 By this point in the biology playlist, you're probably wondering a very natural question. How is gender determined in an organism? And it's not an obvious answer, because throughout the animal kingdom, it's actually determined in different ways. In some creatures, especially some types of reptiles, it's environmental, not all reptiles, but certain cases of it. It could be the temperature in which the embryo develops will dictate whether it turns into a male or a female or other environmental factors. And in other types of animals, especially mammals, of which we are one example, it's a genetic basis. So your next question is, hey, Sal, let me write this down. In mammals, it's genetic. So you're like, hey, maybe there are different alleles, a male or a female allele. But then you're like, hey, there are so many different characteristics that differentiate a man from a woman. Maybe it would have to be a whole set of genes that have to work together. And to some degree, your second answer would be more correct. So let me just draw even more than just a set of genes. It's actually whole chromosomes determine it. So let me draw a nucleus. That's going to be my nucleus. And this is going to be the nucleus for a man. So 22 of the pairs of chromosomes are just regular non-sex-determining chromosomes. So I could just do, you know. That's one of the homologous 2, 4, 6, 8, 10, 12, 14. I could just keep going. And you eventually have 22 pairs. So these 22 pairs right there, they're called autosomal. And those are just our standard pairs of chromosomes that code for different things. Each of these right here is a homologous pair, which we learned before. You get one from each of your parents. They don't necessarily code for the same thing, for the same versions of the genes. But they code for the same genes. If eye color is on this gene, it's also on that gene. The other gene, the homologous pair, you might have different versions of eye color on either one. That determines what you display. But these are just kind of the standard genes that have nothing to do with our gender. And then you have these two other special chromosomes. I'll do this one. It'll be a long brown one. And then I'll do a short blue one. And the first thing you'll notice is that they don't look homologous. How could they code for the same thing when the blue one's short and the brown one's long? And that's true. They aren't homologous. And these we'll call our sex-determining chromosomes. And the long one right here, it's been the convention, is to call that the X chromosome. Let me scroll down a little bit. And the blue one right there, for that as the Y chromosome. And to figure out whether something is a male or a female, it's a pretty simple system. If you got a Y chromosome, you are a male. So let me write that down. So this nucleus that I drew just here, obviously you could have the whole broader cell all around here. This is the nucleus for a man. So if you have an X chromosome, and we'll talk about it in a second why you can only get that from your mom, an X chromosome from your mom, and a Y chromosome from your dad, you will be a male. If you get an X chromosome from your mom and an X chromosome from your dad, you're going to be a female. And so we can actually even draw a punnett square. There's almost a trivially easy punnett square. But it kind of shows what all the different possibilities are. So let's say this is your mom's genotype for her sex determining chromosomes. She's got two X's. That's what makes her your mom and not your dad. And then your dad has an X and a Y. And a Y chromosome. I should do a capital. It has a Y chromosome. We could do a punnett square. What are all the different combinations of offspring? Well, your mom could give this X chromosome in conjunction with this X chromosome from your dad. This would produce a female. Your mom could give this other X chromosome with that X chromosome. That would be a female as well. Or your mom's always going to be donating an X chromosome. And then your dad is going to donate either the X or the Y. So in this case, it'll be the Y chromosome. So these would be female, and those would be male. And it works out nicely that half are female and half are male. But a very interesting and somewhat ironic fact might pop out of you when you see this. What determines whether someone is or who determines whether their offspring are male or female? Is it the mom or the dad? Well, the mom always donates an X chromosome. So in no way does the mom what the haploid genetic make up of the mom's egg of the gamete from the female. In no way does that determine the gender of the offspring. It's all determined by whether, let's say these are all of, let me just draw a bunch of, dad's got a lot of sperm. And they're all racing towards the egg. And some of them have an X chromosome in them. And some of them have a Y chromosome in them. And obviously they have others. And obviously the one, if this guy up here wins the race, or maybe I should say this girl, if she wins the race, then the fertilized egg will develop into a female. If this sperm wins the race, then the fertilized egg will develop into a male. So the reason why I said it's ironic is throughout history, and probably the most famous example of this is Henry VIII. You have these, it's not just the case with kings, it's probably true because most of our civilization is male dominated. That you've had these men who are obsessed with producing a male heir to kind of take over the family name. And in the case of Henry VIII, take over a country. And they become very disappointed. And they tend to blame their wives when the wives keep producing females. But it's all their fault. Henry VIII, I mean the most famous case, was with Ann Bullen, I'm not an expert here, but the general notion is that look, she became upset with her that she wasn't producing a male heir, and then he found a reason to get her essentially decapitated. Even though it was all his fault, his fault, he was maybe producing a lot more sperm that looked like that than was looking like this. I'm going to hijack this video now because I want to clarify one statement. It's not entirely right to think that Henry might have made more of these sperms and as a result he ended up with a girl. As far as we know, the number of sperms having x and y are pretty much equal. So then what decides whether this one fertilizes the egg or this one fertilizes the egg? It's pure chance. It's a 50, 50% chance. If this one goes ahead and fertilizes, you get a girl, which might have happened in his case. And if this one goes and fertilizes, it's gonna be a boy, but it's a purely 50, 50% chance. And before we wind up, just to clarify this statement, imagine a scenario in which a couple have five daughters in a row, all right? Let's say they get five daughters in a row. Now my question is, do you think for their sixth offspring, do you think that there's a higher chance they get a baby boy now? Well, in such case, I might have a tendency to think that in the first five tries, they got a girl, so maybe in the sixth try, there's a higher chance of getting a boy and maybe a lower chance of getting a girl because they already got girls for the first five times, right? But this is wrong way to think about it. The history doesn't matter. Every time, you know, a fertilization happens, it's gonna be 50, 50%. That means even for the sixth time, there's a 50, 50% chance of getting a boy or a girl. And this would be true even if they had hundred daughters. I know it sounds crazy, but even if they had hundred daughters, then for the 111 time, again, it's a 50, 50% chance whether they have a girl or a boy.