Hardy-Weinberg Assumptions

by Georgina Cornwall, PhD

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    00:00 What is this population at equilibrium thing all about, you might ask. You may remember, see if you can for a moment, some of the Hardy-Weinberg equilibrium rules. What constitutes a population that is actually at equilibrium? Take a moment. Think about that. Then I’ll reveal them to you because of course, I know them. Here they are. A theoretical population in Hardy-Weinberg equilibrium says this population has an infinitely large size, right? There is definitely not a small population. We call it infinitely large, so huge. There is no immigration or emigration between this population, into or out of this population with any other population, none.

    00:59 Of course, we know that’s not reality. We also know that equilibrium population has no mutations.

    01:08 It has totally random matings, so no individual choice. There is no natural selection occurring.

    01:19 Again, Hardy-Weinberg equilibrium rules are really not very realistic for an actual population.

    01:28 But it is a great standard by which to compare situations that are not in equilibrium and may be experiencing evolution. The point here for us is changes in allelic frequency.

    01:45 There may be variation in allelic frequency between one population and another.

    01:52 We need to understand how those things are calculated again. Let’s now apply our values that we obtained by looking at the allelic frequencies back into this Hardy-Weinberg equation.

    02:09 Given that the frequency of p is 0.906 and the frequency of q or the mutant form is 0.094, take a moment, pause the video, and stuff those things into this equation. See what you come up with for the genotypic frequencies in this population. Alright, good work. Let’s check the answers to make sure that we’re all on the same page. If we square our p number, this is what we end up with, 0.821. If we calculate in 2pq, pretty easy. Plug and chug all the way through.

    02:51 Then we calculate that for q squared. If you add them all together, poof, they should add up to one.

    02:59 You’ve done it correctly. This is the equation that you need to be familiar with. You need to be able to to plug and chug and determine the numbers of each of the genotypes. You also need to be able to look at the genotypic frequencies and extract what the allelic frequencies would be, respectively p and q. Be sure that you’re very comfortable with that before you go sit for your exams.

    03:31 To prove that this whole thing works in both directions, let’s go ahead and try to once again extract the genotypic frequencies. We can do that by taking the number of individuals in the population multiplied by the frequency. Viola! We end up with exactly the same number. We can then look at the p squared and multiply that frequency by the number of individuals in the population. Guess what? It comes up with exactly the right number. Then let’s take a look at the 2pq which is the heterozygotes.

    04:18 You’ll see that alas, we have the right number of those individuals. It works both ways. Again, be familiar with this equation and how to work with it to make predictions about allelic and genotypic frequencies.

    About the Lecture

    The lecture Hardy-Weinberg Assumptions by Georgina Cornwall, PhD is from the course Population Genetics.

    Author of lecture Hardy-Weinberg Assumptions

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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