 We're going to start out by proving to ourselves that the chromosomes cause chromosomal sex and that the inheritance pattern for those chromosomes is actually the same. So let's start with, let's run through our little Punnett square rules. Like what were the things? I'm just going to say we're going to make babies between an anatomical male and an anatomical female. Make babies. Okay. Do I have to write all that down? I don't, right? We just know that I told you in words that we're dealing with anatomical male and female humans and they're going to make some babies. Maybe we should ask a question like what are the anatomical sexes? How much, what are the possible sexes of the babies that they're going to make? All right. That sounds like a good plan. So let's do our four steps to do a Punnett square. What are all the possibilities? First of all, we have to do phenotype for the parents. We have an anatomically male and an anatomically female. Do you agree with that? What's our phenotype if we're just looking at sex? You know, like what your sex is, your anatomical sex, you know what I'm talking about? What was the next thing we had to look at? We had to look at genotyp, what? Genotype, parental genotype. So around anatomical sex, chromosomal sex, that works. What is the male's genotype? Well, we got to keep track of Xs and Ys. And look at what we're doing now. That doesn't represent alleles. That doesn't represent a gene. That represents an entire chromosome. This is going to be important for our bookkeeping. A female is going to have what's her genotype going to be, or their genotype. That's gendering it. XX. An anatomically female human has XX chromosomes in our example. Now what was the next step in this process? We want to know what the possible gametes are. Are we keeping track of any genes yet? No. We're keeping track of entire chromosomes, but the process is the same because, look, anatomically male humans and their sperm, half the sperm are going to get next chromosome and the other half are going to get a Y chromosome. Do you follow? Like, yes, right? What about anatomically females? And I'm just going to show you right now that's how huge eggs are. They're gigantic compared to the sperm. Like probably 10 times bigger than that compared to the sperm, but I can't draw them that big because I don't have enough room. But all the babies are going to get an X chromosome from their egg parent. They might get an X chromosome and they might get a Y chromosome from their sperm parent. Okay. So far so good. Shall we do a Punnett square? Possible offspring? Now, wasn't our question like what percentage of boys and girls, XXs and XYs, can we get in from these two parents? Was that our question that we asked? Are you seeing the answer here? We only have an X option from the egg. We have two, an X and a Y option from the sperm. And if this sperm, I can't help it, I have to draw the sperm tail just so we keep track that those are sperms, yo. That baby's going to have two X chromosomes, which means it's going to be anatomically female. That tells us nothing about that baby's gender. This baby is going to be XY, right? It got the Y sperm and that means that it's going to be anatomically male. Also tells us nothing about, doesn't tell us anything about anything except what chromosomes it has. Now, do you agree? 50%, there's a 50% chance of having an XY baby and a 50% chance of having an XX baby. Not a 55% chance. Who knows? Done. That's it. That's all there is to it. And it's that simple now. What if there's a condition on the chromosome? What if there's something that we want to keep track of on that chromosome? For example, color blindness. Color blindness is a recessive trait carried on the X chromosome. So I'm going to give you a problem. I don't have any more paper. What? Maybe I'll just do that. What happens? We'll just see what happens here. Let's do a problem just for the fun of it. Let's say we have a colorblind male making babies with a, let's say, carrier. Female. Okay. And this is awesome because we're just rolling to do our example here. What I've just given you is the phenotype. Do you agree with that? Yeah. I told you that the male is colorblind and the female is a carrier. I told you before that color blindness is recessive and it's carried on the X chromosome. You can tell me. What is the genotype of the colorblind male? Well, take a look, my friends. We know he has an X and a Y chromosome. We know that the colorblindness allele is carried on that X chromosome and he's colorblind. So I'm going to say, I'm going to use a little B for my colorblind allele. Again, this is so random, like who knows whatever. You can use whatever letters you want. That's his genotype. He's colorblind, so he has to have the allele. The Y chromosome doesn't have anything to do with colorblindness. So he has one copy of the colorblind allele and he has it. Now, I told you that she is a carrier. First of all, we know that she has two X chromosomes, so she's going to have automatically two alleles for colorblindness. She's a carrier. That right there, those words, those are fatten words, folks. That means they're heterozygous, which means they have one dominant allele and one recessive allele. So her genotype is X big B, X little B. Do you see why it matters? If this were not sex-linked or linked onto the X chromosome, then we would, all we would do is put, I mean, our colorblind person would have to have two copies of the colorblind allele. That's not the case here. Now, also, this messes with our gametes that we're about to form. Let's make some gametes, y'all. We have X little B gametes and we have y gametes. If you get a y, no information from the sperm parent about colorblindness. I'm not going to make my eggs gigantic. But look, it's so easy to figure out what my possible gametes are, especially if you put the circle around them to remind yourself that, oh, they're haploid. I can only have one copy of each chromosome, each gene, in the gamete. And then our last step is just to throw these guys. Now we have four options, right? So let's throw these in there. We've got X little B, y on one side. Those are my gametes possible. And then X big B, X little B, that's my possible gametes on the other side. Oh, what? You can't see that. Oh, geez. Oh, geez. I have to take this to another page, y'all. We're doing it anyway, though. Look at me go. Paste. Okay, we have one little sperm tail in there, so just ignore my sperm tail. Okay. Now, what are the possible babies? Well, this here, baby, tell me about the phenotype of that baby. Anatomically female, chromosomally female, heterozygous for color blindness, a carrier. This is a carrier female. Tell me about this little baby. So anatomically female, colorblind. Two recessive alleles, that is a colorblind female. That is a not colorblind male, and we have a colorblind male. Half of our females are colorblind, half of our males are colorblind. When we do problems like this and I say, look, there's half of the females are this and half the males are that. Does that mean that if these parents had like 20 kids, 10 would be colorblind and 10 wouldn't be? No, this is the probability. And as you know, you can flip a coin and you could flip heads 10 times in a row. You could end up with 10 colorblind boy babies. It's not very likely, but it is possible. These are just probabilities. They do not, they're not predictions or like, they're just probabilities, like what you would expect. Okay, whatever I just said. How do you feel about that? Okay, we're going to do another problem that is related to a sex-linked pattern in fruit flies. So I have some fruit flies coming at you in a second.