Hi. And welcome to the AP Biology Lab 8 Population Genetics and Evolution podcast.
In this podcast we do what's called the Hardy-Weinberg lab. Hardy-Weinberg remember is a way to describe
in a population how the genes will change over time. Or a better way to say that is
how they won't change. And so the reason I put a picture of Mr. Darwin here is that we
can use Hardy-Weinberg equilibrium to observe when evolution is actually taking place in
a population. And so what we're dealing with is microevolution. In other words changes
within the gene frequencies in a population. This graph is kind of confusing if you don't
know what p and q values are. And so p and q values are going to be the frequency of
the dominant and frequency of the recessive allele. And so as the number of big A big
A or homozygous dominant individuals, we'll say right here, increase up to a total of
maybe 100 percent of them, you can see that the frequency of the other two phenotypes
decrease. So if you're really into that take a second to look at that. If not, look to
the next page. So this is Hardy-Weinberg equilibrium or Hardy-Weinberg equation is a better way
to say that. And so first of all we should kind of detail ourselves with what's down
here in the middle. p value is going to be the allele frequency of the dominant. And
q is going to be the allele frequency of the recessive. And so in this lab we'll use a
cup. We'll call it the mating chamber. And inside there we're going to have beads. And
those beads will either be black, and we'll say that's dominant, or they're going to be
white. And we'll call that recessive. And so if I put 50 beads of black and 50 beads
of white in the cup, what's my p value? p value is going to be 0.5. What's my q value?
q value is also going to be 0.5. And so in this lab every time we start we always start
with a p and a q value equal to 0.5. Or 50-50. Half of each. So, what does the equation even
tell us then? If we look at the equation itself, p squared, if we take p squared, that's 0.5
times 0.5, which is 0.25. What that tells us is the frequency of homozygous dominant.
In other words if I shake this up, the odds of me pulling out two beads that are both
black should be 0.25. In other words there's 25 percent chance. What about pulling out
two whites? Well that's also 0.25, or that's q squared. And then what about the odds of
me pulling out one that's heterozygous? Either black white or white black? Well that's going
to be a 50 percent. And so if I shake it up, and pull two out. What are the odds that it's
going to be black white? Should be half of the time. Now I can't look. And so if I pull
those two out, I got black black. What are the odds of that? Well it would 25 percent.
Let me try it again. Pull it out. Black black. So that's pretty rare. What about the next
time? It's black white. And so that should happen 50 percent of the time. Now that's
an incredibly small sample size. And so in this lab we have to make sure we pull a whole
heck of a lot more than just three pairs. In fact we pull forty pairs out each time.
And so what is equilibrium then? Well Hardy-Weinberg equilibrium is more of a mathematical model.
And it was discovered by two different mathematicians at about the same time. Hardy and Weinberg.
This happens a lot in science. But what they said is if we keep these five things the same,
in a population or in beads in a cup. In other words if there's no mutation, the beads don't
magically change to a different color, if there's no selection, I'm not drawing out
ones of a specific color to get rid of. There's no gene flow. I'm not like adding new genes
to the cup and I'm not pulling genes out. If it's a large enough population size and
if we do random mating, in other words I shake it up each time and I randomly pull it out,
it should stay at 0.5 that whole time all the way across. And so does it ever do that?
No. Not really. But look at this graph which is pretty cool. Over here what we have is
a population where the n value is 20. The n value is 200. And the n value is 2000. And
n is the number of individuals in that population. And you can see that the bigger number we
get the closer it stays to that equilibrium. And as we decrease the number then it's just
going to get random. In other words the law of large number says stats don't really work
unless you have enough of them. And so what do we do in this lab? Well the mating chamber
is going to represent sex from generation to generation. And so what we're doing is
putting all our alleles inside here. It makes it what's called the gene pool. And then we're
simply pulling them out. And each of those pair that we pull out represents a new organism.
And that sets the next generation. And so what are we studying in this lab? We're studying
four things. First thing is just trying to hold Hardy-Weinberg equilibrium the same.
In other words we're trying to make sure we have large sample, it's random mating, no
mutations, all of those 5 things are exactly the same. And we should see that those 0.5
values stay the same. In the next round we do selection. What do we do there? If we pull
out a black and a white, then we're okay. If we pull out a black and black we're okay.
But if we pull out a white and a white, then that dies. So we could say that's a recessive
disease for example. We remove that and then we calculate the next generation on that.
And so I don't want to tell you what happens. So you'll have to figure it out. The next
thing we model is heterozygous advantage. An example of that might be sickle cell anemia.
Sickle cell anemia is an awful disease if you're homozygous recessive for it. But if
you're heterozygous, you're actually protected against malaria. And so heterozygous are actually
protected. And so that's an advantage. And so we model that and see what happens to our
frequencies. And then finally we do genetic drift. And so genetic drift is some kind of
event where we reduce the numbers from a large sample size to a smaller sample size. And
then when we're done with that we figure out what happens once we return that sample size
to a larger number. And so those are the four things that we'll study in that. And basically
we're trying to model what happens in a population as far as genetics go. And that's about it.
And so hopefully that's helpful. And the end.