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21. Population Genetics

Hardy Weinberg


Hardy Weinberg

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Hi in this video we're gonna be talking about hardy Weinberg. So I know Hardy Weinberg is probably your favorite topic because it's just so much fun to do. So hopefully you won't have too much to review. But for most people, most people do not really like Hardy Weinberg. So let's delve in. So Hardy Weinberg, what is it all it is is a formula. And this formula is looking at what are the frequencies? So how frequent are different alleles and gina types in a population at a specific time. Right? So you're looking at a population of fish and some fish are brown and some fish are blue. And so what are the frequencies of those brown and blue alleles in that population? So we can measure this in two ways using two formulas. First is the allele frequency and that's measuring the alleles the frequency of the alleles. And so that formula is here P. Plus Q equals one for P. Represents dominant and the Q. A little represents recessive, which makes sense. You add up all the dominant, you add up all the recessive that's going to total the number of alleles in the population. Now you may be asking you say, well, we've gone over a lot in the genetics and we realized that not all of leo's are dominant and recessive does hardy Weinberg take that into account. Well it does. So if you have more alleles than just dominant and recessive then you can act actually add those into. So you have the dominant, you add the recessive and you add any other type of deal that you may have in these sort of weird genetic cases that we've talked about and all of them will add up to equal one. But for simplicity sake we're just going to focus on the P plus Q equals one because that's probably what you're familiar with. And so the allele frequency is looking at the gene pool of the population and that is some of the lille. So people ask you in the breeding members of the population. So these are the alleles that can be passed on at that specific time, then we have Jenna typical frequencies and these are looking at Jenna types. Right? So either home a zygote or hetero zygote. And so this formula is here P squared plus two PQ plus Q squared equals one. So P squared is dominant hama zygotes. So that would look like this to P. Q. R. Hetero zygotes and then Q squared are recessive, homocide, goats. And so by using this formula we can calculate what's the frequency of hetero zygotes in the population. What's the frequency of dominant homo zygotes in the population? It's a really great way to measure frequencies. Now, hardy Weinberg to do it. There has to be a bunch of assumptions net and we're gonna talk about those assumptions on the next page. But before we get to them, I just want to say that because of all these assumptions that we're about to go over the hardy Weinberg says that the genes the alleles and the genotype frequency. So whatever these numbers are do not change, underline this from one generation to the next. So if you get a question that says something like you know, assuming hardy wine equilibrium, here's P. And Q. For this generation, what P. And Q. For the next generation? Well assuming hardy Weinberg, they're the exact same. All the alleles get passed on to the next generation according to hardy Weinberg. And so all the frequencies that you calculate using the genetic formula or the formula do not change. So here's an example and I typically I'm using this example because it's a little bit more difficult than some of the questions that you'll see. Usually some of the easier questions will give you the value for P. Or the value for Q. And then ask you to solve any of the other you know questions. But sometimes I feel like professors throw out weird wordings and they don't necessarily tell you what it is. So hopefully this is one like this. So this question says a recessive disease has a frequency of 12 1100 in the population, assuming Hardy Weinberg. So assuming we can use the formulas above calculate all these values. And so this is maybe a little bit difficult because we're not telling you P. Is this or Q. Is this or two PQ is this? We're saying that there's a recessive disease and here's its frequency now calculate hardy Weinberg. So because we know that this is a recessive disease and then only one person out of 1100 in the population have it, then this is Q. Two or Q squared. Right? We know that because it's a recessive disease. So it's got to be a home as I get this one person here has to be home as I guess recesses for that disease. So if we were to put that in the formula, what we would get is this nice number here. So without giving us saying you know Q. Is this, we can determine Q squared by looking at the frequency of the disease in the population. Now this type of question is often that one that professors give because they like you to take that next step and sort of figure that out for yourself. But luckily I'm here. So if you know if either it says recessive disease then you know that the Q square value is whatever the frequency of the disease in that population. So here here here we have Q. Square. Now if we're gonna solve Q. What do we have to do to get for a Q. We take the square root. Right? And that's gonna equal Q. Which in this case if we take the square root this we get this. So now we have Q squared and we have Q. Right? So now we can start solving for P. So we know that P plus Q equals one. So if we do P plus zero three equals one. And we solve for P. What we get is this? So here's Q square, here's Q. And here's P. Let me disappear. So I can use this other part. So now we have P. And Q. Right? So now that we have P we can square it. So P squared is equal 0.97 of squared. Right? Which if we put that into the calculator, what it will be somewhere around here. So now we have P squared equals this. And so now we have um now we have P squared. We have Q squared. We have Q. We have P. So all we need now is to P. Q. So you just fill in the equation. So P is 0.97 and Q is 0.3. And so the answer to two P. Q. Is this. So now we have all of these values here we have Q squared. Which is what it asked for. We have P squared. Which is what it asked for. Two P. Q. Q. And P. And it all started out with just this bit of information. The recessive disease has a frequency of 1 to 1100 a population. Because this is Q squared. We can use that value to calculate all the other values in hardy Weinberg equilibrium. Now often times they'll tell you, okay, well tell me the number of individuals that are homeless. I guess dominant. Well, if you do that, we figure out which one's home is, I guess dominant. That will be P square. Right? So we take 941. We multiply it by the number of the population, which is what we're given is 1100. And if we do that, what we get is and that is the number of individuals who are homos, I guess dominant. So P squared in the population and you can do this for any value here. These would be hetero zygotes, right? Two PQ. If I ask you for the number of hetero zygotes, you could do that if it asked you for the recessive recessive, it gave you that it's one, but you could use this to calculate it if you'd like. Um And so all of this will allow you to calculate all these problems. So let's now move on to the really important part of hardy Weinberg and that is understanding the assumptions and so Hardy Weinberg exists kind of in this ideal world. So what does this ideal world mean? So I have abbreviated it to Samir. So you may know someone named Samir. But essentially each one of these is going to represent an assumption for hardy Weinberg. So the first is s and that stands for no selection. So this is talking about natural selection, which we haven't gone over a lot. We'll go over a little bit more in the next video. But essentially natural selection is just the ability of genes or leal's to give a competitive advantage. The ability to contain to survive and reproduce at this really great advantage. But if there's no selection that means all gina types and all alleles have equal viability. None of them give the organism more chance to survive and none of them make it worse. They have equal viability, they will be passed on equally. None of them affect the ability to make or passed on to the generation. So no selection means all the alleles are equal in their ability to be passed on. Now that's s so now we have a Samir that stands for no new alleles. So this means that if there's no new alleles then there's no mutations creating those alleles. So sometimes people see no new alleles and some they'll see that the assumption is no mutations. But either way it's the same way mutations create new alleles, no new alleles can be creative. There's no mutation going on at all. The third and Samir is m and that stands for no migration. You may also see this as gene flow but it's the same. It means the exact same thing. So this means that individuals are not coming into or leaving the population. The population that exists is the pop That will exist. There's no changes in that number. There's also no sub population. So out of it there's 10,000 in the population. They won't be divided or separated anyway. All 10,000 have an equal ability to make all other 10,000 then we get to. So all three of these are nose first through your nose and the second two are not nose. The 4th 1 is I. And this means that the population is infinitely large. So it's just this huge population is not small. And what this means is that there's no genetic drift occurring. I know we haven't talked about genetic drift yet. It's kind of a complicated concept which we're going to talk about in the next video. Um, but just know that the the population is very large, infinitely large, right eye and no genetic drift is occurring. And then finally we have random mating are and so this says, mates are completely chosen at random. They're not influenced at all by the genes or the illegals. So, if we look back over here, no selection, no new alleles, no mutation, no migration. Infinite population. And always random mating. You can see that this is not reality right? There's always selection going on. There's always mutations going on. People populations are changing in numbers because they're migrating, no population is infinite. All populations have a finite number in their population and random mating is occurring or doesn't occur very often. Usually genes and alleles very much impact the mates that are chosen. And so hardy Weinberg. We can use it in these ideal situations where these five things are true. but these five things are true and really no population on earth. So we use Hardy Weinberg to estimate alleles and genotype frequencies. In these ideal situations, we say, okay, if everything is ideal, if everything is perfect, if all five of these similar assumptions are met, then we can use hardy Weinberg. So hardy Weinberg is 150 billion%. Something that's used to estimate these frequencies in an ideal population but does not represent what's really happening because these five assumptions never happen in a real population. So important part of this. Make sure you understand those formulas, how to calculate all the values with those formulas and then also make sure you have memorized these five assumptions because I guarantee you you will be asked about the assumptions of hardy Weinberg on a quiz or test. So with that let's not move on.

Which of the following is NOT an assumption made when using the Hardy-Weinberg formula?


Which of the following formulas can be used to calculate heterozygote frequency in a population?


In a random mating population of Drosophila, 5% of the flies have black bodies (encoded by recessive b) and 95% have brown bodies (encoded by B). Assuming Hardy-Weinberg equilibrium what is the allele frequency of B in the population?


In a random mating population of Drosophila, 5% of the flies have black bodies (encoded by recessive b) and 95% have brown bodies (encoded by B). Assuming Hardy-Weinberg equilibrium what is the genotypic frequency of BB in the population?