If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:7:27
EVO‑1 (EU)
EVO‑1.K (LO)
EVO‑1.K.1 (EK)
EVO‑1.K.2 (EK)

Video transcript

what I want to do in this video is explore the idea of allele frequency allele frequency and just as a reminder an allele is a variant of a gene you get a variant of the G of a gene from your mother and you get another variant of the gene from the father and so when we're talking about the allele we're talking about that specific variant that you got from your mother or your father and we've seen this before but now let's dig a little bit deeper and to to help us get our heads around this well we'll start with a a fairly common a fairly common model for this and we're going to think about eye color and obviously this is a very large simplification but let's just assume that where 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 and listen there's two variants one variant one allele form for eye color look we'll use the shorthand capital B let's say that's the allele for brown brown eye color and we're going to assume that this one is dominant its 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 lowercase B so that is blue eye color and we're going to assume we're going to assume that this is recessive so 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 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 where they're going to show brown eyes so 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 the only way to have blue eyes is to be a homozygote for the recessive for the recessive allele and all of that of course is review we've seen that before but now let's think about allele frequency and to think about that I'll set up a very artificially small population so let's say our s 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 so person 1 let's say has a capital B allele has a brown allele and a blue allele well person 2 has two blue two blue alleles now given that we know the genotypes in this artificially small population now we can start thinking about the allele frequencies or the frequencies of the different alleles so what do you think is going to be the frequency the frequency of the brown allele in this population and I encourage you to pause this video and think about this on your own so I'm assuming you've had a go at it so you might be tempted say oh what 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 and allele frequencies you want to dig a little bit deeper and look at the individual alleles and when you look at that you say okay there's four individual alleles in this population of or there's four variants in this or this there's literally four chromosomes I guess you could say that they're carrying that gene in this population and out of them one of them carry one of them is the capital B is the capital B allele and so we could say that that is going to be 0.25 or 25% so once again 20 25 percent of the genes for I color have the capital B allele have the brown allele now we can do the same we can ask ourselves the same question for the lowercase B allele what fraction of the genes in this population are code for or represent the lowercase B the blue allele and once again I encourage you to pause the video and think about it well very similar idea there's four there's four there's four genes in the population that are coding for eye color of them one two three one two three code for or are the lowercase blue are the lowercase blue allele so that's 0.75 or 75% 75% of the genes code for the lowercase bleep the blue and allele while 25 are the brown are the brown allele and I really want to hit this point home how this is different than say the phenotype frequency if I if I asked you in the population if I asked you the percent of brown-eyed people brown-eyed people so now I'm 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 and similarly similarly if I were to ask you what is the percentage of people who are blue-eyed that two would be one half this person is one of the two people they're exhibiting blue eyes but allele frequency we're digging deep or we're looking at the genotypes and we're saying well out of the four out of the four genes here one of them is the Big B allele so that's 25% of so 25% of the of the olia of the gene population codes for is the brown allele and 75% is the blue allele and this is really important to internalize because once we internalize this then as we'll see that the idea is in the hardy-weinberg principle start to make a lot of sense and I'll do a little bit of foreshadowing we can denote we can denote this this is just a convention that's often used by the lowercase letter P and we can use Q lowercase Q to denote the frequency so P lowercase P is the frequency of the dominant allele lowercase Q the frequency of the recessive allele but what's true here what's true of P what's true what's going to be true of P plus Q what's going to be what's P plus Q going to be equal to and I encourage you to pause the video again and think about that what is this 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 assume there's only two alleles in this population and in kind of the allele population for this in the gene population for this trait so the frequency of the of the dominant ones plus the frequency of the recessive ones well everyone's going to have one of those too so there if you add those two frequencies it's going to have to add to one hundred one hundred percent we see that they're 1/4 plus 3/4 is one is 1 or 100% and 25% plus 75% is also 100% so we could say P plus Q is equal to a hundred percent or we could say that P plus Q is equal to 1 is equal to one and so in the next video we're going to start from the seemingly fairly simple idea to get to a more a richer and fairly neat idea that's expressed in the hardy-weinberg equation
Biology is brought to you with support from the Amgen Foundation