Allele frequencies in populations and how they differ from genotype frequencies. Created by Sal Khan.
Voiceover: What I want to do with this video is explore the idea of allele frequency. Allele frequency. Just as a reminder, an allele is a variant of a gene. You get a variant of a gene from your mother, and you get another variant of the gene from the father. So, when we're talking about the allele, we're talking about that specific variant that you got from your mother or your father. We've seen this before, but now let's dig a little bit deeper. To help us get our heads around this, we'll start with a fairly common model for this. We're going to think about eye color. Obviously, this is a very large simplification, but let's just assume that we have a population where there's only two variants of an eye color gene. Let's first assume there is an eye color gene. Let's assume there's two variants. One variant, one allele for eye color, we'll use the shorthand, capital B. Let's say that's the allele for brown. Brown eye color. We're going to assume that this one is dominant. It's dominant over the other allele. Now the other allele, we're going to assume is for blue eye color, and we'll represent that with a lower case B. So that is blue eye color, and we're going to assume that this is recessive. Once again, this is review. Someone who has one of the big B alleles, the brown alleles, it doesn't matter what their other allele is going to be, because it's either going to be another brown or it's going to be a blue, they're going to show brown eyes. This is going to be brown eyes, and this is going to be brown eyes, because the capital B is dominant. The only way to get blue eyes is to be a homozygote for the recessive allele. All of that, of course, is review. We've seen that before. Now let's think about allele frequency. To think about that, I'll set up a very artificially small population. Let's say our population has exactly two people in it. Population has exactly two people in it, Person 1 and Person 2, and let's say we're able to look into their DNA and figure out their genotypes. Person 1, say, has a capital B allele, has a brown allele and a blue allele, while Person 2 has two blue, two blue alleles. Given that we know the genotypes in this artificially small population, we could start thinking about the allele frequencies. Or the frequencies of the different alleles. What do you think is going to be the frequency the frequency of the brown allele in this population? I encourage you to pause this video and think about this on your own. I'm assuming you've had a go at it, so you might be tempted to say, "Looks like one out of two people "have it, maybe it's 50%." But that wouldn't be the right way to think about allele frequencies. In allele frequencies, you want to dig a little bit deeper and look at the individual alleles. When you look at that, you say, "Okay, there's "four individual alleles in this population, or there's "four variants, or there's literally four chromosomes "that are carrying that gene in this population." Out of them, one of them carry, one of them is the capital B allele, so we could say that that is going to be zero point two five, or 25%. Once again, 25% of the genes for eye color have the capital B allele, have the brown allele. Now we can do the same, ask ourselves the same question for the lower case B allele. What fraction of the genes in this population are code for or represent the lower case B, the blue allele? Once again, I encourage you to pause the video and think about it. Well, very similar idea. There's four genes in the population that are coding for eye color. Of them, one, two, three code for or are the lower case blue allele. So that's zero point seven five or 75%. 75% of the genes code for the lower case blue allele, while 25 are the brown allele. I really want to hit this point home, how this is different than, say, the phenotype frequency. If I asked you, in the population, if I asked you the percent of brown-eyed people, so now I'm talking about phenotype, what would that be? Well, there's two people in the population. One of them is exhibiting brown eyes, so that's going to be one-half. Similarly, if I were to ask you what is the percentage of people who are blue-eyed that, too, would be one-half. This person is one of the two people, they're exhibiting blue eyes. But allele frequency, we're digging deeper, we're looking at the genotypes. We're saying out of the four genes here, one of them is the big B allele, so that's 25% of the gene population codes for the brown allele and 75% is the blue allele. This is really important to internalize. Because once we internalize this, then as we'll see, that the ideas in the Hardy-Weinberg principle start to make a lot of sense. I'll do a little bit of foreshadowing. We can denote this, this is just a convention that's often used, by the lower case letter P, and we can use lower case Q to denote the frequency. So lower case P is the frequency of the dominant allele, lower case Q the frequency of the recessive allele. What's true here? What's true of P, what's going to be true of P plus Q, what's P plus Q going to be equal to? I encourage you to pause the video again and think about that. What is this going to be equal to? Well, when we started off, we said that there's only two potential, that's one of the assumptions we assumed, we assumed there's only two alleles in this population, in kind of the allele population for this gene population for this trait. The frequency of the dominant ones plus the frequency of recessive ones, well everyone's going to have one of those two, so if you add those two frequencies, it's going to have to add to 100%. We see that there. One-fourth plus three-fourths is one, or 100%. And 25% plus 75% is also 100%. So we could say P plus Q is equal to 100%, or we could say that P plus Q is equal to one. Is equal to one. So, in the next video, we're going to start from the seemingly fairly simple idea, to get to a more richer and fairly neat idea that's expressed in the Hardy-Weinberg equation.