—
Mr. Andersen explains Hardy-Weinberg equilibrium and describes the bead lab.
—
—
Transcript Provided by YouTube:
00:01
Hi. And welcome to the AP Biology Lab 8 Population Genetics and Evolution podcast.
00:07
In this podcast we do what’s called the Hardy-Weinberg lab. Hardy-Weinberg remember is a way to describe
00:15
in a population how the genes will change over time. Or a better way to say that is
00:19
how they won’t change. And so the reason I put a picture of Mr. Darwin here is that we
00:23
can use Hardy-Weinberg equilibrium to observe when evolution is actually taking place in
00:27
a population. And so what we’re dealing with is microevolution. In other words changes
00:31
within the gene frequencies in a population. This graph is kind of confusing if you don’t
00:37
know what p and q values are. And so p and q values are going to be the frequency of
00:42
the dominant and frequency of the recessive allele. And so as the number of big A big
00:48
A or homozygous dominant individuals, we’ll say right here, increase up to a total of
00:55
maybe 100 percent of them, you can see that the frequency of the other two phenotypes
00:58
decrease. So if you’re really into that take a second to look at that. If not, look to
01:03
the next page. So this is Hardy-Weinberg equilibrium or Hardy-Weinberg equation is a better way
01:07
to say that. And so first of all we should kind of detail ourselves with what’s down
01:11
here in the middle. p value is going to be the allele frequency of the dominant. And
01:16
q is going to be the allele frequency of the recessive. And so in this lab we’ll use a
01:20
cup. We’ll call it the mating chamber. And inside there we’re going to have beads. And
01:24
those beads will either be black, and we’ll say that’s dominant, or they’re going to be
01:29
white. And we’ll call that recessive. And so if I put 50 beads of black and 50 beads
01:37
of white in the cup, what’s my p value? p value is going to be 0.5. What’s my q value?
01:43
q value is also going to be 0.5. And so in this lab every time we start we always start
01:47
with a p and a q value equal to 0.5. Or 50-50. Half of each. So, what does the equation even
01:54
tell us then? If we look at the equation itself, p squared, if we take p squared, that’s 0.5
02:00
times 0.5, which is 0.25. What that tells us is the frequency of homozygous dominant.
02:06
In other words if I shake this up, the odds of me pulling out two beads that are both
02:13
black should be 0.25. In other words there’s 25 percent chance. What about pulling out
02:19
two whites? Well that’s also 0.25, or that’s q squared. And then what about the odds of
02:23
me pulling out one that’s heterozygous? Either black white or white black? Well that’s going
02:28
to be a 50 percent. And so if I shake it up, and pull two out. What are the odds that it’s
02:33
going to be black white? Should be half of the time. Now I can’t look. And so if I pull
02:38
those two out, I got black black. What are the odds of that? Well it would 25 percent.
02:42
Let me try it again. Pull it out. Black black. So that’s pretty rare. What about the next
02:48
time? It’s black white. And so that should happen 50 percent of the time. Now that’s
02:52
an incredibly small sample size. And so in this lab we have to make sure we pull a whole
02:57
heck of a lot more than just three pairs. In fact we pull forty pairs out each time.
03:02
And so what is equilibrium then? Well Hardy-Weinberg equilibrium is more of a mathematical model.
03:08
And it was discovered by two different mathematicians at about the same time. Hardy and Weinberg.
03:14
This happens a lot in science. But what they said is if we keep these five things the same,
03:20
in a population or in beads in a cup. In other words if there’s no mutation, the beads don’t
03:25
magically change to a different color, if there’s no selection, I’m not drawing out
03:29
ones of a specific color to get rid of. There’s no gene flow. I’m not like adding new genes
03:34
to the cup and I’m not pulling genes out. If it’s a large enough population size and
03:39
if we do random mating, in other words I shake it up each time and I randomly pull it out,
03:43
it should stay at 0.5 that whole time all the way across. And so does it ever do that?
03:48
No. Not really. But look at this graph which is pretty cool. Over here what we have is
03:52
a population where the n value is 20. The n value is 200. And the n value is 2000. And
03:58
n is the number of individuals in that population. And you can see that the bigger number we
04:04
get the closer it stays to that equilibrium. And as we decrease the number then it’s just
04:09
going to get random. In other words the law of large number says stats don’t really work
04:14
unless you have enough of them. And so what do we do in this lab? Well the mating chamber
04:18
is going to represent sex from generation to generation. And so what we’re doing is
04:23
putting all our alleles inside here. It makes it what’s called the gene pool. And then we’re
04:28
simply pulling them out. And each of those pair that we pull out represents a new organism.
04:34
And that sets the next generation. And so what are we studying in this lab? We’re studying
04:37
four things. First thing is just trying to hold Hardy-Weinberg equilibrium the same.
04:42
In other words we’re trying to make sure we have large sample, it’s random mating, no
04:46
mutations, all of those 5 things are exactly the same. And we should see that those 0.5
04:51
values stay the same. In the next round we do selection. What do we do there? If we pull
04:55
out a black and a white, then we’re okay. If we pull out a black and black we’re okay.
04:59
But if we pull out a white and a white, then that dies. So we could say that’s a recessive
05:03
disease for example. We remove that and then we calculate the next generation on that.
05:07
And so I don’t want to tell you what happens. So you’ll have to figure it out. The next
05:12
thing we model is heterozygous advantage. An example of that might be sickle cell anemia.
05:17
Sickle cell anemia is an awful disease if you’re homozygous recessive for it. But if
05:21
you’re heterozygous, you’re actually protected against malaria. And so heterozygous are actually
05:26
protected. And so that’s an advantage. And so we model that and see what happens to our
05:30
frequencies. And then finally we do genetic drift. And so genetic drift is some kind of
05:35
event where we reduce the numbers from a large sample size to a smaller sample size. And
05:39
then when we’re done with that we figure out what happens once we return that sample size
05:44
to a larger number. And so those are the four things that we’ll study in that. And basically
05:48
we’re trying to model what happens in a population as far as genetics go. And that’s about it.
05:55
And so hopefully that’s helpful. And the end.
—
This post was previously published on YouTube.
—
Photo credit: Screenshot from video.