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20. Quantitative Genetics

1

Calculating Heritability

7m

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Hi in this video, we're gonna be talking about heritability. So heritability is a measurement and what it's measuring is the proportion of variation in a population that's due to genetics. So that's saying how much of a trait can actually be inherited. So there's this ton of variation. So how much of height can you actually inherit it? And how much has to be, how much it depends on environment. And so the important thing to, I feel like people a lot of times, especially in the news articles, they hear these heritability statistics and they attribute that to something that it's not. So it's important to remember that it's actually a very specific specific measurement and it's only true for a certain population. So that population that you're measuring in a certain environment, it is not widely applicable to any individual that is of the same species. It's only in this population in this environment. And so what it measures is a measure from 0-1. So most of the time it's some kind of decimal point. And the larger the value, the more the variation is explained by genetics. So if you have a heritability which is heritability equals .65, then you're what you're saying. And this is important to understand, you're saying 65% of the overall variation. So it's not 65% of an individual's variation, 65% of the entire variation found in that population is explained by genetic differences found in individuals. So this is where pop news sort of messes up the statistic and says, you know, so much of your inheritance or your intelligence for instance is dependent on genetic factors is not true. Often these statistics um are misinterpreted that you know so 65% of the overall variation and intelligence is due to genetic differences in individuals. But it says nothing about a particular individual at all. There are two types of heritability. The first is broad since and what this is is it measures the contribution of genotyping variants to total variance. So here's the formula. So V. G. And V. P. Um if you remember from our analyzing trait variance this is the same exact um calculation here. And so if the H. Two is close to one this means that environmental conditions had little impact. If it's close to zero environmental conditions had a major impact. So an H. Two close to wood meaning that it's mostly genetic and it's very little genetic. And so if we're going to calculate the broad sense heritability for each of these traits what do you do? So if we have body fat you scroll down so we have body fat here and this is and the formula is here. Right so we're using V. G. And V. P. And so uh the formula here for a body that is going to be what it's going to be 16.9 divided by 40.5. So you can either do that long hand or you can input it into a calculator. If you input it into a calculator it's 417. So you can kind of round that 2.42 let me disappear so you can actually see that. And so what does this mean? This means 42% of the overall variants in the population for body fat is attributed to genetic factors. If we do body length we can calculate the same thing except this time at 17.9-43.6. And so again we put it into a calculator which is what I'm doing right now. Um what you get is which is uh one or 41%. And so these are very similar for this for broad sense heritability. The second type is narrow sense heritability and narrow sense. Heritability provides a proportion of fanatic variation due to additive genetic variance. So you'll notice that this is different and the additive is important. So what an additive variation? Well genetic variation you just think oh it's just genes right? It's just genetic variation but it's actually not. It's composed of two different and so the genetic variation is composed of attitude variation which stands for V. A. And this is genetic variance caused by differences between alleles. So this is things like the differences between dominant and recessive right? They have differences. They present different characteristics so that this is variation according to different alleles either dominant recessive or whatever alleles that exist. The second is dominant variation and this is variants caused by hetero zygotes not being intermediate. And so when hetero zygotes aren't intermediates there's intense variation, right? Because then intermediates may the hetero zygotes maybe closer to the dominant um phenotype or they could be closer to the recessive or it could entirely vary depending on the hetero zygotes found in the population. So these are the two types of genetic variants that make up that V. G. Calculation that we've been using just this one thing is just total genetic variation. So it's important I wrote here. So V. G actually equals the attitude variants versus plus the dominant variants. And so that's an important formula to know. Um So here's all your important formulas. And therefore when calculating narrow sense heritability. Which is this here. So this is the formula for narrow sense heritability. You use the additive genetic variance over the total variance. So if we are calculating this right? So we have body fat again and body length. This time we're using the additive value. So for body fat that's going to be 7.66 over 40.5. And if you put that into a calculator, what you get is .18 or 18 The same for body length. So this is 5.12 divided by 43.6 and that is gonna be .11 and that's 11%. So that means in this case the body length although it was what? 42 or 43?? What was it up here? Yeah, it was 41% attributed to genetics, but it's only 11% attributed to additive genetics. And that is the fact that alleles have different characteristics, dominant and recessive. So with that, let's now turn the page.

2

Artificial Selection

4m

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Okay, so now we're gonna talk about artificial selection and the reason we're gonna talk about this is because this is actually an applicable reason to use the calculations we talked about before heritability. So what is artificial selection? That's the process of choosing specific individuals for some type of fanatic breeding purpose. So for instance if a a farmer like a dairy farmer who produces cows for milk um and they wanted to increase the milk production of their cows. So they would try to make specific individuals in order to increase that milk production. But before they start making these cows right that's a huge undertaking to make these cows for bigger or for more milk production. They want to make sure that it's actually going to work. And so geneticists can actually tell you, you know, is this likely to increase milk production and how they do that is they use narrow sense heritability to predict the impact of breeding. Now why narrow sense heritability? Well because narrow sense heritability is measuring the likelihood that a trait is actually attributed to genetics and not inheritance and it's also the ability to because narrow sense heritability measures additive right? Then that is measuring the the ability to inherit this dominant or recessive allele that's really going to impact the production of milk. So the higher the H. Two value. So because H. Two is a measure between zero and one. So that's gonna be the closer to one. Um So if you calculate this value, the closer to one then the more likely that the breeder will observe a change, the more likely that that's going to result in potentially milk production. So there are two formulas the first we talked about before and that's just the narrow sense heritability formula right? This is what we talked about previously but and sometimes you know this information. It's like on a test or quiz setting you might be actually given this information but if you're a breeder you probably don't already just have the amount of additive variance that's contributed to the production of milk right? Like you most likely don't have that. So there's a second formula that you can use to calculate narrow sense heritability and that actually is this one and it's R. Over S. And so are is the mean of the offspring. So how much milk and volume the offspring produces versus the overall mean of the population that you started with divided by the mean of the parents. So how much the mother produced essentially versus the overall mean. So you do have to do at least one mating to get this right. And this is called the selection response. Um And the selection differential. And so um sometimes on I don't know which one you'll be given right. Like if you're asked to do this on a test or quiz, you could be given this or you could be given the means and asked to solve it this way. But essentially the question would be how likely it is that this trait could be inherited or that a breeder could select for this trait in the population. Either way what you're looking for is an HD value close to one. The closer to one it is the more likely that it can be um uh inherited. So if we go back to these numbers which we talked about before right we had body fat had an H. Two value of 20.18 and we had body length As an H. two value of .11. I think this is right now um Which of the following two traits will respond best to a selection by breeder. Right? So it's the higher H. Two value. So in this case it would be this one which would be body fat because it's closer to one. But really to be honest with you both of these are very like very low. Right? So you would need much higher H. Two values before a breeder is like okay I will try to actually select for this trait. So um make sure you understand this formula and this one and how you use that to determine whether or not a breeder you know whether or not that trait could be inherited or not for artificial selection purposes. So with that let's now turn the page

3

Twin Studies

3m

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Okay, so now let's talk about twin studies. And the reason we talk about twin studies is the fact that humans can't be bread, right? We can't be bred to determine in different environments. And with different genetics to be able to determine what genetic variation, what environmental variation, what's additive genetic variation, dominant genetic variation. So how we actually have to do this, we can't set up these control experiments. So what we do is we use twin studies and twin studies are taking twins that are produced and studying them for genetic or environmental variation. So there's two types of twins. There's mono psychotic twins. And how this happens is there's a single zygote. So there's a single sperm that fertilizes a single egg. But what happens is that it does this weird division thing. It methodically divides and splits into two sales. And so when it does that there's now two babies that are produced from these two cells, but they're genetically identical. So they have the same genetics. Um, so therefore when you study them, they all the variation that they have is environmental variation. That's kind of lying. If you look at identical twins when they're babies, they look very similar, right? Because they haven't had a lot of exposure to the environment. But if you look at those same twins and then they're in their seventies, they actually can look quite different even though they're identical and that's because that environmental variation has built up throughout the years to make changes in, you know, potentially epigenetic or made them more likely to get a disease or various things right, that impact phenotype. And so um Monet's I got a twin studies are extremely important and being able to identify how much environmental variation plays on human genetics. However it's not ideal and there's actually some genetic changes that occur very early in development. So an example of this is copy number variants can actually change a lot in very early development. And this is a way that mono psychotic twins which would otherwise be genetically identical may actually not be 100% genetically identical ba because each twin could could actually carry different numbers of copy variants. And there's actually been cases of mono psychotic twins who are genetically identical. Pretty much of one twin actually developing a genetic disease and the other one not very early in their life because they had variations and copy numbers so that it's not a perfect system but it's really the best. We have a second type of twin is dies. I got twins and this is actually two fertilization. So there's two eggs and two sperm. And um these are all the other twins. They are just as close genetically as any other sibling. So if you have a sibling that isn't a twin they're just as close genetically to you as a fraternal or dies. I got a twin. Um But because they often share a similar environment. Right? They're raised in a similar situation. We can kind of use that to um at least in part study genetics. it's not perfect. Obviously the models I got a twin is definitely the gold standard. But we can also use the psychotic twins, especially ones that have been separated and raised in different environments to study um various components of human genetic variation. And so um an interesting part of expression of twin expression is that sometimes traits aren't expressed equally and so on. Concordant trait is a trait that both twins expressed. Either so they either both express it or they both do not express it. The second type is discordant, and that is when one twin expresses a trait and the other doesn't. And this can happen in both mono psychotic and dies. A narcotic twins. Um And so just some fun bow cap for you as we finish out this topic. So with that let's now move on.

4

Problem

A chicken breeder has a population of chickens where the average number of eggs laid per hen per month is 34. The narrow-sense heritability is 0.75. With this information is it likely that a breeder could select for an increase in eggs per hen laid each month?

A

No, breeders never know whether they can select for a trait

B

No, the breeder will need to know the broad-sense heritability to determine whether selection could cause an increase in eggs?

C

Yes, because the narrow-sense heritability is 0.75, this means selection is likely to occur

5

Problem

The narrow-sense heritability of the number of seeds per flower is 0.9. The mean of the population is 6.0 seeds per flower. A flower breeder crosses one flower with 7 seeds to another plant with 9 seeds. What is the expected number of seeds per flower in the offspring of this cross?

A

5

B

6

C

7

D

8

6

Problem

Heritability calculations were calculated for a variety of different traits. Which of the following traits would respond best to selection?

A

H_{2} = 0.8, h_{2} = 0.3

B

H_{2} = 0.3, h_{2} = 0.3

C

H_{2} = 0.9, h_{2} = 0.8

D

H_{2} = 0.5, h_{2} = 0.9

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