 Okay, so here we are looking at some examples of sex-linked inheritance. First of all, as we've already implied, essentially, if you get a Y chromosome, you will, okay, I say this, if you get a Y chromosome and everything functions properly, you will be a boy. There is a, I'm going to draw you a picture of a Y chromosome. This is a Y. I'm going to draw you a picture of a gene on the Y chromosome called the S-R-Y gene. Now, this gene actually turns on the Y chromosome, the rest of the Y chromosome genes. So if, I'm going to write that down for you, the S-R-Y gene turns on the Y chromosome. And this happens at, I want to say, six weeks in utero, the S-R-Y gene in the Y chromosome. So six weeks when you're in your mama's belly, fellas, this specific gene on the Y chromosome turns on and begins activating the rest of the genes on the Y chromosome. As they get activated, again, all they're doing is producing proteins. And those proteins have consequences. So as they get activated and they, essentially, cause the testes, the gonads, to descend down into the scrotum. The gonads at six weeks in utero, if there isn't a Y chromosome around, those gonads will stay in the abdominal cavity and will basically be ovaries. So the default is to have ovaries. But if you have a Y chromosome with a functioning S-R-Y gene, you will become male. Now, the reason why I said this doesn't always work this way is because you can have a Y chromosome and be genetically male, X, Y. But if you have a wackadusical S-R-Y gene that isn't working, then your Y chromosome will never get turned on, which means that you will not, your testes, your gonads, will not lower. They'll stay, they'll become ovaries. You'll basically look like a female. In fact, the number of supermodels, super skinny, super tall, very boyish figures, but drop dead gorgeous, especially by our societal standards, those, not always, but there is a higher percentage of supermodels with a messed up S-R-Y gene. They're actually men. They're genetically male, but phenotypically female. And that's because they're missing that S-R-Y gene. How cool is that? If we have a trait that is say carried on the X chromosome, I'm going to show you what we're going to talk about, something like this. So we would have, let's say, colorblindness is a condition carried on the X chromosome. So colorblindness, I'm going to do it like this. We'll make it a B. Big X, okay, hold on a second. This is representing my X chromosome. This is representing the allele, the colorblind allele on the X chromosome. Do you see what I just did here? This is my Y chromosome. Do I have a colorblind allele on the Y chromosome? No, because the colorblind allele is only found on the X chromosome. So there's nothing on my Y chromosome. So let's say I have a woman who is not colorblind. This is her phenotype. What's her genotype? What do you know? We know, first of all, that she's XX and she's female. So what are the possibilities? Well, would you say this is a dominant or a recessive allele for colorblindness? She's not colorblind. So she has to have at least one dominant allele in that colorblindness gene, which means that she'll be able to see color. She could be heterozygous. She could be a carrier of the colorblind allele. Before she could be homozygous dominant and both situations leave her not colorblind at all. If she's a carrier of the condition but she doesn't express it, she's fine. She's not going to be colorblind at all. Now let's look at a fella. A fella, let's look at a male not colorblind. What is his genotype going to be? So his phenotype is not colorblind. If he's not colorblind, he has to have a big B but he also has to have a Y. Do you get that? So now watch what happens. One little B. Just like her, what's his phenotype now? Now he's colorblind. Do you get that? He's colorblind now because he doesn't have a big B allele to cover up, he can't be heterozygous. I think they call it hemizygous when you have, you actually don't have two copies of the gene in the first place, which this is the only opportunity to be hemizygous if you're a dude with your X chromosome. What this means is that when we do problems with inheritance of a sex-linked condition, our phenotypic ratios are going to be different than what you would expect if we were just doing straight up Mendelian inheritance. And the reason why they're different is because boys are more likely to express these sex-linked traits. So let's do a sample problem and we'll make sure that you understand what the heck I'm talking about.