Why are tortoishell cats mostly female? Christmas Lectures 1984 with Walter Bodmer #shorts #science

She's a patchwork, isn't she? She's a patchwork of black and yellow. And her parents were probably uh I don't know, maybe one of them was black and one was slightly brownish. She's a really lovely cat. I'd better give her back to you. She's getting a bit worried here. There's a patchwork there of both types of colors of fur. And you can have patchwork mice, too, Bill. Let's have a look at these patchwork mice. Look here. We've got some mice here. And here you see the uh one of the parents is black, right? The other is white. And here we've got the offspring. Look, do you see that? A patchwork. Can you see that? Both of them. Both of them are a patchwork of black and white fur. They're a patchwork just like the cat, the tortois shell cat. The tortois shell cats are female. And these are females. They're always patchwork females, not patchwork males. Now, why do you think that is? The reason for that is in the sex chromosomes again, the X and the Y. Remember, two X's in the female, one X and a Y in the male. And you see the Y is a little chromosome. It only determines sex basically, whether you're a male. There isn't, as there is for the other chromosomes, which occur in pairs like that, a matching set of genes in the male for the X's. there's only one set of X genes, not two sets of X genes. Now, nature doesn't like that sort of imbalance. Nature tries to arrange it so that the number of active X chromosomes, the number that are at work in every cell is the same. There's only one in the males, what happens in the females is either one is at work or the other. Never both chromosomes at work. So if one of those chromosomes determines the black color, the X chromosomes, and the other X chromosome determines the white color, you get this patchwork. Some of have one of the X's at work and some have the other of the X chromosomes at work. And that's why you get a patchwork in the females and not in the males.