Hello, and welcome back to Introduction to Genetics and Evolution. In the previous video we talked about basic single-gene inheritance, or basic Mendelian inheritance. It's called this because this was elaborated first by Austrian monk Gregor Mendel. We saw how you get single copies of genes or alleles from one parent and then they combine with single copies of genes or alleles from another parent. In this video we'll talk about a special case of inheritance that of X-linked inheritance. What happens when genes are on the X chromosome, some of you may have heard of. The other thing we'll talk about is following what happens with multiple genes. This is something we'll doing quite a bit later in the course. Where we will begin at now looking at the principle of independent assortment. This is something that was also elaborated by Austrian Monk Gregor Mendel. Now some chromosomes have slightly different patterns of inheritance from what I described previously. In terms of the single copy from males, single copy from females going to offspring. And this was first hypothesized partially because there are some hereditary diseases that pop up much more frequently in males than in females. And this is true even when both sexes are likely to get the disease. You just see a lot more men having it than women having it. There are some cases of muscular dystrophy, for example, which fit this description. Now Drosophila geneticist Thomas Hunt Morgan studied this around the turn of the 20th century. He was working with fruit flies, and specifically working with a white-eyed mutant fly. He found that inheritance was sometimes different depending on the sex of individual. If you tracked what was happening with particular genes in males versus in females you're getting completely different pattern. He inferred from this the principle of sex linkage or X-linkage. We now know that there are some genes that are actually not present in two copies in all of them. That quite often you'll find that, in humans, males may have one copy of a gene, and females may actually have two copies. And these are genes that are on what's referred to as the X chromosome. So often across species, males are XY. So they have one copy of the X chromosome, and instead of having a second copy have this other chromosome called Y. Whereas females have two copies, so females are called XX. You've probably heard this one. Now, there's not the same set of genes on the X as on the Y despite the fact these two chromosomes pair and separate. You'll see the principle using them is very similar to what we discussed before. Except that the X has one set of genes and the Y often has a different set and in fact, often has very few functions. In humans, one of the few genes on the Y chromosome is a gene which basically makes the embryo become a male. Now again, in this case, you get different patterns of inheritance depending on who's the mom, who's the dad. And whether you're looking at male or female offspring from them. Now I mentioned muscular dystrophy. Another example is green color blindness. Now this is an X-linked recessive in humans. Meaning that if you have, if you're a female and you have two copies of this particular mutation then you will be colorblind. Males only get one because it is on the X chromosome, so if they have the one copy, they are colorblind. This picture on the screen depicts what somebody with green color blindness may actually see when you're looking at the pictures. So on the, in the picture on the left, it has the number 74 very clearly visible. In the picture on the right, if you're green color blind, you actually won't see the 74, in fact, you may see a different number in there. Now inheritance of genes on the X chromosome can be studied very similarly to what we described. And again, using the Punnet square, just like we did before. And what happens in this case is you just insert a Y instead of the second allele that would go into males. So let's try this now. Let's imagine that you're studying red-green colorblindness, something that I just mentioned is something that's inherited on the X chromosome. Let's say hypothetically that you have a male which is red green color blind. So here he is. Here's his X chromosome and I'll put a little rcb on there for red green color blind. For the female, let's say that she is just normal, she does not have this. She just has two regular Xs. So we can follow what happens in this case just like we did before. So again, the male is equally likely to either give this X, which has this mutation, or the Y. I've already drawn in the Ys there for us, so here, let's put this in. Here's the male's X rcb, X rcb, okay? The female will always give her just regular X, regular X, regular X, regular X. Well what do we see here? The male offspring are the two on the bottom because those are the ones that have the Y chromosome. You don't become a male unless you have a Y chromosome. So this is true in humans, it's not true in all species. So looking at these individuals on the bottom, would they be red-green colorblind or no? So look at it carefully. There is no red-green colorblind mutation on them, right? So the two males here would be just fine, write the word fine. What about the females? Well the females do get the red-green colorblind mutation, but I mentioned it's recessive. So they have one normal copy and one sort of broken copy. So in this case, again, the females would also be [INAUDIBLE]. Well that was interesting. Well, let's try something a little bit different. Let's switch it around. This time, let's say the mom was red-green colorblind, and the dad had perfect vision. Okay, so, this case here is dad, he's got just a regular X. Mom's got X with a rcb mutation. X with a rcb mutation. So let's follow patterns of inheritance. Again, the dad's always giving a Y, in this case, he's giving an X. And mom is always giving an X rcb. So now what do we see? Well let's look first at the girls. So the girls are the two X's. So they have one normal X chromosome and one with the mutation for red-green colorblind. So again, the daughters in this case would be fine. So this is similar to what we saw last time. However, look at the boys. The only X chromosome the boys have has this mutation. So in fact the boys are actually red-green colorblind. So we see a different pattern of inheritance in this case depending on whether your looking at the daughters or the sons. And depending on who is the mom and who is the dad. So this illustrates some principles of X-linked. Now let's look a little bit more broadly at the gene. Now everything we've focused on so far has been looking at single genes in isolation. But the genome's a very big place. A human genome for example has on the order of 23,000 protein coding genes. These are genes that actually make protein. It has many other important things in there. There are other things that affects how much a protein are made, etc, these are called regulatory regions, etc. Now, again, we have 23 pars of chromosomes, one pair being that X and Y. The other pairs being ones that follow just regular Mendelian inheritance, as we described before. Interestingly, you see all these pairs of 23 chromosomes in the picture on the bottom left here, they're actually all inherited independently. So, let's say, for example, I have a particular chromosome two from my dad, I have a particular chromosome two from my mom. I have a particular chromosome three from my dad, I have a particular chromosome three from my mom. If I give to my daughter, my dad's chromosome two, there is no way that you can know which copy of my chromosome three I'm may be giving. I'm equally likely to give my dad's or my moms chromosome three. And basically what's happening with the chromosome is independent of what's happening on these other chromosomes. You're not more likely to give both your dad's copies or both your mom's copies when you're looking at different chromosomes. This is referred to as Mendel's Law of Independent Assortment. So what happens when you're studying two traits that are inherited from genes are different chromosome. Well, let's look at two traits. In this case I'm looking at two that show patterns similar to being single-gene inheritance. They're not actually inherited through single genes. One of them is what happens when you put your hands together like this. Watch. Put your hands together like that and see which thumb is on top. In my case my right thumb is on top, for a lot of people the left thumb is on top. For a long time people though this was inherited as single gene. That actually probably isn't' true, but just pretend that's the case for right now. If you study the inheritance of this, it looks like the left one on top one is dominant, the right thumb on top is recessive. The other trait we'll look at, is again dealing with thumbs because they're easy to show. It's straight thumb, which looks like this. Or hitchhiker's thumb, mine is definitely hitchhiker, it comes really far back. Well, it's thought that straight thumb is dominant, so I have both recessive in this case, whereas the hitchhiker's thumb is recessive. So, let's say you were following these two traits together, let's say you're following inheritance of them. Let's do a hypothetical example. Let's say that you have a dad who has left thumb on top and straight thumb. Having kids with a mom that has right thumb on top and hitchhiker. So let's call them just using these letters right here to make it a little easier to follow, big T big T, big S big S crossed with little t, little t, little s, little s. In this case this is referring to straight versus hitchhiker for the S and left versus right where capital T is a left. So let's follow what happened. Well the offspring will obviously get one big T, one little t, one big S, one little s, right? Because there's no alternative. So here we go, here is the kids. Big T little T, Big S little S. And let's say the kids marry each other and have kids. That's a little gross, I'm sorry but let's pretend this just for a moment. What will we see in their offspring? Well, the kids can give different alleles right? This boy right here could give his big T or he could give his little t. He could give his big S or could give his little s. And the same thing's happening with the girl here. So it's a little bit more complicated then what's going to happen. These T and S alleles are going to be inherited independently from the parents. But again if you give the big T, you are not more or less likely to get the big S or the little s. So, I showed here on the picture for possible gametes, big T, big S, big T, little s, little t, big S and little t with little s. Those are the four possible gametes you can get. Now there's two ways you can try to predict what the offspring would be like. The better way which I'll show you second is to actually multiply probabilities. We'll come back to that in just a moment. The other thing you can do which is maybe more intuitive but definitely more laborious is to follow what's happening with all the gamete. Workout all sixteen possibilities. So let's do that one first. I won't do the whole thing, I'll just show you how it would go. So again, here's the thing. We can fill this in, remember all four gametes we have are big T big S, big T little s, little t big S, little T little s, so that's true for one of these. Let's do the other one. Big T little s, little t big S, and you can follow what's going to happen with each of these things. This case mom will always be giving a big T, always be giving a big S. Dad will be giving a big T, he'll be giving a big S. So there's one individual. This is gonna take forever if I do this so let me just fill in what all the answers would be. Boom! [LAUGH] So here is all the possible outcomes there. And I have here up in the upper left corner a little cheat sheet. So you can see what, what happens in terms of what they would look like or their genotype. But if you have a big T you got your left thumb top. If you have a big S then you're straight rather than checkered. So let's look at the ratio here. How many individuals would be left and straight? Remember, left and straight are both dominant. Let's count them. This one, this one, this one, this one, this one, This one. Am I missing some? This one. [INAUDIBLE] there's one. There's one. So one, two, three, four, five, six, seven, eight, nine. So we have nine that are dominant for both. How many would be right straight? Right thumb on top, straight. So in this case, right thumb on top straight would be little t but big S. Little t but big S would be this one, this one. There, those three, okay? So you have three that are dominant for one, recessive for the other. The opposite would also be three. This would be what happened if you're left thumb on top, but hitchhiker. That would be these three over here, I'll put X in this time. I misspoke sorry, this would be left thumb on top and hitchhiker The other one was right thumb on top and straight. So again, three for when you have one dominant and one recessive and finally one that's the double recessive. So the ratio you get is 9 to 3 to 3 to 1. So again, there's 16 possible outcomes here and this is the ratio we expected. That was laborious. [LAUGH] So let's see how we could do this a little bit more straightforward. Well there's a very simple thing you could use for basic probability. That if you have two independent events, all you have to do is to multiply the probabilities with for the joint probability. Just like when you're flipping a coin. What are the odds you flip a coin and get heads twice? Well, the odds of you flipping a coin and getting heads the first time is one half, odds of you flipping it a second time and getting heads is also one half. So it'd be one quarter. We can apply this same principal to this problem. Here is all the possible outcomes when you are looking at just one gene in isolation. So we have here, this what we would see if we were just looking at what's happening with the T gene. The one that controls left versus right thumb on top. Using the four possibilities when you are looking at hitch-hiker versus straight thumb. So, all you have to do is just multiply the probabilities to get these different genotypes. So we say big T big T, big S, big S. That would be one sixteenth. Big T, big T, big S, little s, that will be one quarter times one-half. Half, quarters, this would be this case one-eighth. And we can fill in this whole thing. There we go. We filled them all in. Now, if we do the same thing, let's add which ones will be left thumb on top straight. Well, we can just add them up, let's do it by 16ths so it's a little bit clear. So there's one here, this'll be plus another two, cuz one-eight is two-sixteenths, right? So this would be three. So one plus two is three. Plus two is five. In this case plus four is nine. So again we have 9/16 have left thumbs straight. So again if you do this whole thing it becomes 9/16, 3/16, 3/16 and the last one over here being the 1/16. We get that same overall ratio and this time we can do it just by multiplying probability. So let me give you one to try on your own and then I'll give you a problem at the end of it. So again, assume independent assortment between A and B genes. What will the genotypes and proportions be of the offspring if you're looking at big A, little a, big B, little b? So this is the double-header zygote again. Crossed with big A, little a, little b little b? It's a little bit different in this case. So what I suggest you do is to multiply the probabilities. So look at what's happening just with the A gene, and look at what's happening just with the B gene. And multiply those probabilities together. Let me give you a moment to try this out and solve this problem.