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Sources of genetic variation

In sexual reproduction, chromosomes can sometimes swap sections during the process of meiosis (cell division), thereby creating new genetic combinations and thus more genetic variation. Although DNA replication is tightly regulated and remarkably accurate, errors do occur and result in mutations, which are also a source of genetic variation. Environmental factors can also cause mutations in genes, and viable mutations are inherited. Created by Sal Khan.

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Video transcript

- [Instructor] In this video, we're going to talk about sources of genetic variation, which is key for evolution and natural selection to happen. Just as a little bit of a primer, natural selection, you can have a bunch of different organisms with different genetics, different genotypes, and they can express themselves as different phenotypes. And I'll just do this as different colored circles right over here. So there are all of these different phenotypes, and I'm just expressing different phenotypes of one trait. And then depending on what's going on in the environment, some of these phenotypes might be more favorable for survival and reproducing and therefore passing on those genes to the next generation. And if you do that over many, many, many, many generations, you can have a change in your gene pool because the genes that provide the variants of phenotypes that are more successful will exist more. But an interesting question is, where does this variation come from? And there's several sources of it. So one of the key and probably the most primitive version of genetic variation is mutation. Cells are incredibly accurate when they are copying DNA, but there are going to be some errors. Now, most of these errors can oftentimes break the organism in some way or might not matter at all, but every now and then, some of these errors, either as an individual base pair change, or maybe cumulatively can produce a different phenotype and potentially a phenotype that has an advantage. And so this has always been the case. Now, another major source of genetic variation is sexual reproduction. And to remind ourselves of sexual reproduction, I will show you this diagram of meiosis. Now sexual reproduction is the process by which we form gametes. So for a male organism, that would be producing the sperm cells, or for a female organism, that would be producing the egg cells. This meiosis diagram is for an organism that has two pairs of chromosomes, while we know that human beings actually have 23 pairs. But if we saw a diagram with 23 pairs, it would get very complicated, very fast, so the two pairs help us understand what's going on and help us understand where some of this genetic variation is going to come from. So I've already pre-labeled the homologous chromosomes here. And just as a reminder, homologous chromosomes are ones that have the same genes on them. Now they could have different versions of the genes on them, but they're fundamentally coding for the same genes. You can view chromosomes as really long stretches of DNA that has all been rolled in and bunched in together, something like this. A human chromosome can have on the order of 100 million base pairs in it. Now, if you were to straighten that string of DNA, if you were to unwind it, you would see different section's code for different genes. So that might be one gene there, that might be another gene there. You might have one long gene right over there. On average, the genes are about 27,000 base pairs in length but some of them could be millions of base pairs. So on one of these chromosomes, you can actually have thousands of genes being coded. And so if you were to pick this chromosome and this chromosome right over here, they would be coding for the same genes because they're homologous. But once again, they could have different alleles, different versions of those genes on them. And similarly, this chromosome and this chromosome are also homologous. They're also coding for the same genes. Now, as we go into meiosis, the first step is that the chromosomes are essentially going to copy themselves into two sister chromatids. So, for example, this one right over here has now copied itself and it has that telltale X shape. But this side of this now chromosome, which we would call a chromatid, and this sister chromatid should be identical. Now there might be some errors that got introduced through mutation. But if we don't assume mutation, they would be identical. Likewise, this side and this side, assuming no mutations, they would be identical. Now what's interesting about this is what happens in the next phase. In the next phase, you have the independent assortment of homologous chromosomes. So as we said, this and this might be coding for the same genes, it might just have different versions. But as we go into this phase, as we do meiosis I, as it's often known right over here, this blue chromosome could go here, while the homologous red chromosome would go there. The purple chromosome is going here and the light blue chromosome is going there. And this is really interesting because there's a lot of different ways this could happen. In this situation, you have two pairs. Each of these intermediary steps in meiosis could randomly have one from each pair. So just in this example, you have two to the number of pairs combinations at this stage right over here. Now this was only when we have two pairs. If we're talking about a human being, we're talking about two to the 23rd different combinations of which of the two homologous chromosomes you get. So there's a lot of variation here. Now on top of that, some of y'all might have noticed something interesting. If you just follow the colors here, it looks like a little chunk of this chromosome got swapped with a little chunk of this chromosome. You could see it here. The red is now on the big blue X and the blue is now on the big red X. This is another source of genetic variation and it is known as crossover. And what it does is, it can actually mix DNA between chromosomes. Once again, these are homologous chromosomes, they are encoding the same genes, but now alleles that were sitting on the blue one could now sit with the rest of the red one and the alleles that was sitting with the red one can now sit with the rest of the blue ones. And crossover is actually reasonably common during meiosis. So once again, it's mixing things up even more than this two to the 23rd combinations. So a lot of variation that you can produce through sexual reproduction. And then as we go into this last phase into meiosis II, and we're actually producing the gametes, if this meiosis is going on in the gonads of a male, this would be the chromosomal makeup of the sperm cells. If this is going on within the female, then this would be the DNA makeup of the egg cells. And what you see, and just to make it clear what's happened here is that your sister chromatids have now spread apart, although they're no longer identical, especially if you have the crossover. So for example, this one went over here and this one went over here as well. And then you have another scenario where you have this one and this one ended up in this gamete, and we can go on and on. So actually you can have, especially if you consider crossover, more than two to the 23rd possible combinations. Now two to the 23rd power is approximately a little bit more than eight million combinations. And if do you want a little math trick for estimating powers of two, you can just recognize that two to the 10th power is a little bit more than 1,000. So this is going to be two to the 20th, which is about a million, and then two to the third, which is eight, so a little bit more than 8 million. And once again, that's before considering crossover and mutation, which is going to make the combinations even larger. And I'll also point out these are the combinations for the gametes, and the gametes are haploid. They have half the DNA of a full organism. And so when the gametes combine, you're actually going to have two to the 23rd times two to the 23rd combinations, just from sexual reproduction, and you'll have even more from mutation and crossover. And so that's going to lead you to more than 70 trillion combinations just from these two parents.