Biology: Polygenic Inheritance

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Let's continue to talk about some of the genetic anomalies that we've discovered post-Mendelian. Now, what do I mean by that? Well, remember Mendel's whole idea of dominant and recessive, segregation of alternate alleles, the independent assortment? All of these form a very firm basis for our knowledge of genetics, but more and more, we're finding out that it's never as simple as all of that. And whether we're talking about epistatic interaction, the fact that there are more than two alleles for a given trait and other aspects, just make this more of a conundrum if you will than, oh look, it's so simple. And what I want to talk to you about is things where you have what are called quantitative characteristics. In other words, you have different amounts of expression of something.
A good example would be skin color. Look around you. Look in your classes. Look in your places of work. Even people considered members of the same race have enormous amounts of skin variation, and that doesn't even include going across races. And so why is that? How do you explain the genetics of skin color? Now, one of the things is, and this is a topic all unto itself, environment affects expression of genes at times. I know for myself, I am much darker at some times than others, and that's because I have this genetic ability to produce melanin when I'm exposed to ultraviolet radiation. It's a protective device.
We have in our skin a pigment called melanin, and when we pound our skin with ultraviolet radiation, we defend ourselves from this carcinogenic radiation, cancer-causing radiation, by actually providing a pigment that absorbs that UV. And the more we put that UV into our skin, the more it absorbs. So you're saying, "Oh, it's safe to go out and lie in the sun." No! If you think about it, that's one of the dumbest things you can do because you're saying, "I think I'll go lie out in this cancer-causing radiation and make myself dark so that I can protect myself from the cancer-causing radiation." But meanwhile, you're getting some in there. The more you expose yourself, the more likely you are to lose that war because remember, your bodies did not evolve to spend eight hours lying out in the sun. But that's not what I want to talk to you about.
What I want to talk to you about is this whole idea of skin color and polygenic inheritance. It's nice to talk about rolling your tongue, and it's nice to talk about all the different traits that are so simple to trace. But a lot of genes are not that simple. A lot of traits are controlled by more than one gene, and that makes it tough. It's very easy to do a demographic study of a family history and build a pedigree chart when we're just following one gene because the patterns fall into place, and we can say, "Oh look. These two people didn't have the trait, and this kid did, and this kid didn't, so I'll shade that one in." That's a little trick we do on pedigrees to show whether they have the trait or not. And so we say, "Ah ha! These people both must have been carriers for that trait, and this one got the tt, and this got the T, whatever." That's easy. But with polygenes, you can't do that. It's very difficult. So a lot of our knowledge of polygenic inheritance is a bit sketchy. And in fact, what I'm about to propose to you, there's some evidence for it, but I'm not ready to take it to the bank yet, and it has to do with skin color.
To make it simple, and I've seen estimates anywhere between 3 and 5, we're going to talk about genes present. And I'm going to use a word now that is going to keep coming up, I'm feeding you a little bit at a time, gene loci or locus. Locus is Latin for place, and I almost said this word without explaining it to you. So we are going to suggest that there are three pairs of alleles or three loci that control skin color, and I'm going to make up letters. I'm going to say A, B, C, and here's the way I want to explain it in terms of expression. Let's look at these loci.
Let's make a person, so we're assuming a 3-locus model for skin color--Aa, Bb, Cc. Let's say the way this works in terms of expressivity is that the capital letters are genes that function and produce melanin, and the small letters do not function and produce melanin. So this particular person would be moderately skinned. And let's set them up here, introduce them to someone who is about the same skin shade. I have a question for you. If these people are fairly dark, moderately dark, can they have a very light-skinned child? Sure they can. Could they have something this light-skinned? That's pretty light. But of course they can.
Let's look at the numbers. What are the odds they're going to get an a from dad, 1 out of 2; an a from mom, 1 out of 2; 1 out of 2; 1 out of 2; 1 out of 2; 1 out of 2. 2 x 2 = 4 x 2 = 8 x 2 = 16 x 2 = 32 x 2 = 64. They have a 1 out of 64 chance of having a child that light. They also have other possible combinations, too, and we're not going to go into all of them because I have a nice chart to show this. But there are many different ways you could get the same skin color out of this couple. Let me show you.
So we have an Aa, Bb, Cc crossed with an Aa, Bb, Cc. Let's look at this. We could come up with a combination that looks something like this--AA, BB, CC, very dark, 1 out of 64. But look what else we could come up with. We could come up with this--AA, Bb, cc--phenotypically, the very same as their parents--3 dark genes, 3 nonfunctioning genes. This child will have the exact same phenotype as their parents, so they'll look the same skin color, but genotypically, they're going to be very different. So if you were to do some statistics and probability with this or a rather involved 64-square Punnett square, you would find out what I'm about to show you.
What I'm about to show you is this, why we use this to explain gradations. Because if you take a look at this, and we put this bar graph together, this histogram together, of sheer numbers of possibilities, and here are my parents--Aa, Bb, Cc together. Check the genetics. You have a 1/64^th chance out of getting a child very light and a 1/64^th chance of getting a child pretty dark. And then the numbers go up. So for example, with one unit of darkness, it's up to 6 out of 64, or with one unit of lightness, 6 out of 64. And so the thing goes on and on. And we're going to color this one in because it has to be dark. And what happens is, eventually you get up to the point where half of their genes are dark and half are light, and that's 20 out of 64. Remember, there are a lot of different combinations for that. I showed you the one where you had AA, Bb, cc. There are 20 out of 64 of those. And so this histogram ends up looking like this.
But one more question, why do we get gradation? Why isn't it just that we have 7 populations of skinned people, and everybody fits into this one? And that's where the effect of environment comes in because environmental factors such as how much you expose yourself to the sun, maybe nutrition, and a whole lot of other things are going to go into this to smooth this curve out and provide an actual gradation within there.
What's the lesson I want you to get out of this? The lesson I want you to get out of this is there are an awful lot of traits that are, indeed, polygenic, more than one gene locus controls the tree, and it makes the prediction of phenotype, I don't want to say impossible, but very difficult.
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Polygenic Inheritance Page [2 of 2]