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exploring genetics – Mendel, alleles and stuff

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Canto: So I’d like to know as much as I can about genetics before I die, which might be quite soon, so let’s get started. What’s the difference between genetics and genomics?

Jacinta: Okay, slow down – but I suppose that’s as good a place to start as anywhere. I recently listened to a talk about the human genome project, which was completed around 2003, and the number I heard the guy mention was 3 billion genes, or something. But according to videos and other sources, each human has between 20,000 and 25,000 genes – though I’ve found another FAQ which estimates 30,000. So I gather from this that our genome is the number of genes we might possibly have – in the whole human population? Which raises the question, how do we know that the human genome project has captured or mapped all of them.

Canto: So there’s an individual genome, peculiar to each of us, and a collective genome?

Jacinta: Errr, maybe. We’re 99.9% genetically identical to each other, supposedly. And if this sounds very paradoxical, we need to zoom in on the detail. And with that, I’ve discovered that the 3 billion refers to base pairs, sometimes called ‘units of DNA’. So what’s a base pair? Well, we need to start with the structure of DNA, the genetic molecule. That’s deoxyribonucleic acid, which is made up of basic components called nucleotides. A nucleotide of DNA consists of a sugar molecule, a phosphate group and a nitrogenous base. The bases come in four types – adenine, guanine, thymine, and cytosine (A, T, G and C). The sugar and phosphate groups provide structure, allowing the bases to form a long string of DNA. Bonds form between the bases to create a double strand of DNA – hence base pairs.

Canto: Here’s how the World Health Organisation defines genomics, obviously from a health perspective:

Genomics is the study of the total or part of the genetic or epigenetic sequence information of organisms, and attempts to understand the structure and function of these sequences and of downstream biological products. Genomics in health examines the molecular mechanisms and the interplay of this molecular information and health interventions and environmental factors in disease.

Now you might think that this definition could cover genetics too, and maybe we shouldn’t be too worried about the distinction. Maybe, in general, genomics is about sequences of genes, especially in detailing whole organisms, while genetics is more about individual genes.

Jacinta: Genomics is the much more recent term, first coined in the 1980s, whereas genetics and genes date back to before we knew about DNA as the genetic molecule. Going back to Mendel and all, though I don’t think he used the term, he talked about ‘factors’ or some such.

Canto: So we know that there’s DNA, and there’s also RNA, another building block of life. How old are they, and which came first? And can species replicate without these molecules?

Jacinta: Oh dear – we’ll get there eventually, maybe. Genomics deals with the whole complement of genes in an organism, which we’ve gradually realised is necessary to evaluate, say, how prone that organism is to contracting a disease, or developing some immuno-deficiency, because individual genes often don’t tell us much. And there’s also the matter of dominant and recessive genes. Which takes us to inheritance. All those genes are combined together on chromosomes, of which there are 23 pairs in humans, which we inherit from our parents, 23 chromosomes each.

Canto: Combined together? Can you  be more specific?

Jacinta: Okay, a chromosome is a thread-like structure, in which DNA is coiled around structural proteins called histones. Each chromosome has two ‘arms’, flowing from a constriction point called a centromere. These arms are labelled p and q. The p arm is shorter than the q. And these chromosomes contain genes, which may or may not code for proteins. The genes, as mentioned, consist of base pairs, which vary in number from hundreds to millions.

Canto: Okay, so what’s the difference between a gene and an allele?

Jacinta: Well, genes are codes for making proteins – and those proteins affect all sorts of things, to do with taste, smell, hair colour and type, height, and predisposition to various diseases, among many other things. You can call these things ‘traits’, which show up in our phenotype, our physical characteristics. And it should be pointed out that many of these traits are the results of not just one gene but different genes in combination. Now, as mentioned, these genes are in pairs of chromosomes – 23 pairs in humans. Now, say we isolate an area in a chromosome that codes for a particular trait. What about the other chromosome in that pair? Remember, each chromosome comes from a male or female parent, and they are different, genetically – or likely to be. That’s where alleles come in, and it takes us back to Mendel, who found that with pea plants, traits such as colour, or the alleles that carried those traits, could be dominant or recessive. So, for that trait, they could carry two dominant alleles, or two recessive alleles, or one of each. If one or both of those alleles is dominant, the trait will be expressed, but if both are recessive, it won’t be. But as I say, it’s more complicated than that, as traits expressed in phenotypes are generally carried by many genes.

Canto: So alleles are? – how to define them?

Jacinta: Google it mate. Here’s a quickly found definition: “each of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome”. So let’s continue with the work of Mendel. When we find a dominant trait, we use a capital, T. It might be paired with another dominant trait, TT, or with a recessive trait, Tt. On the other hand, both traits might be recessive, tt, and that’s all the combos you have, for single traits. Now, in noting this, and the way that alleles combine, Mendel came up with a ‘law of segregation’. Or rather, he noticed a process, which later became recognised as a law. In fact, he observed three fundamental processes, ‘segregation’, ‘independent assortment’, and ‘dominance’, which we now describe as laws. Now, I’ve used the term ‘trait’ but perhaps I should’ve used the term ‘allele’. So TT combines two dominant alleles. The law of segregation has been stated thus:

During gamete formation, the alleles for each gene segregate from each other such that each gamete formed carries only one allele for each gene.

Canto: Right. Uhhh, what’s a gamete again?

Jacinta: Sex cells, which carry only one copy of each chromosome. They’re created during meiosis, after which we end up with four cells each with only one allele for each gene. So indeed, alleles are segregated during gamete formation.

Canto: Oh dear. I’ll have to brush up on meiosis.

Jacinta: So now we have these segregated alleles, which will be recombined. The law of independent assortment comes next. This also occurs during meiosis. In the fourth or metaphase period of cell division, the chromosomes align themselves on the equatorial plane, also called the metaphase plate. This alignment is random, and that’s the key to the law of independent assortment – ‘genes for different traits assort independently of each other during gamete formation’. But obviously Mendel knew nothing about meiosis, though it was first observed in his lifetime, in sea urchins . Anyway, this law allows for many different combinations of alleles depending on how chromosomes become aligned on the metaphase plate. A dihybrid cross will provide more such combinations.

Canto: A dihybrid cross? Please explain.

Jacinta: Well, a monohybrid cross will be like this – TT x tt. Not much to be assorted there. A dihybrid cross might be like this – TtCc x TtCc, creating four different assortments for each cross. So now to the third law, of dominance. This law simply states that ‘some alleles are dominant while others are recessive. An organism with at least one dominant allele displays the effect irrespective of the presence of the recessive one’. So the phenotype will present the dominant allele regardless of whether it’s double-dominant or single-dominant. Though the terms used are homozygous (TT), or heterozygous (Tt).

Canto: So are we going to look at punnett squares now? I’ve heard of them…

Jacinta: Well it might help. They were named after a bloke called Punnett back in 1905, the early days of Mendelian genetics. They’re neat little tables, that can start to get quite complicated, for determining the genotypes of offspring, when you breed dominant with recessive, heterozygous with homozygous and so on. It’s useful for simple genotypes, but when genotypes are multifactorial, as they often are, other methods are obviously required.

Canto: Okay, that’s more than enough to absorb for now.

Jacinta: I think, since we’ve started with Mendel, we might do a historical account. Or maybe not….

References

https://www.google.com/search?client=safari&rls=en&q=alleles&ie=UTF-8&oe=UTF-8

https://byjus.com/biology/mendel-laws-of-inheritance/

https://www.yourgenome.org/facts/what-is-meiosis

 

 

Written by stewart henderson

May 29, 2022 at 8:04 am

Posted in alleles, Mendel

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