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# Allele frequency

Allele frequencies in populations and how they differ from genotype frequencies. Created by Sal Khan.

Video transcript

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.