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20. Quantitative Genetics

Traits and Variance



Traits and Variance

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Hi in this video, we're gonna be talking about traits and variants. So there are many different types of inherited traits. The first category that we're gonna talk about is actually called continuous traits and these are traits that could take a potentially number or infinite number of states have been arranged. What do I mean by that? So we're looking at human height, which is an example of a continuous trait. There's the shortest person in the world and the tallest person in the world. But any height in between there can kind of be any number. Right? So we can have someone who's four ft 1.6298 inches. Right? You can have somebody who's seven ft and 70.92346 inches. Right? So anything that you can add these like extended number of decimal points on is typically a continuous trait. And this differs from categorical traits which are traits that have to be sorted into discrete categories. So an example of this is a flower that is either purple or white. There's no mixture either your purple or white. Another example of this is um Spots on a Dalmatian. For instance, you can have one spot, you can have two spots. You have 30 spots but you can't have 30.67 spots. Right? It doesn't make sense. You can't have .67 spots on a Dalmatian and therefore that's going to be a categorical trait because it has to be divided into those categories. Now, within categorical traits, there's two kind of classification is the first is a little bit confusing. Two graphs, but it's called a threshold trait and these are traits that need a variety of different genetic and environmental factors to be expressed. And so some people can have some of these genetic or environmental factors, but only people who reach a threshold number of those factors expressed the traits of an example of this is type two diabetes. It's a categorical trait because you either have type two diabetes or you don't. And so um type two diabetes is a combination of genetic and environmental factors, but you have to reach a threshold number of those like you have to have so many genetic and so many thresh or environmental factors to become a Type two diabetic, which is why it's considered a trait. Um It's also a disease, but it's like one of those interesting disease trait mixtures because type two diabetes can be at least in part genetic but not everyone who carries carries genetic markers for type two diabetes will be type two diabetic because they haven't reached that threshold of environmental and genetic factors required to become a Type two diabetic. A second type of categorical trait is much easier to wrap your mind around. That is a metastatic trait or otherwise accounting trait and these are things that are divided into a range of discrete values. So an example of this would be birds, they can lay 12347 eggs, but they can't lay 1.26 eggs, right? Or 2.58 eggs. That doesn't make any sense. They can like one egg. They can lay three eggs. They can't lay 2.56. And so that is a categorical trait. So let's do an example and classify each one of these into categorical or continuous. So number of spots on a Dalmatian. Is it continuous or is it categorical? What do you think? Right. It's categorical reason. It's categorical because you can have 50 spots. You can have 57 spots. You can't have 50.36 spots on a Dalmatian human weight, continuous. Categorical. Right? It's gonna be continuous. And the reason it's continuous is because you can have somebody that weighs 130. You can have somebody that weighs 130.00000001. If as long as you have a way to detect it, you can have somebody who actually ways that. So it's this continuous trait of human, wait, what about foot size, continuous categorical? What do you think? Right. That's continuous. And that is because foot size. It can be a range of values between the smallest foot to the largest foot that exists in that organism. And then we have cat litter sizes. Um So what do you think? Is that categorical or continuous. Right? Categorical. Because you can have a cat with that will produce one kitten or two kittens or three kittens. But you can't have a cat that produces 2.58 kittens. And so that is categorical. So now that we've divided into the continuous or categorical, let's talk about how these traits are inherited. And there's two main ways. The first is called complex inheritance and complex inheritance is a trait that that is involves multiple genes or environmental factors. So we're looking at something, for instance, human height, human heights controlled by multiple genes in that way, human height is undergoes complex inheritance. It's inherited in a complex manner. This differs from simple inheritance which involves one trait and how we know and typically like how you can figure out whether it's one trade or more is just look at Mandali in ratios. So if you can do crosses, let's say it's plants or flies or something. If you can do crosses and look at mandolin ratio. If you can get a 3 to 1 or 9 to 3 to 3 to 1 ratio that we've talked about so much, that's going to be something that's simple, simply inherited. It's usually just involving one trait. So for complex, So the simple inheritance, we've talked a ton about, right and other things that we're gonna pretty much leave that there, but not more about complex inheritance and how that typically works. And so um for polly genic inheritance, this is also called complex inheritance, right? Because polly genic means multiple genes and that's right there and complex inheritance. And so let's talk about that. So actually what happens is that typically in apologetic inheritance, there are multiple genes but each gene acts as behaves in an alien fashion. So if you were able to isolate you know every single gene and were able to study it in a model ian fashion, what you would get is model Ian numbers. But the thing is that we're looking at phenotype and all of them come together and contribute to the phenotype. And so all of these alleles and these different genes can come and add. So that phenotype now not every allele in the phenotype is actually contributing to that phenotype. So there's two types of alleles and added to the level that contributes to the phenotype, meaning it's added to the phenotype when it's present. And a non additive liel that does not contribute to the phenotype. And these um almost seems similar to dominant and recessive. And we'll see that we talk about that a lot. But it's not exactly because in a recessive gene you only need two recessive alleles to show that phenotype. But in the case of additive and non additive because it's multiple genes you can actually need a lot more non additive genes to actually not see that dominant phenotype or added a phenotype. So let's go through an example of this. So example you're going to see in your book is this cross parental cross of white corn and purple corn. And I'm giving you the genotype here. So it's four non additive leal's. So lower cases non additive and upper case is additive and it's for both um genes A. And B. So in order to get purple you have to have four additive alleles. And if you have if you don't have all four you have four non additive levels then you see white. So we have to do this cross. We actually need to look at the F. Two generation. So we do this cross we do the F. One. We get this sort of intermediate phenotype which is read. It contains two additives and two non additives. But what we're really interested in is this F. Two generation. And the reason that we're in this F. Two generation is because of all the phenotype. So we have some that look like the parents. So we have some that are purple and white. Right? We have some that look like the F. One generation that's red. But we also have this these um different phenotype which are dark red and light red. And you'll notice that these have to do with how many alleles or how many additive alleles are in um the genotype. So for a purple kernel it has four additive alleles for a dark red, purple Colonel. It has three additive alleles. Now notice it doesn't matter what combination of additive levels. You can have two bees in one A. Or you can have two A's and one B. Does not matter. These jeans don't do anything different. It's just that the additive alleles, there's three of them and that's creating this dark red, almost purple phenotype for the red kernels. You have two additives. Now I wrote this as A. And B. As almost like hetero zygote. But because it's too additives it can be written other ways as well. What would you say Those ways could be right. It could be written like this or like this right? Because all we need is to add a devil eels so we can have it like this, we can have it like this or this because it doesn't matter. The genes aren't doing anything different. Just that they're adding to the genotype for light red kernels. You have one additive alil and that can either be A. Or B. Does not matter. And then for why you get four non additive alleles. And so when you get a question, you know something like how can you tell this is apologetic inheritance? Well usually you can tell because you have a variety of phenotype in the F. Two generation. This isn't your typical 9 to 3 to 3 to 1 ratio either write 1 to 4 to 6 to 4 to one. Um And so that is how we begin to tell that something else is going on. And it could be apologetic inheritance. Now another question that you're often given in response to these types of inheritance question is that you're given this you're given the F two phenotype and the ratios but then you're asked something really difficult and you're asked how many genes are involved in this phenotype. And so um there's actually a formula to predict how many genes contribute to this trait. So if I didn't tell you that it was too jeans with A. And B. How would you know this, luckily there's a formula and here's the formula wrote it here. But it's also this it's one to the fourth to the end equals that none of the ratio of parental pena types. So if we look up here there are two ratios with parental phenotype since one to the 16th. Right? Because our parental remember where these they were white and purple and so on. Are F. Two generation. We look at purple and white and both of them are 1 16. So if we were to if we didn't know if you you had no idea that there were two genes involved in this. We could use this formula and it would be written as one to the fourth to the N. Equals 1/16. And you solve for N. Because N. Is the number of poly genes involved. So if you could put this in the calculator you could do it yourself for doing it ourself. You say if n equals two. So if there are two genes then this is what this looks like and this is true. Right? So there are two genes involved in this um phenotype. So that is a common question you'll see on an exam or a quiz is that you're given these ratios and these phenotype and asked to calculate the number of genes. So as long as you know this this formula here one to the force of the end equals the ratio of parental phenotype. Since then you will be able to figure that out fairly easy. A second number a second formula and a second question that you might be asked is to calculate the number of fanatic pick categories observed. So if you're given the number, apologies say that. Say the question is something like uh trait a or you know some type of trait is involved is controlled through four genes. How many phenotype will be present in the f two generation? Well how you would do that is you would use this formula. So we were given, right I told you that there was a trait controlled by four genes. So how many phenotype use to in plus one? So two to the two times four plus one. So that's going to be eight plus one equals nine. So in a trait that is controlled by four genes you're gonna have nine phenotype sis. So these two formulas are really important and you will vary is a good chance that you will be asked to solve some type of math problem. Using one. If not both of these problems. So make sure you understand these two formulas on these math, it's fairly simple, as long as you understand what the formula is and what it stands for and how to solve for it. So with that let's now move on.

A trait controlled through polygenic inheritance was observed in a series of experiments. A brown eyed rabbit was mated with a blue eyed rabbit. 130 F2 offspring were produced. 2 offspring had brown eyes and 2 offspring had blue eyes. How many polygenes control eye color in rabbits?


If a trait is controlled by 5 polygenes, how many phenotypic categories will be observed in the F2 generation?


Polygenic inheritance is what type of inheritance?