-nucleotide polymorphisms associated with complex vs. Mendelian disease: Evolutionary evidence for differences in molecular effects. Paul D. Thomas, Anish Kejariwal. Proceedings of the National Academy of Sciences Oct 2004, 101 (43) 15398-15403; DOI: 10.1073/pnas.0404380101 The 1000 Genomes Project Consortium. A global reference for human genetic variation. Nature 526, 68–74 (2015). https://doi.org/10.1038/nature15393 “Germline Mutation.” National Cancer Institute, www.cancer.gov/publications/ dictionaries/cancer-terms/def/germline-mutation. Accessed 22 Feb. 2022. Genetic Alliance; District of Columbia Department of Health. Understanding Genetics: A District of Columbia Guide for Patients and Health Professionals. Washington (DC): Genetic Alliance; 2010 Feb 17. Appendix B, Classic Mendelian Genetics (Patterns of Inheritance) Available from: https://www.ncbi.nlm.nih.gov/books/NBK132145/ “Exome Sequencing.” Science Direct, 2018, www.sciencedirect.com/topics/ agricultural-and-biological-sciences/exome-sequencing. Accessed 22 Feb. 2022. “What are whole exome sequencing and whole genome sequencing?” MedlinePlus, 28. July 2021, medlineplus.gov/genetics/understanding/testing/sequencing/. Accessed 22 Feb. 2022. Bamshad, M., Ng, S., Bigham, A. et al. Exome sequencing as a tool for Mendelian disease gene discovery. Nat Rev Genet 12, 745–755 (2011). https://doi.org/10.1038/nrg3031 Byron, S., Van Keuren-Jensen, K., Engelthaler, D. et al. Translating RNA sequencing into clinical diagnostics: opportunities and challenges. Nat Rev Genet 17, 257–271 (2016). https://doi.org/10.1038/nrg.2016.10 Wang, Zhong et al. “RNA-Seq: a revolutionary tool for transcriptomics.” Nature reviews. Genetics vol. 10,1 (2009): 57-63. doi:10.1038/nrg2484 Chial, H. (2008) Rare Genetic Disorders: Learning About Genetic Disease Through Gene Mapping, SNPs, and Microarray Data. Nature Education 1(1):192 Benowitz, Steven. “Centers for Mendelian Genomics uncovering the genomic basis of hundreds of rare conditions.” National Human Genome Research Institute, 6 Aug. 2015, www.genome.gov/news/news-release/ Centers-for-Mendelian-Genomics-uncovering-the-genomic-basis-of-hundreds-of-rare-conditions. Accessed 22 Feb. 2022. Craig, J. (2008) Complex diseases: Research and applications. Nature Education 1(1):184 Heguy, A et al. “Gene expression as a target for new drug discovery.” Gene expression vol. 4,6 (1995): 337-44.

Zhiyuan Shi BASIS International School Hangzhou Hangzhou, China Teacher: Dr. Dongchen Xu

Mendelian theories provided the foundations for the contemporary understanding of heredity. Mendel’s legacy has been particularly beneficial to medical sciences, where research on inheritance patterns of Mendelian disorders has been made possible through utilizing Mendel’s theory. Mendelian theories serve as robust models for evaluating and verifying the inheritance patterns of particular diseases. Even though our current understanding of genetics has moved beyond the Mendelian model, studying certain Mendelian disorders such as oculocutaneous albinism can lead to an improved understanding of complex disorders with polygenic inheritance.

Oculocutaneous albinism is an autosomal-recessive condition caused by the extremely low level of melanin biosynthesis due to mainly four genes (1, 2). Individuals with this illness will also experience whitening of the skin, certain degrees of vision deterioration, and a higher risk of contracting skin cancer due to the lack of dermal melanin (1, 2); understanding the underlying inheritance pattern of albinism would be advantageous towards the prevention of skin cancers. The genetic cause of oculocutaneous albinism can be explained by Mendelian genetics. The disorder is autosomal, meaning neither the gender of the parents nor the gender of the offspring plays a role in its inheritance. The disorder is recessive, meaning both parents must be carriers for birthing an Albino child (3). Through examination of information like the ones above and specific pathology of the disorder, one can establish critical predictions of an offspring’s genotype based on the family’s history. Such analyses enable us to speculate and reconstruct pedigrees for Mendelian disorders using family history. Information regarding Mendelian disorders running in the family and the possible genotypes for offsprings (50% risk of being carriers and 25% risk of being affected) are important to parents seeking family planning suggestions, reinforcing prevention.

Mendelian and non-Mendelian diseases are often regarded as segregated families of genetic disorders. Complex non-Mendelian disorders involve polygenic traits that don’t follow Mendelian disorders’ monogenic properties. However, genes responsible for monogenic diseases correspondingly contribute to the expression of polygenic traits (4). Mendelian disorders are key in providing the individual monogenic components that contribute to complex disease’s polygenic causes. Some of the gene variants responsible for skin pigmentation disorder and skin cancer are the exact genes responsible for the pigment deficiency in the Mendelian disorder oculocutaneous albinism. The 2 most notable ones are variants of the gene TYRP1, a gene coding for the protein tyrosinase-related protein 1, which contributes to melanosome integrity; and gene SLC45A2, which code for a cation exchange protein that transports material required for melanin synthesis into the melanosome (2, 6, 7). Variants of these genes are inherited as monogenic traits, and studies show they contribute to the formation of polygenic skin cancers such as squamous skin cell carcinoma (8). Mendelian inheritance of other variants of the 2 listed genes can even cause other polygenic skin cancers such as melanoma, exhibiting excessive melanin levels. Research showed that heterozygous variants of TYRP1 and SLC45A2 are overrepresented in families with multiple cases of melanoma (9).

Although overrepresentation of SLC45A2 is found in cases of melanoma, variants of the gene can have the opposite effect. A meta-analysis conducted by Ibarrola-Villava et al., 2012, revealed that the SLC45A2 p.Phe374Leu variant had an odds ratio of 0.41 for melanoma (p = 3.50 * 10^-17), enough for concluding that SLC45A2 p.Phe374Leu negatively correlates with melanoma formation (13). This and the previous evidence suggest that factors affecting melanin concentration, one of the key determinants for the presence of different types of polygenic skin cancers, could be partially attributed to the variants of TYRPI and SLC45A2 genes that involve Mendelian inheritance mechanisms.

Another polygenic disorder with Mendelian roots is growth disorder, in which several genes that contribute to the complex disorder of growth disorders are monogenic. For instance, one factor contributing to the common short stature in growth disorders such as dwarfism is the autosomal dominant Mendelian disorder achondroplasia, resulting from the Mendelian inheritance of the mutated FGFR3 gene (10,11). Another monogenic disorder that contributes to growth disorders such as dwarfism is growth hormone deficiency, an autosomal recessive disorder resulting from the mutation and Mendelian inheritance of the mutated GH1 or GHRHR gene (12).

The Mendelian factors underlying both skin cancer and growth disorders demonstrated the value of studying Mendelian inheritance patterns in complex disorders. Although Mendelian diseases only contribute to a small proportion of all known human disorders, understanding their underlying mechanism and pattern, and utilizing them alongside conventional methods for the investigation of complex diseases is of great importance(5), and would produce spectacular innovations in the field of genetics.

  • Marçon, C. R., & Maia, M. (2019). Albinism: Epidemiology, genetics, cutaneous characterization, psychosocial factors. Anais brasileiros de dermatologia. Retrieved March 1, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6857599/
  • Grønskov, K., Ek, J., & Brondum-Nielsen, K. (2007, November 2). Oculocutaneous albinism. Orphanet journal of rare diseases. Retrieved March 1, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2211462/
  • Gulani, A. (2021, May 8). Genetics, autosomal recessive. StatPearls [Internet]. Retrieved March 1, 2022, from https://www.ncbi.nlm.nih.gov/books/NBK546620/
  • Franić, S., Groen-Blokhuis, M. M., Dolan, C. V., Kattenberg, M. V., Pool, R., Xiao, X., Scheet, P. A., Ehli, E. A., Davies, G. E., van der Sluis, S., Abdellaoui, A., Hansell, N. K., Martin, N. G., Hudziak, J. J., van Beijsterveldt, C. E. M., Swagerman, S. C., Hulshoff Pol, H. E., de Geus, E. J. C., Bartels, M., … Boomsma, D. I. (2015, October). Intelligence: Shared genetic basis between Mendelian disorders and a polygenic trait. European journal of human genetics : EJHG. Retrieved March 1, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4592100/
  • Lango Allen, H., Estrada, K., Lettre, G., Berndt, S. I., Weedon, M. N., Rivadeneira, F., Willer, C. J., Jackson, A. U., Vedantam, S., Raychaudhuri, S., Ferreira, T., Wood, A. R., Weyant, R. J., Segrè, A. V., Speliotes, E. K., Wheeler, E., Soranzo, N., Park, J.-H., Yang, J., … Hirschhorn, J. N. (2010, October 14). Hundreds of variants clustered in genomic loci and biological pathways affect human height. Nature. Retrieved March 1, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2955183/
  • Del Bino, S., Duval, C., & Bernerd, F. (2018, September 8). Clinical and biological characterization of skin pigmentation diversity and its consequences on UV impact. International journal of molecular sciences. Retrieved March 1, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6163216/
  • Federico, J. R. (2021, August 27). Albinism. StatPearls [Internet]. Retrieved March 1, 2022, from https://www.ncbi.nlm.nih.gov/books/NBK519018/
  • Board, P. D. Q. C. G. E. (2009, July 29). Genetics of Skin Cancer (PDQ®). PDQ Cancer Information Summaries [Internet]. Retrieved March 1, 2022, from https://www.ncbi.nlm.nih.gov/books/NBK65895/
  • Nathan, V., Johansson, P. A., Palmer, J. M., Howlie, M., Hamilton, H. R., Wadt, K., Jönsson, G., Brooks, K. M., Pritchard, A. L., & Hayward, N. K. (2019). Germline variants in oculocutaneous albinism genes and predisposition to familial cutaneous melanoma. Pigment Cell & Melanoma Research, 32(6), 854–863. https://doi.org/10.1111/pcmr.12804
  • Krakow, D., & Rimoin, D. L. (2010, April 27). The skeletal dysplasias. Nature News. Retrieved March 1, 2022, from https://www.nature.com/articles/gim201054
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  • U.S. National Library of Medicine. (2020, August 18). Isolated growth hormone deficiency: Medlineplus Genetics. MedlinePlus. Retrieved March 1, 2022, from https://medlineplus.gov/genetics/condition/isolated-growth-hormone-deficiency/
  • Ibarrola-Villava, M., Hu, H.-H., Guedj, M., Fernandez, L. P., Descamps, V., Basset-Seguin, N., Bagot, M., Benssussan, A., Saiag, P., Fargnoli, M. C., Peris, K., Aviles, J. A., Lluch, A., Ribas, G., & Soufir, N. (2012). MC1R, SLC45A2 and Tyr genetic variants involved in melanoma susceptibility in southern European populations: Results from a meta-analysis. European Journal of Cancer, 48(14), 2183–2191. https://doi.org/10.1016/j.ejca.2012.03.006

Audric Thakur Reading School Reading, United Kingdom Teacher: Ms. Francis Howson

Mendel’s research intended to determine how characteristics of an individual were inherited by their offspring. At the time, the scientific community lacked the genotypic knowledge required to explain how genetic information was transferred to an individual¹. Only in 1826 did Augustin Sageret discover the idea of trait dominance³ (amid a cultural resurgence of Preformation Theory²), and so it was through observational study that Mendel developed the laws of heredity which ground our understanding of Mendelian disorders today.

Most famous of Mendel’s work are those regarding the rugosus locus and the presence or absence of the SBE1 gene⁴, phenotypically expressed by the distinctive ’round’ or ‘wrinkled’ shapes of pea pods respectively⁵. Specifically, he determined the recessive nature of the wrinkled trait through his monohybrid crossing of a uniformly heterozygous generation of pea plants (which themselves were the progeny of a homozygous-dominant and homozygous-recessive cross)⁵. Naturally, this uniform generation of heterozygous peas all possessed the round characteristic. However, Mendel proved that these peas retained their parents’ ‘elementen’⁵ (or more accurately, DNA), since they went on to produce offspring with characteristics from the grandparent generation, evidenced by the 3:1 ratio of round to wrinkled offspring – clear to us now through use of a Punnett square⁶. Of course, these results were the aggregate of a large sample size across several iterations⁵, and therefore incredibly precise (to the point of controversy⁷). As such, they formed the basis for his laws of heredity.

Deriving Mendel’s laws from his work on pea plants is critical to understanding monogenic conditions because their inheritance patterns are often identical⁸, enabling us to make accurate comparisons between the two. This is demonstrated by the Mendelian condition phenylketonuria (PKU)⁹-¹⁰, an autosomal recessive disorder caused by an absent PAH gene at the genetic locus 12q23.2¹⁰.

Citing national Newborn Screening Reports¹¹, 1.7606% of Caucasian-Americans (1996-2000) are heterozygous carriers of PKU. If I apply some simplified mathematics (i.e. ignoring lifestyle factors), the probability of both parents in a Caucasian-American household being carriers of PKU is 0.0310% (0.017606²). Therefore, as per the rules of inheritance followed by Mendel’s pea plants, 0.0077% (0.0310*0.25) of the Caucasian-American population should be expressors of PKU. According to the National Library of Medicine¹¹, the official estimate is 0.0075% – a remarkable example of the accuracy and utility of Mendel’s work, and how understanding and implementing his discoveries has relevant real-world significance, being comparable to large-scale medical statistics to this day.

Unfortunately, it must be noted that Mendelian disorders are an exceptional minority of genetic conditions – the emerging consensus that most exist on a spectrum from Mendelian conditions¹² (high gene penetrance and low gene-environment interaction¹³) to increasingly complex conditions (incomplete or varying gene penetrance and high gene-environment interaction¹³), and that complex disorders are influenced by a multitude of interconnected factors¹⁴. This is why scientists approach complex disorders by assessing risk of onset, rather than applying Mendelian rules of inheritance. Nevertheless, links between the genotypic expression of Mendelian conditions in an individual and the onset of associated complex disorders have been established in the last decade or so of scientific inquiry¹³.

Studies regarding Mendelian comorbidities alongside complex disorders have proved that genetic loci containing causal variants for both Mendelian disorders and complex disease tend to have a greater influence on the onset of a complex disorder compared to genes that pertain to risk factors for only that complex disorder¹³. This means, for an individual afflicted by a series of Mendelian disorders, the probability that they will develop a complex disorder whose determinant genes are simultaneously involved in expressing those Mendelian disorders is significantly higher¹³. For example, an increased risk of schizophrenia is involved with patients who carry genetic variants of Lujan-Fryns and velo-cardio-facial syndromes¹⁷ (clear correlation), and a higher likelihood of developing type-2 diabetes mellitus if the patient suffers from Huntington’s disease, Friedreich’s ataxia and beta-thalassemia¹⁵-¹⁶ (partially supported correlation). This demonstrates that Mendelian-associated genes are certainly influential in determining emergence of a complex disorder. Therefore, understanding inheritance patterns of these Mendelian conditions is essential to create an accurate way of ascertaining the risk of onset for more complex conditions.

Despite the elusive nature of inheritance patterns surrounding several complex disorders, insight can nevertheless be found in studying genes associated with Mendelian conditions. Due to their high penetrance and straightforward inheritance patterns¹³, these monogenic conditions are easy to diagnose and engage in research with, providing a unique foothold to better understand many complex conditions, and allowing us to form more realistic models to predict their onset¹⁸.

Note: citations from sources published prior to 2015 have been used for historical knowledge or to explain/discuss historical scientific experiments only. The exception to this is reference 15.

  • Durmaz, A. A., Karaca, E., Demkow, U., Toruner, G., Schoumans, J., & Cogulu, O. (2015). Evolution of genetic techniques: Past, present, and beyond. BioMed research international. Retrieved February 28, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4385642/
  • Maienschein, J. (2005, October 11). Epigenesis and Preformationism. Stanford Encyclopedia of Philosophy. Retrieved February 28, 2022, from https://plato.stanford.edu/entries/epigenesis/#8
  • Zirkle, C. (1951, June). Gregor Mendel & his Precursors. Retrieved February 28, 2022, from https://www.mun.ca/biology/scarr/Zirkle_%281951%29_Gregor_Mendel_&_his_Precursors,%20Isis_42,97-104.pdf
  • Smith, A., & Martin, C. (2020, December 11). A history of wrinkled-seeded research in PEA. John Innes Centre. Retrieved February 28, 2022, from https://www.jic.ac.uk/advances/a-history-of-wrinkled-seeded-research-in-pea/
  • Miko, I. (2008). Gregor Mendel and the Principles of Inheritance. Nature news. Retrieved February 28, 2022, from https://www.nature.com/scitable/topicpage/gregor-mendel-and-the-principles-of-inheritance-593/
  • (while the citation doesn’t reference the SBE1 gene in particular, it does discuss other recessive pea plant traits, making it useful nevertheless) LibreTexts, O. S. (2021, September 22). 8.2: Laws of inheritance. Biology LibreTexts. Retrieved February 28, 2022, from https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_Concepts_in_Biology_(OpenStax)/08%3A_Patterns_of_Inheritance/8.02%3A_Laws_of_Inheritace
  • Radlick, G. (2015, October 9). Beyond mendelfisher – eprints.whiterose.ac.uk. Beyond the “Mendel-Fisher controversy”. Retrieved February 28, 2022, from https://eprints.whiterose.ac.uk/91201/2/BeyondMendelFisher091015%5B1%5D.pdf
  • Chial, H. (2008). Mendelian Genetics: Patterns of Inheritance and Single-Gene Disorders. Nature news. Retrieved February 28, 2022, from https://www.nature.com/scitable/topicpage/mendelian-genetics-patterns-of-inheritance-and-single-966/
  • NHS. (2019, December 3). Phenylketonuria. NHS choices. Retrieved February 28, 2022, from https://www.nhs.uk/conditions/phenylketonuria/
  • Hillert, A., Anikster, Y., Belanger-Quintana, A., Burlina, A., Burton, B. K., Carducci, C., Chiesa, A. E., Christodoulou, J., Đorđević, M., Desviat, L. R., Eliyahu, A., Evers, R. A. F., Fajkusova, L., Feillet, F., Bonfim-Freitas, P. E., Giżewska, M., Gundorova, P., Karall, D., & Blau, N. (2020, July 14). The genetic landscape and epidemiology of phenylketonuria. The American Journal of Human Genetics. Retrieved February 28, 2022, from https://www.sciencedirect.com/science/article/pii/S0002929720301944
  • Arbesman, J., Ravichandran, S., Funchain, P., & Thompson, C. L. (2018, July 1). Melanoma cases demonstrate increased carrier frequency of phenylketonuria/hyperphenylalanemia mutations. Pigment cell & melanoma research. Retrieved February 28, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6013363/
  • 12.Freund, M. K., Burch, K. S., Shi, H., Mancuso, N., Kichaev, G., Garske, K. M., Pan, D. Z., Miao, Z., Mohlke, K. L., Laakso, M., Pajukanta, P., Pasaniuc, B., & Arboleda, V. A. (2018, October 4). Phenotype-specific enrichment of mendelian disorder genes near gwas regions across 62 complex traits. The American Journal of Human Genetics. Retrieved February 28, 2022, from https://www.sciencedirect.com/science/article/pii/S0002929718302854
  • Spataro, N., Rodríguez, J. A., Navarro, A., & Bosch, E. (2017, February 1). Properties of human disease genes and the role of genes linked to mendelian disorders in complex disease aetiology. Human molecular genetics. Retrieved February 28, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5409085/
  • Yong, S. Y., Raben, T. G., Lello, L., & Hsu, S. D. H. (2020, July 21). Genetic architecture of complex traits and disease risk predictors. Nature News. Retrieved February 28, 2022, from https://www.nature.com/articles/s41598-020-68881-8
  • Blair, D. R., Lyttle, C., Mortensen, J., Bearden, C., Jensen, A., Khiabanian, H., Melamed, R., Rabadan, R., Bernsdam, E., Brunak, S., Jensen, L., Nicolae, D., Shah, N., Grossman, R., Cox, N., White, K., & Rzhetsky, A. (2013, September 26). A Nondegenerate Code of Deleterious Variants in Mendelian Loci Contributes to Complex Disease Risk. Define_me. Retrieved February 28, 2022, from https://www.cell.com/fulltext/S0092-8674(13)01024-6
  • (not disproving, but cautioning the results of 15) Montojo, M. T., Aganzo, M., & González, N. (2017, September 29). Huntington’s disease and diabetes: Chronological sequence of its association. Journal of Huntington’s disease. Retrieved February 28, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5676851/
  • Rizvi, S., Khan, A. M., Saeed, H., Aribara, A. M., Carrington, A., Griffiths, A., & Mohit, A. (2018, August 14). Schizophrenia in digeorge syndrome: A unique case report. Cureus. Retrieved February 28, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6188160/
  • Jordan, D., &amp; Do, R. (2018, April 11). Using full genomic information to predict disease: Breaking down the barriers between complex and Mendelian Diseases. Annual Reviews. Retrieved February 28, 2022, from https://www.annualreviews.org/doi/10.1146/annurev-genom-083117-021136

Emma Tse Cheltenham Ladies’ College Cheltenham, United Kingdom Teacher: Ms. Helen Stuart

Between 1856 and 1865, Gregor Mendel conducted experiments on garden peas to investigate inheritance (1). His observations, notably his three principles of inheritance, form the basis of scientists’ grasp of monogenic (Mendelian) disorders today, which are caused by mutations in a single gene (2). Before Mendel’s discoveries, it was widely accepted that traits of progeny were a combination of those of each parent. However, when he cross-pollinated smooth-seeded peas with wrinkled-seeded peas, the offspring (F1 generation) only had smooth seeds as opposed to semi-wrinkled seeds. This gave rise to the concept of dominant traits, as well as his first principle: the principle of uniformity, which states that all offspring of parents with two distinct traits will inherit the same (dominant) trait of one parent (3). Mendel discovered recessive traits by self-pollinating a plant from the F1 generation, noting that its offspring (F2 generation) displayed a 3:1 ratio of smooth to wrinkled seeds (3). This proportion indicated that there was a hidden form of the trait, which Mendel acknowledged passed down to the F2 generation. Mendel also proposed the idea of each parent giving their offspring one heritable unit which he called “elementen”, and scientists now recognise this as genes – more specifically, alleles (2). Sickle-cell anaemia is a well-characterised autosomal recessive disease; those affected inherit two copies of a mutant beta-globin gene (1). Huntington’s disease, on the other hand, is an autosomal dominant disorder in which affected individuals possess at least one copy of the mutant HTT gene (1).

Mendelian disorders are relatively uncommon; on the other hand, complex diseases such as asthma and multiple sclerosis are more prevalent and arise from a combination of genetic, environmental and lifestyle factors (4). Therefore, complex diseases do not entirely adhere to Mendelian inheritance. They can be oligogenic or polygenic, meaning there are multiple genes each with their own mutations contributing to the disease’s phenotype (5). Studying Mendelian disorders allows researchers to examine the mutant gene’s effects on human biochemistry and physiology, thus furthering our understanding of the aetiology of complex, multifactorial diseases (4). An example is obesity, an increasingly pressing medical issue in developed countries. In congenital leptin disorder, a rare disease exhibiting an autosomal recessive inheritance pattern, severe obesity is a typical clinical feature. Affected individuals are unable to produce leptin because of mutations in the leptin encoding gene. Leptin acts on the hypothalamus to halt the production of neuropeptide Y, a neurotransmitter responsible for stimulating food, specifically carbohydrate, intake (6). Thus, studying congenital leptin disorder and other related Mendelian obesity disorders has helped scientists gain deeper insight into the complexity of the underlying causes behind obesity, one of which is the effects of leptin on the human body.

Another example is Van der Woude syndrome, an autosomal dominant condition caused by mutations in the IRF6 gene. It is characterised by a cleft lip and palate, hypodontia and lower lip pits (7). Interestingly, IRF6 mutations were also shown to be associated with non-syndromic isolated cleft lips and palates, which are complex traits and more prevalent in the general population than Van der Woude syndrome (8). This illustrates how the same defective gene could be responsible for rare inherited diseases and common medical conditions simultaneously. In essence, this shows Mendelian disorders and complex diseases that share overlapping phenotypes could be caused by the same sets of genetic aberrations (4).

Furthermore, systematic analyses using statistical methodologies have demonstrated that certain Mendelian disorders and complex diseases share a common genetic foundation. A study examining patients with concomitant Mendelian disorders and cancer revealed genetic connections between the two (9). The researchers’ initial hypothesis was that genetic mutations responsible for certain Mendelian disorders may predispose to the development of cancer. They found that genes associated with melanoma (MC1R and TYR), for instance, are also mutated in patients with oculocutaneous albinism, a Mendelian recessive disorder in which patients lack pigment in their skin, hair or eyes (10). Identifying cancer-driving genes that are found in Mendelian disorders enables scientists to understand the genetic basis of cancer development as well as various clinical presentations in cancer patients.

Although Mendel’s legacy has undoubtedly shaped our present understanding of inheritance, his discoveries alone cannot fully encapsulate the science behind complex diseases. The study of Mendelian disorders has given scientists a strong grounding for further research using advanced technologies such as whole genome sequencing and genome-wide association studies (11, 12), enhancing our knowledge of the genetic mechanisms and pathogenesis underlying polygenic diseases which would have been impossible in the 19th century.

  • Molnar, Charles. Concepts Of Biology – 1st Canadian Edition. 1st ed., 2019, pp. Chapter 8.1.
  • Chial, Heidi. “Gregor Mendel And Single-Gene Disorders | Learn Science At Scitable”. Scitable By Nature Education, 2008, https://www.nature.com/scitable/topicpage/mendelian-genetics-patterns-of-inheritance-and-single-966/.
  • Miko, Ilona. “Gregor Mendel And The Principles Of Inheritance”. Scitable By Nature Education, 2008, https://p75fz1.nbcnews.top/scitable/topicpage/gregor-mendel-and-the-principles-of-inheritance-593.
  • Reid, Jeremy. “Rare Disease Research Helps Us Understand Medicine For All Diseases – On Biology”. Biomed Central, 2016, https://blogs.biomedcentral.com/on-biology/2016/02/26/rare-disease-research-helps-understand-medicine-diseases/.
  • Collins, Samuel et al. The Genetics Of Allergic Disease And Asthma. 4th ed., Elsevier, 2016, pp. 18-30, https://www.sciencedirect.com/science/article/pii/B9780323298759000033, Accessed 25 Feb 2022.
  • Beck, B. “Neuropeptide Y In Normal Eating And In Genetic And Dietary-Induced Obesity”. Philosophical Transactions Of The Royal Society B: Biological Sciences, vol 361, no. 1471, 2006, pp. 1159-1185. The Royal Society, https://doi.org/10.1098/rstb.2006.1855. Accessed 25 Feb 2022.
  • Chial, Heidi. “Human Genetic Disorders: Studying Single-Gene (Mendelian) Diseases | Learn Science At Scitable”. Nature.Com, 2008, https://www.nature.com/scitable/topicpage/rare-genetic-disorders-learning-about-genetic-disease-979/.
  • Craig, Johanna. “Complex Diseases: Research And Applications”. Nature.Com, 2008, https://www.nature.com/scitable/topicpage/complex-diseases-research-and-applications-748/#:~:text=To%20comprehend%20the%20intricacies%20of,passed%20from%20generation%20to%20generation.
  • Melamed, Rachel D. et al. “Genetic Similarity Between Cancers And Comorbid Mendelian Diseases Identifies Candidate Driver Genes”. Nature Communications, vol 6, no. 1, 2015. Springer Science And Business Media LLC, https://doi.org/10.1038/ncomms8033. Accessed 26 Feb 2022.
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  Hannah Wilson Raphael House Rudolf Steiner School Lower Hutt, New Zealand Teacher: Ms. Sarah McKenzie

From his study of pea plants, Gregor Mendel developed three fundamental principles of inheritance: the principle of uniformity, the principle of segregation, and the principle of independent assortment (1). All monogenic traits follow these principles and are thus called Mendelian traits (1,2). Therefore, Mendel’s principles can be used to study Mendelian diseases, notably through pedigree analysis (1,2). The study of Mendelian diseases can in turn provide valuable insight into complex (non-Mendelian) diseases due to genetic correlations between Mendelian and complex diseases (3-6).

Mendel’s principles enable us to both decipher the past inheritance and predict the future inheritance of Mendelian diseases through pedigree analysis. Pedigree charts are diagrams based on Mendel’s principles that visually represent a family’s inheritance history of a Mendelian trait (1,2). Analysis of pedigree charts reveals whether the allele responsible is dominant or recessive, autosomal or sex-linked, due to the specific inheritance pattern exhibited by each allele type (2). Autosomal recessive diseases such as phenylketonuria (PKU) and sickle cell anemia can skip generations because two heterozygous (carrier) parents can give rise to progeny with either the affected or wild-type phenotype (2). Autosomal dominant diseases never skip generations unless random mutation occurs (2). Conversely, sex-linked Mendelian diseases display unique inheritance patterns depending on whether the disease is X-linked or Y-linked, dominant or recessive (2).

Pedigree analysis is applied in genetic counselling (7). Genetic counsellors presented with the family history of two individuals can predict the probability of each possible genotype and phenotype occurring in future offspring (7). These probabilities equip individuals with the information they need to make an informed reproductive decision. Furthermore, the simplicity of Mendel’s principles makes them accessible to the general public, better enabling individuals to understand the nature of their or their loved one’s disease. Nowadays, fetuses can be screened for common genetic defects during pregnancy, however, pedigree analysis maintains its value in that it can provide preliminary information before conception (8).

Although Mendel’s principles form the foundation of inheritance, most human diseases are complex, meaning they violate Mendel’s principles of inheritance (3). Examples of complex diseases include schizophrenia, hypertension, multiple sclerosis, and Alzheimer’s disease (3). Complex diseases are polygenic, meaning they are influenced by multiple genes, and are subject to environmental influence (3). Some also exhibit pleiotropy and epistatic interactions (9,10). Thus, unlike Mendelian diseases, complex diseases lack distinct inheritance patterns (3,4). This poses a challenge to geneticists when attempting to predict an individual’s risk of developing a complex disease.

In addition, there is now evidence that Mendelian and complex diseases are more interconnected than scientists formerly believed (11). For example, cystic fibrosis, typically categorized as an autosomal recessive Mendelian disease, is now believed to involve multiple loci (5,6). A mutation in the CTFR gene, which codes for a membrane channel protein for chlorine ions, forms the primary genetic basis for cystic fibrosis (6,12). However, variation in the severity of cystic fibrosis has been linked to potential modifier genes separate from the CTFR gene (5,6). As eukaryotic gene expression involves transcription factors as well as the structural gene(s) underlying a trait, it is highly likely that other Mendelian diseases also have complex aspects (13).

The study of Mendelian diseases can directly inform the study of complex diseases when a Mendelian disease acts as a model for a complex disease. Such is the case for Van der Woude syndrome, a rare autosomal dominant Mendelian disorder caused by mutations in the IRF6 gene (3,14). Symptoms of Van der Woude syndrome include cleft lip, a birth defect where the tissue in the lip does not join up completely before birth (3,14). Statistical studies provide evidence that one of the genes responsible for isolated cleft lip, a complex disorder, is IRF6, the same gene underlying Van der Woude syndrome (3). The discovery of links between other phenotypically similar Mendelian and complex diseases would be highly beneficial when considering that complex diseases are simultaneously challenging to study in isolation and highly prevalent in the general population (3,4).

Mendel’s abstract but fundamental principles of inheritance have paved the way for modern genetics. These principles directly enable both scientists and the general public to comprehend the inheritance of Mendelian diseases (1). The study of Mendelian diseases can also inform our understanding of complex diseases, especially in cases where a complex disease shares an element of its genetic basis with a Mendelian disease (ref). Therefore, despite their rarity, humankind as a whole is certain to benefit from the continued study of Mendelian diseases.

  • Miko, I. (2008). Gregor Mendel and the Principles of Inheritance. Nature Education. https://www.nature.com/scitable/topicpage/gregor-mendel-and-the-principles-of-inheritance-593/
  • Chial, H. (2008). Mendelian Genetics: Patterns of Inheritance and Single-Gene Disorders. Nature Education. https://www.nature.com/scitable/topicpage/mendelian-genetics-patterns-of-inheritance-and-single-966/
  • Craig, J. (2008). Complex Diseases: Research and Applications. Nature Education. https://www.nature.com/scitable/topicpage/complex-diseases-research-and-applications-748/
  • MedlinePlus. (2021, May 14). What are complex or multifactorial disorders? https://medlineplus.gov/genetics/understanding/mutationsanddisorders/complexdisorders
  • O’Neal, W. K., & Knowles, M. R. (2018). Cystic Fibrosis Disease Modifiers: Complex Genetics Defines the Phenotypic Diversity in a Monogenic Disease. Annual review of genomics and human genetics, 19, 201–222. https://doi.org/10.1146/annurev-genom-083117-021329
  • Buschman, H. (2019, December 10). Modifier Gene May Explain Why Some with Cystic Fibrosis are Less Prone to Infection. UC San Diego Health. https://health.ucsd.edu/news/releases/Pages/2019-12-10-modifier-gene-may-explain-why-some-with-cystic-fibrosis-less-prone-to-infection.aspx
  • NBIAcure. (2014). Genetic Counselling. http://nbiacure.org/learn/genetic-counseling/
  • MedlinePlus. (2021, September 29). Prenatal Testing. https://medlineplus.gov/prenataltesting.html
  • Nagel R. L. (2005). Epistasis and the genetics of human diseases. Comptes rendus biologies, 328(7), 606–615. https://doi.org/10.1016/j.crvi.2005.05.003
  • Gratten, J. & Visscher, P.M. (2016). Genetic pleiotropy in complex traits and diseases: implications for genomic medicine. Genome Med 8, 78. https://doi.org/10.1186/s13073-016-0332-x
  • Jin, W et al. (2012, April 1). A systematic characterization of genes underlying both complex and Mendelian diseases. Human Molecular Genetics, Volume 21, Issue 7, Pages 1611–1624. https://doi.org/10.1093/hmg/ddr599
  • MedlinePlus. (2021, July 6). Cystic Fibrosis. https://medlineplus.gov/genetics/condition/cystic-fibrosis/#causes
  • Urry, Meyers, N., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Reece, J. B. (2018). Campbell Biology: Australian and New Zealand Version (11th edition. Australian and New Zealand version.). Pearson Australia.
  • Children’s Hospital of Philadelphia. (2022). Van der Woude Syndrome. https://www.chop.edu/conditions-diseases/van-der-woude-syndrome

Emma Youngblood St. John Paul the Great Catholic High School Dumfries, Virginia Teacher: Dr. Clare Kuisell

“I am convinced that it will not be long before the whole world acknowledges the results of my work.” Gregor Mendel published the results of his pea plant experiments in 1865, but it wasn’t until the 1900s that people began to rediscover his work, and even then, it was controversial (Williams & Rudge, 2015). Now, nearly 200 years later, he is known as the father of the science of genetics, and students throughout the world learn about the laws of segregation and independent assortment which originated from Mendel’s observations. Mendel’s discoveries allow us to understand Mendelian disorders because they have been used to identify patterns of inheritance, which can be applied to genes that are known to have influence in complex diseases.

Single gene diseases are often referred to as Mendelian diseases–or disorders–and may be inherited in one of several patterns (Genetic Alliance, 2010). An example of such a disease is Marfan syndrome. With an incidence of approximately 1 in 5000 individuals, Marfan syndrome is an autosomal dominant disease that affects the body’s connective tissue (Coelho & Almeida, 2020). Using Mendel’s law of dominance and uniformity, which differentiates dominant and recessive alleles (Lewis & Simpson, 2021), one can predict the inheritance pattern of Marfan syndrome using the same calculations and ratios Mendel discovered in his pea plants. Because the mutated allele of the gene is dominant, a child who inherits Marfan syndrome must have a parent who also has it. This also means that Marfan syndrome, like other autosomal dominant diseases, would occur in every generation until the dominant allele is not inherited from either the mother or the father. Mendel’s work has allowed the identification of different types of inheritance patterns of single gene disorders to be very simple.

Complex diseases, while much less predictable than Mendelian disorders, are still influenced by genetics. Almost all complex diseases are affected by multiple genes and environmental factors, and examples include heart disease, cancer, and diabetes (National Human Genome Research Institute, 2013). Another well-known complex disease is Alzheimer’s Diseases (AD). Approximately 44 million people currently live with AD, and that number is expected to triple by 2050 (Lane et al., 2018). Aside from age, one of the highest risk factors for AD is the presence of the ε4 allele of the gene that codes for apolipoprotein E, also called ApoE (Yin & Wang, 2018). Recent studies have also shown that two of the most reliable biomarkers for AD are Aβ protein deposits and phosphorylated tau proteins (Mantzavinos & Alexiou, 2017). By studying the genes that code for these proteins and the gene that codes for, scientists may be able to identify a better way to treat or even cure AD. The multiple factors that affect complex diseases make it nearly impossible to determine exact patterns of inheritance, but if single genes that influence them can be isolated, the same patterns used to predict inheritance patterns in Mendelian disorders can be used to predict a high or low likelihood of developing or inheriting a complex disease.

Mendel’s discoveries have been essential in determining the inheritance patterns of Mendelian disorders, which can also be used to form a more accurate prediction of the inheritance of complex diseases. Interest in genetics-related careers is rapidly growing; the U.S. Bureau of Labor Statistics shows a job outlook of 26% from 2020 to 2030. This compares to the outlook of 14% for other healthcare occupations and 8% for all occupations (2021). Increased interest in the field of genetics may lead to new ways of applying the discoveries Mendel made nearly 200 years ago to solve modern questions and problems. It might have taken longer for the world to acknowledge the results of his work than he believed it would, but there is no doubt that once it did, Gregor Mendel’s work opened a realm of new scientific possibilities that will certainly endure for 200 years more.

Boyle, E. A., Li, Y. I., & Pritchard, J. K. (2017). An Expanded View of Complex Traits: From Polygenic to Omnigenic. Cell, 169(7), 1177–1186. https://doi.org/10.1016/j.cell.2017.05.038 Coelho, S. G., & Almeida, A. G. (2020). Marfan syndrome revisited: From genetics to the clinic. Síndrome de Marfan revisitada – da genética à clínica. Revista portuguesa de cardiologia, 39(4), 215–226. https://doi.org/10.1016/j.repc.2019.09.008 Genetic Alliance. (2010, February 17). Classic Mendelian Genetics (Patterns of Inheritance). Understanding Genetics: A District of Columbia Guide for Patients and Health Professionals. Retrieved January 20, 2022, from https://www.ncbi.nlm.nih.gov/books/NBK132145/ Lane, C. A., Hardy, J., & Schott, J. M. (2018). Alzheimer’s disease. European journal of neurology, 25(1), 59–70. https://doi.org/10.1111/ene.13439 Lewis, R. G., & Simpson, B. (2021). Genetics, Autosomal Dominant. In StatPearls. StatPearls Publishing. Mantzavinos, V., & Alexiou, A. (2017). Biomarkers for Alzheimer’s Disease Diagnosis. Current Alzheimer research, 14(11), 1149–1154. https://doi.org/10.2174/1567205014666170203125942 National Human Genome Research Institute. (2013, May 3). Genetic Analysis Tools Help Define Nature and Nurture in Complex Disorders. Genome.gov. Retrieved January 20, 2022, from https://www.genome.gov/10000865/complex-disorders-background U.S. Bureau of Labor Statistics. (2021, September 8). Genetic counselors : Occupational outlook handbook. U.S. Bureau of Labor Statistics. Retrieved January 21, 2022, from https://www.bls.gov/ooh/healthcare/genetic-counselors.htm Williams, C. T., & Rudge, D. W. (2015). Mendel and the Nature of Science. The American Biology Teacher, 77(7), 492–499. https://doi.org/10.1525/abt.2015.77.7.3 Yin, Y., & Wang, Z. (2018). ApoE and Neurodegenerative Diseases in Aging. Advances in experimental medicine and biology, 1086, 77–92. https://doi.org/10.1007/978-981-13-1117-8_5

  Vivian Yuan Ridgewood High School Ridgewood, New Jersey Teacher: Mr. Ryan Van Treuren

Complex Diseases Through the Lens of Mendelian Genetics

In 2001, the Human Genome Project reported that the human genome contains 20,000 to 25,000 protein-coding genes (1, 2). Among those genes, less than 10% are related to single gene diseases, also known as monogenic or Mendelian disorders (2). With the recent advances of genome-wide association studies (GWAS) and single nucleotide polymorphism (SNP) sequencing approaches, interest in human genetics has shifted from rare Mendelian disorders to more common complex diseases, which involve both genetic components and environmental factors (2, 3, 4). Although Mendelian disorders affect a small portion of the population, studying them has contributed greatly to our understanding of genetic mutations and the risk factors underlying the aetiology of complex diseases.

The foundation of all modern human genetic studies relies upon Gregor Mendel’s study with pea plants. Through his experiments, Mendel discovered three laws: the law of dominance, the law of segregation, and the law of independent assortment (5, 6). Mendelian laws aptly dictate Mendelian disorders, which allows scientists to better determine the inheritance pattern of diseases. Disease inheritance genes can be classified as autosomal or sex linked, dominant or recessive. Huntington’s disease, a progressive neurodegenerative disorder, is an example of autosomal dominant Mendelian disorder, because only one copy of the defective gene from one parent is needed for disease manifestation. Conversely, phenylketonuria (PKU), which causes the accumulation of the amino acid phenylalanine, is an autosomal recessive disease. Both parents must give the defective gene to the child for the disease to appear. If only one parent carries the mutated gene, the child will not be affected, but they could still be a carrier of the mutated gene. Luckily, doctors are now able to predict the genotype and phenotype of an individual using pedigree analysis. Now, PKU could be confirmed within three days after birth, and PKU babies will be switched to a low protein and phenylalanine diet, preventing cognitive abnormality.

Although complex diseases do not follow Mendelian inheritance, the mechanisms learned from Mendelian diseases can help scientists understand complex diseases (2). Initially, cystic fibrosis was characterized as an autosomal recessive monogenic disease because of the mutations in the Cystic Fibrosis Trans-membrane conductance Regulator (CFTR) gene. However, recent studies showed that not all CFTR mutations produce the same disease, and disease severity is associated with modifier genes (7, 8). The interactions between modifier genes and different CFTR mutations heavily affect the phenotypic complexity and expressivity of CFTR genes. Due to the discovery of these modifier genes, cystic fibrosis is now classified as an oligogenic disease, involving a few genes. In a study of several families with epilepsy, multiple members carrying the same SCN1A gene mutations showed varying phenotypes and disease severity. Like the case in cystic fibrosis, modifier genes were also identified in epilepsy. While they may not be pathogenic, those genes still account for the variability in SCN1A-related phenotype (9).

In addition, study of Mendelian diseases can provide useful information about individual gene’s contribution to the phenotypes in complex diseases. When comparing two databases, Online Mendelian Inheritance in Man database (OMIM) and Genetic Association database (GAD), scientists found that among the 968 Mendelian genes identified, 524 genes are also genetic risk factors for complex diseases (3); hence, those genes are called complex-Mendelian genes (CM genes). CM genes were found to have higher allelic Odds Ratios (ORs) than genes associated only with complex disease, suggesting that CM genes have stronger effects on the complex phenotypes they affect (10).​ ​​​​​​

Furthermore, some complex diseases, such as breast cancer and hypertension, have Mendelian subtypes that clearly display the inheritance patterns typical of monogenic diseases. Hereditary breast cancer, accounting for 5%-10% of all breast cancer, is mainly caused by a mutation in BRCA1 and BRCA2 genes (11). The inheritance of BRCA1 and BRCA2 follows an autosomal dominant pattern, and carriers of those two genes are at higher risk of developing other cancers, especially ovarian cancer. Similarly, scientists have found that some types of hypertension, called monogenic hypertension, are caused by distinct genetic mutations resulting in gain-of-function or loss-of-function in the mineralocorticoid, glucocorticoid, or sympathetic pathways (12).

The knowledge gained from studying genetic inheritance is surely invaluable to understanding diseases and finding treatments. Future applications of these basic principles laid out by Mendel over 150 years ago will lead doctors to predict disease manifestation and severity, working towards prevention and early treatment for all diseases, simple or complex.

  • International Human Genome Sequencing Consortium (2004) Finishing the Euchromatic Sequence of the Human Genome. Nature 431: 931-945
  • Antonarakis S.E. and Beckman J.S. (2006) Mendelian disorders deserve more attention. Nature Reviews Genetics 7: 277-282
  • Jin WF, Qin PF, Lou HY and Xu SF. (2012) A systematic characterization of genes underlying both complex and Mendelian diseases. Human Molecular Genetics 21 (7): 1611-1624
  • Craig J. (2018) Complex Diseases: Research and Applications. Nature Education 1 (1): 184 https://www.nature.com/scitable/topicpage/complex-diseases-research-and-applications-748/
  • Miko I. (2008) Gregor Mendel and the Principles of Inheritance. Nature Education 1 (1): 134. https://www.nature.com/scitable/topicpage/gregor-mendel-and-the-principles-of-inheritance-593/
  • Chial H. (2008) Mendelian Genetics: Patterns of Inheritance and Single-Gene Disorders. Nature Education 1 (1): 63. https://www.nature.com/scitable/topicpage/mendelian-genetics-patterns-of-inheritance-and-single-966/
  • Buratti E., Brindisi A., Pagani,F. & Baralle F. E. Nuclear factor TDP-43 binds to the polymorphic TG repeats in CFTR intron 8 and causes skipping of exon 9: a functional link with disease penetrance. Am. J. Hum. Genet. 74, 1322–1325 (2004).
  • O’Neal W.K. and Knowles M.R. Cystic Fibrosis Disease Modifiers: Complex Genetics Defines the Phenotypic Diversity in a Monogenic Disease. Annu. Rev. Genom. Hum. Genet. 2018. 19:201–22
  • de Lange I.M., Mulder F., Slot R, et al (2020). Modifier genes in SCN1A-related epilepsy syndromes. Mol Genet Genomic Med. 8: e1103
  • Spataro N., Rodriguez J., Navarro A., Bosch, E. (2017) Properties of Human Disease Genes and the Role of Genes Linked to Mendelian Disorders in Complex Disease Aetiology. Human Molecular Genetics 26 (3): 489-500
  • Mehrgou A. and Akouchekian M. (2016) The Importance of BRCA1 and BRCA2 gene mutations in breast cancer development. Med J Islam Repub Iran 30: 369
  • Raina R, Krishnappa V, Das A, et al (2019) Overview of Monogenic or Mendelian forms of Hypertension. Frontiers in Pediatrics 7: 263

Xinyi Zhang South Brunswick High School Monmouth Junction, New Jersey Teacher: Ms. Jessica Pagone

Genetic mutations lend each person their individuality, but certain variations can cause adverse health effects. Mendelian, or monogenic, disorders arise from variations in just one of the over 4,000 protein-coding genes that are currently associated with these diseases (2). Using Mendel’s principles to trace the inheritance pattern and phenotypes of a specific genetic mutation forms the basis of studying monogenic disorders. In turn, these findings can elucidate the role of various genetic mutations in diseases with more complex causes (8).

Gregor Mendel’s laws of genetic inheritance establish the framework for Mendelian patterns of inheritance. Given that each parent provides an allele for every gene in their offspring, if one parent has a genetic mutation that may cause a certain monogenic disorder, their offspring may inherit the mutant allele (5). Whether the child will develop the disorder or be a carrier depends on the dominance of the alleles they inherit (11).

Coupling Mendel’s principles with pedigree analysis reveal predictable modes of inheritance that bring light to the genetic nature of Mendelian diseases (5). Consider, for example, the realization of the inheritance pattern of sickle cell disease (SCD). Both parents need to have at least one mutant allele in the hemoglobin beta (HBB) gene to produce offspring with SCD (6). However, if their offspring only has one mutant allele, they will not be afflicted with SCD (6). With these observations, scientists determined that SCD is an autosomal recessive disorder in which it could only develop in people with two mutant alleles of the HBB gene (11). The inheritance pattern of a Mendelian disease would be different in an autosomal dominant disorder, where one mutant allele is enough to cause the disease, or in a sex-linked disorder, where diseases are inherited through the X or Y chromosome (11). Using Mendel’s principles to identify Mendelian inheritance patterns often serves as the first step in assessing disease risk and pinpointing the responsible genotype.

In actuality, Mendelian disorders are much rarer than complex disorders, which are distinguished from monogenic conditions because many genes, environmental interactions, and lifestyle choices all contribute to disease development (8). These variables complicate the determination of inheritance patterns or causative factors of a complex disease.

Despite their inherent differences, some connections have been uncovered between Mendelian and complex diseases. Many monogenic diseases are comorbid with complex ones (4). Furthermore, over 20% of the gene variations that cause Mendelian disorders have been implicated in at least one complex disorder (8). For instance, mutations in the IRF6 gene can lead to Van der Woude syndrome, a rare Mendelian disorder that causes cleft lip, cleft palate, and other facial deformities (10). Intriguingly, IRF6 mutations have also been implicated in complex, isolated forms of cleft lip and palate (12). These overlaps highlight the importance of utilizing Mendelian diseases to understand complex disease etiology.

Techniques such as whole-exome sequencing can link the characteristics of a Mendelian disease with the mutant gene that causes them (9). These findings are recorded in the Online Mendelian Inheritance in Man (OMIM), an accessible catalog of thousands of genotype-phenotype links for monogenic disorders (3). Studying this data has led to the identification of mutations and pathways that play a role in producing similar phenotypes in complex diseases (3,4). To better understand the complexity of essential hypertension, researchers studied many Mendelian disorders that are associated with high blood pressure, such as Liddle’s syndrome (7). Many of these disorders are caused by genetic mutations that alter proteins involved in renal salt balance (7). These studies brought attention to the importance of the kidneys and adrenal glands in regulating blood pressure and revealed the genetic mutations that may be associated with essential hypertension (7). Better knowledge of the molecular pathways behind essential hypertension has opened up new targets in drug development, such as ROMK, a renal potassium channel that is altered by a monogenic disorder known as Bartter syndrome type II (1).

Overall, while insights gleaned from studying Mendelian disorders cannot account for the environmental or lifestyle risks that contribute to complex diseases, they can guide research on pinpointing the pathophysiological processes and susceptibility alleles that bring about complex disorders. Thus, despite the rarity of Mendelian disorders, research on them should not be undercut to prioritize the study of prevalent complex diseases. A more comprehensive understanding of Mendelian disorders allows for more efficient risk assessment, prevention measures, and diagnoses for Mendelian and complex diseases alike, rendering it a valuable tool that should be further explored in the field of medical genetics.

  • Abdel-Magid, A. F. (2016, November 22). Potential of renal outer medullary potassium (ROMK) channel as treatments for hypertension and heart failure. American Chemical Society. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5238487/
  • Antonarakis, S. E. (2021, June 23). History of the methodology of disease gene identification … Wiley Online Library. Retrieved from https://onlinelibrary.wiley.com/doi/10.1002/ajmg.a.62400
  • Brownlee, C. (n.d.). OMIM turns 50: A genetic database’s past, present, and future. Johns Hopkins Medicine. Retrieved from https://www.hopkinsmedicine.org/research/advancements-in-research/fundamentals/in-depth/omim-turns-50-a-genetic-databases-past-present-and-future
  • Kumar Freund, M. (2018, October 4). Phenotype-Specific Enrichment of Mendelian Disorder Genes near GWAS Regions across 62 Complex Traits. Cell. Retrieved from https://www.cell.com/ajhg/fulltext/S0002-9297(18)30285-4
  • Lewis, R. G. (2021, May 7). Genetics, autosomal dominant. StatPearls [Internet]. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK557512/
  • Mangla, A. (2021, December 19). Sickle cell anemia. StatPearls [Internet]. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK482164/
  • Seidel, E., Scholl, U. I. (2017, November 1). Genetic mechanisms of human hypertension and their implications for blood pressure physiology. Physiological Genomics. Retrieved from https://journals.physiology.org/doi/full/10.1152/physiolgenomics.00032.2017
  • Spataro, N., Rodríguez, J. A., Navarro, A., & Bosch, E. (2017, February 1). Properties of human disease genes and the role of genes linked to mendelian disorders in complex disease aetiology. Human molecular genetics. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5409085/
  • Suwinski, P., Ong, C. K., Ling, M. H. T., Poh, Y. M., Khan, A. M., & Ong, H. S. (2019, February 12). Advancing personalized medicine through the application of whole exome sequencing and Big Data Analytics. Frontiers. Retrieved from https://www.frontiersin.org/articles/10.3389/fgene.2019.00049/full
  • U.S. National Library of Medicine. (2020, August 18). Van der Woude Syndrome: Medlineplus Genetics. MedlinePlus. Retrieved from https://medlineplus.gov/genetics/condition/van-der-woude-syndrome/
  • 11. U.S. National Library of Medicine. (2021, April 19). What are the different ways a genetic condition can be inherited?: Medlineplus Genetics. MedlinePlus. Retrieved from https://medlineplus.gov/genetics/understanding/inheritance/inheritancepatterns/
  • Zhao, H., Zhang, M., Zhong, W., Zhang, J., Huang, W., Zhang, Y., Li, W., Jia, P., Zhang, T., Liu, Z., Lin, J., & Chen, F. (2018, July 20). A novel IRF6 mutation causing non-syndromic cleft lip with or without cleft palate in a pedigree. OUP Academic. Retrieved from https://academic.oup.com/mutage/article/33/3/195/5056500

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The DNA (abbreviation for deoxyribonucleic acid) molecule has a very special place in life science, making a DNA essay a worthy study. Essays on DNA teach us how DNA stores complete information about the structure and properties of the organism. Therefore, knowledge of all the structural features of DNA is fundamentally important. DNA essays often explore the structure of DNA – a famous double helix It was discovered by Watson and Crick in 1953, which started a new era in the history of human civilization – the era of molecular biology and genetics, biotechnology, and molecular medicine. Our DNA essay samples will make you well-equipped for writing your own essay. Simply check out samples of DNA essays below.

The use of biometric identifiers in security systems is an enticing idea and has been embraced by the public following its use in banking systems where the data from iris scanners, facial recognition, and fingerprint scanners is utilized. Biometric identifier locates the user’s details from the central database, whereas the...

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DNA Evidence and its Application in Forensic Investigations DNA evidence is among many scientific tools that have been provided for the investigation of forensic evidence via the analysis of DNA which is a material that makes up one's genetic code. DNA can be retrieved from their hair, blood, skin cells as...

Words: 1083

CRISPR was first discovered by Ishino and coworkers at Osaka University in Japan while cloning a peculiar gene-repeat (Ishino et al. 5429-30). Nearly two decades later, a group of researchers at the food ingredient producer Danisco in Madison, United States, discovered that these CRISPR repetitions in bacteria offered defense against...

From its humble beginnings, forensic science has come a long way. For identification purposes, fingerprints have been used for a very long period. The breadth of forensic discoveries and advancements will be extended by this discipline. The tools used today to identify criminal offenders include DNA testing, impressions, and even...

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Recently, it has been proposed that human DNA and RNA are structurally distinct. As the scientific theory of the origin and evolution of man indicates, the structure, a double helix, of these two salts, i.e., both the RNA and the DNA, has actually been present for billions of years. Numerous...

Words: 1225

Introduction After the use of fingerprints, the use of DNA testing for forensic investigations can be considered the most important invention in the area of criminal investigations. The word "DNA" stands for deoxyribonucleic acid. DNA utilizes biological components like skin, hair, blood, and bodily fluids to identify people. A distinctive genetic...

Words: 1020

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The film Gattaca is set in the future, where DNA plays an important part in defining social class and genetic modification of people is common. Vincent, according to Brezina, Enrico, and Amelia, is born and conceived without the assistance of technology (338). Vincent is subjected to strong genetic prejudice and...

The jurors made a critical error in their evaluation of the evidence. With all of the evidence shown to the jurors, it is easy to conclude that there was no tampering with or framing of the evidence in order to frame "the kid" of the crime against his blood father....

The term "signature-tagged mutagenesis" (STM) refers to a technique that allows for the simultaneous screening of numerous different mutants. The method is accomplished by applying a special DNA sequence known as a signature tag to each of the implicated bacteria. Initially, this method was referred to as TN mutagenesis screening...

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An organism classified as a genetically modified organism (GMO) has had its DNA altered or transformed. Recombinant DNA is a process that includes transferring genetic material from one plant or animal to another to create these organisms. The genetic engineering method produced GMOs (Zhang, 2016). The method involves inserting the...

DNA sequence changes can result from cell mutations passed down from a parent organism to its children. Some cell mutations can be advantageous, but the majority are damaging because they result in the cell's ability to perform a specific function being lost. Bacteria naturally experience base pair mutations at a...

Introduction Media, scientists, and governmental agencies have all expressed interest in and opinions about the use of genetic engineering and biotechnology. There are no definitive solutions to the question of what lies ahead for genetically engineered creatures. Organisms that have had their genetic makeup altered by genetic engineering are referred to...

Words: 1107

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Write an essay on DNA replication.

Dna replication is the process of copying a double-stranded dna molecule. this process is observed in both eukaryotes and prokaryotes. it occurs in the nucleus in the eukaryotes and in the cytoplasm in the prokaryotes. there is only one active replication site in prokaryotes whereas eukaryotes have many replication sites. the process is a semi-conservative method. during the initiation of dna replication, the dna unwinds with the help of the helicase enzyme and becomes single-stranded. the structure thus formed is called a replication fork. the single-stranded dna strand acts as a template strand. with the help of a polymerase enzyme, the new dna strand starts synthesizing from an rna primer. this new dna strand is complementary to the template strand. in this, adenine (a) pairs with thymine (t) and guanine (g) pairs with cytosine (c). since dna is anti-parallel, the two strands are formed, which is the leading strand and lagging strand. the elongation takes place in 5' to 3' direction as p olymerase can only add the nucleotides in a 5'-3' direction. hence, the leading strand is synthesized continuously. the lagging strand is in a 3'-5' direction and must require okazaki fragments to attach to the corresponding code and be joined by dna ligase so that the lagging strand becomes continuous. the termination takes place when dna polymerase reaches the end of the strand..

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  1. DNA

    The configuration of the DNA molecule is highly stable, allowing it to act as a template for the replication of new DNA molecules, as well as for the production (transcription) of the related RNA (ribonucleic acid) molecule.A segment of DNA that codes for the cell's synthesis of a specific protein is called a gene.. DNA replicates by separating into two single strands, each of which serves ...

  2. Essay on DNA: Meaning, Features and Forms

    Essay # 4. Components of DNA: DNA molecule is a polymer which is composed of several thousand pairs of nucleotide monomers. Union of several nucleotides together leads to the formation of polynucleotide chain. The monomer units of DNA are nucleotides, and the polymer is known as a "polynucleotide." Each nucleotide consists of a 5-carbon ...

  3. DNA explained: Structure, function, and impact on health

    DNA is a two-stranded molecule that appears twisted, giving it a unique shape referred to as the double helix.. Each of the two strands is a long sequence of nucleotides. These are the individual ...

  4. Discovery of the structure of DNA (article)

    The components of DNA. From the work of biochemist Phoebus Levene and others, scientists in Watson and Crick's time knew that DNA was composed of subunits called nucleotides 1 . A nucleotide is made up of a sugar (deoxyribose), a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G) or cytosine (C).

  5. Introduction: What is DNA?

    Introduction: What Is DNA? Deoxyribonucleic acid, more commonly known as DNA, is a complex molecule that contains all of the information necessary to build and maintain an organism. All living ...

  6. The structure of DNA

    Nature PastCast: The other DNA papers. Host: Kerri Smith. This is the Nature PastCast, each month raiding Nature's archive and looking at key moments in science. In this show, we're going back ...

  7. Locating an Essay's DNA

    Locating an Essay's DNA. An essayist always writes two essays simultaneously, overlapped as transparencies, one exploring what Vivian Gornick calls the situation, the other what she terms the story. Poet Richard Hugo talks about a piece's "triggering subject" and its generated, or real, subject. Phillip Lopate describes the "double ...

  8. DNA as the genetic material

    Today, we know that DNA is the genetic material: the molecule that bears genes, is passed from parents to children, and provides instructions for the growth and functioning of living organisms. But scientists didn't always know this. In fact, for many years, researchers thought that protein would turn out to be the genetic material!

  9. 9.1 The Structure of DNA

    Now let's consider the structure of the two types of nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The building blocks of DNA are nucleotides, which are made up of three parts: a deoxyribose (5-carbon sugar), a phosphate group, and a nitrogenous base ( Figure 9.3 ). There are four types of nitrogenous bases in DNA.

  10. The Structure and Function of DNA

    Biologists in the 1940s had difficulty in accepting DNA as the genetic material because of the apparent simplicity of its chemistry. DNA was known to be a long polymer composed of only four types of subunits, which resemble one another chemically. Early in the 1950s, DNA was first examined by x-ray diffraction analysis, a technique for determining the three-dimensional atomic structure of a ...

  11. DNA function & structure (with diagram) (article)

    DNA structure and function. DNA is the information molecule. It stores instructions for making other large molecules, called proteins. These instructions are stored inside each of your cells, distributed among 46 long structures called chromosomes. These chromosomes are made up of thousands of shorter segments of DNA, called genes.

  12. 13.1: Introduction to DNA Structure and Function

    This page titled 13.1: Introduction to DNA Structure and Function is shared under a CC BY license and was authored, remixed, and/or curated by OpenStax. The three letters "DNA" have now become synonymous with crime solving, paternity testing, human identification, and genetic testing. DNA can be retrieved from hair, blood, or saliva.

  13. 12.2: DNA organization inside a cell

    DNA Organization in Eukaryotes. Eukaryotes have much more DNA than prokaryotes. For example, an E. coli bacteria contains roughly 3 million base pairs of DNA, while a human contains roughly 3 billion. In eukaryotes such as humans and other animals, the genome consists of several double-stranded linear DNA molecules (Figure \(\PageIndex{2}\)), which are located inside a membrane-bound nucleus.

  14. DNA and Evolution

    DNA, or deoxyribonucleic acid, is a molecule found in the nuclei of cells. DNA contains genes, the building blocks of all organisms. THE STRUCTURE OF DNA The most important function of DNA is its ability to replicate itself repeatedly. DNA must be copied when new cells are formed, when genetic material is passed from parents to offspring, and ...

  15. Annual DNA Day Essay Contest

    ASHG is proud to support National DNA Day through the Annual DNA Day Essay Contest. DNA Day commemorates the completion of the Human Genome Project in April 2003 and the discovery of the double helix of DNA in 1953. This contest is open to students in grades 9-12 worldwide and asks students to examine, question, and reflect on important ...

  16. 163 DNA Topic Ideas to Write about & Essay Samples

    The Discovery of the Deoxyribonucleic Acid (DNA) Structure. Watson and Crick are independent; they come up with the idea of building a DNA structure on their own. Chadarevien argues that the image of Crick, Watson, and the double-helical DNA model has a great […] Transfer of Beta-Carotene via DNA Techniques.

  17. 2022 DNA Day Essay Contest: Full Essays

    2022 DNA Day Essay Contest: Full Essays. April 25, 2022 DNA Day. 1st Place : Man Tak Mindy Shie, Grade 12. Teacher: Dr. Siew Hwey Alice Tan. School: Singapore International School (Hong Kong) Location: Hong Kong, China. Many would say that the most significant stride in recent genetics has been the completion of the human genome, which laid the ...

  18. Essay on DNA Replication

    ADVERTISEMENTS: In this essay we will discuss about:- 1. Definition of DNA Replication 2. Mechanism of DNA Replication 3. Evidences for Semi-Conservative DNA Replication 4. Models for Replication of Prokaryotic DNA. Essay # Definition of DNA Replication: DNA replicates by "unzipping" along the two strands, breaking the hydrogen bonds which link the pairs of nucleotides. […]

  19. Free Essays on DNA, Examples, Topics, Outlines

    DNA essays often explore the structure of DNA - a famous double helix It was discovered by Watson and Crick in 1953, which started a new era in the history of human civilization - the era of molecular biology and genetics, biotechnology, and molecular medicine. Our DNA essay samples will make you well-equipped for writing your own essay.

  20. DNA Damage and DNA Repair: Types and Mechanism

    DNA Damage and DNA Repair: Types and Mechanism. DNA is the basic unit of inheritance that maintains the integrity and function of living organisms. However, it is constantly exposed to damaging agents which can cause DNA damage. Additionally, errors can occur during DNA replication and repair processes, leading to harmful mutations.

  21. PDF M1. Essay Using DNA in science and technology

    M1. Essay Using DNA in science and technology . DNA and classification . 2.2 Structure of DNA . 2.3 Differences in DNA lead to genetic diversity . 2.9 Comparison of DNA base sequences . Genetic engineering and making useful substances . 2.5 Plasmids . 5.8 The use of recombinant DNA to produce transformed organisms that benefit humans . Other ...

  22. AQA A-Level Biology

    Importance of DNA structure and proteins. - DNA codes for unique sequence of amino acids, which determines location of bonds in the tertiary structure and therefore, the role of the protein. - Enables unique binding sites and active sites (without Rubisco photosynthesis wouldn't occur) Recombinant DNA. - Genetic code is universal so DNA can be ...

  23. Write an essay on DNA replication.

    During the initiation of DNA replication, the DNA unwinds with the help of the helicase enzyme and becomes single-stranded. The structure thus formed is called a replication fork. The single-stranded DNA strand acts as a template strand. With the help of a polymerase enzyme, the new DNA strand starts synthesizing from an RNA primer.