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.
Of course, we know that’s not reality. We also know that equilibrium population has no mutations.
It has totally random matings, so no individual choice. There is no natural selection occurring.
Again, Hardy-Weinberg equilibrium rules are really not very realistic for an actual population.
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.
There may be variation in allelic frequency between one population and another.
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.
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.
Then we calculate that for q squared. If you add them all together, poof, they should add up to one.
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.
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.
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.