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Allelic Frequencies

by Georgina Cornwall, PhD
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    00:00 Now, we are looking at calculating allelic frequencies. Here is a whole table of numbers.

    00:09 I’d like to first let you know you don’t need to know the numbers specifically.

    00:15 These are numbers for an example of calculating allelic frequency. Let’s get to work on that.

    00:22 In the left column here, you can see denoted whether we are homozygous for the CCR5 receptor, whether there is heterozygosity at the CCR5 receptor and then homozygous for the ΔCCR5, which is the one that offers protection or resistance against HIV. Right now, we care about the population, so the number of individuals column. We can see that there are a number of individuals with each of those different genotypes in this particular population. Now, as we consider allelic frequency, because we’re looking at how many alleles of each type are in the population, we need to consider that there are 788 individuals in this population. So, how many actual alleles are in this entire population.

    01:20 Well, you’ve got it. You multiply that number by two and you end up having this number of alleles.

    01:26 The key there is to look at allelic frequency. First of all, we must multiply the number of individuals by two because everybody has two chromosomes for each particular trait and thus two alleles.

    01:39 Moving on, let’s look at how we might start to calculate the frequency of alleles in the population based on the number of alleles in the population. Let’s first look at the CCR5 regular wild type form of a gene. We’ll look at the individuals who have that genotype. We know there are 647 of them.

    02:10 We multiply that by two because each of them contributes two of the same kind of allele.

    02:16 Then we end up with this number of CCR5 alleles. Again, the numbers here are not what’s important, it’s the method to calculating the frequency or number of alleles in this population. Let’s move on to the next opportunity or next alternative of genotype. We have the heterozygote that has a wild type allele and a mutant allele. They have one of each. So, 134 individuals, that means that there are 134 wild type alleles and then 134 mutant type alleles. We’re just tabulating the totals here.

    03:01 Moving on to the third genotype option which is the protective form where we have the mutant CCR5 at both loci. Again, we’re going to multiply the number of individuals, times two because both alleles are the same allele. We will turn out as having 14 mutant CCR5 alleles from the homozygous mutant form.

    03:29 Now, in order to get the total frequency of each allele, we will add up how many of the CCR5 wild type alleles there are in both the homozygous form and the heterozygous form and we’ll get some totals.

    03:47 Let’s look at how we get these eventual numbers. Frequency of CCR5 is going to be how many CCR5 alleles there are divided by the total number of alleles in the population. We can take our 1294 CCR5 from the homozygous wild type individuals. Then we will take the number of 134 that are showing up in the heterozygous form. We add them together, divide them by the total number of alleles in the population. We end up with the frequency of 0.906 as you see in the table.

    04:31 Now, we could do this really simply and just subtract that frequency from one.

    04:37 But to go through and try this ourselves again, why don’t you take a moment. Pause the video and add up the numbers. Do the math for yourself just to verify that this is how things work out.

    04:52 Give it a pause. I’ll see you in a moment. Okay, hopefully you took the time to do that.

    04:59 We can prove that we come up with the same reciprocal number by taking the mutant number of alleles that we find in the homozygous mutant form and the mutant alleles from the heterozygous form.

    05:17 Adding up both of those and we see that we have exactly the right number to fill out a round even 1.00, 100%, right? So, we’re in great shape here.


    About the Lecture

    The lecture Allelic Frequencies by Georgina Cornwall, PhD is from the course Population Genetics.


    Author of lecture Allelic Frequencies

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD


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