 Now, as I think I said, I think I said before, we have 22 pairs of autosomes and one pair of sex chromosomes. Nobody knows what that says, but do you accept that that says chromosome? Thank you very much for being patient with me. All right, our autosomes Everybody has them. That's cool. Does it matter what your anatomical sex is or your chromosomal sex is? Everybody has them, but the sex chromosomes, we've got two flavors. We have an X chromosome and we have a Y chromosome. Everybody has an X, at least one X, and super interesting. You get your X from your mom. Everybody gets their X chromosome from their mama. Mamas are generous. They give you all their mitochondria, too. I mean, you get all your mitochondria from them. They didn't give you all their mitochondria because that would be bad, but that might explain why I'm so tired. I don't know. Two possible sex chromosomes. Anatomically or chromosomally female humans have XX and chromosomally male humans have XY. And I, holy cow, there are exceptions all over this place. So, and again, this is anatomy. Now, the Y chromosome is actually going to it's small. In fact, it has the Y chromosome has about 55 genes and let's contrast that to the X chromosome, which has like 900 genes. You can actually see down here. I've got an X chromosome there, and look at the little itty-bitty Y chromosome. Y chromosome is significantly smaller than the X chromosome. If anatomically female humans have two X chromosomes and anatomically male humans have just one and one Y chromosome. Anatomically male humans have less DNA than anatomically female humans. Interestingly, I'm just going to say this out loud. One X gets shut. That said, I won't say it out loud what I said. It gets shut down. One of those X's gets shut down. It gets shut down because to the body sort of manages the fact that X X humans have more DNA. So, one of those gets kind of like quieted, which is fine. On the Y chromosome are genes. In fact, there's a gene called the SRY gene. And this gene, it's on the Y chromosome and it turns on the Y chromosome. In utero at like six weeks. So like a six-week-old embryo, which is like this little tiny fish-looking thing, not even fit, like it's a little... I should show you a picture. That thing, if the SRY gene is present on the Y chromosome, then it turns on the Y chromosome and all of a sudden you get a plethora of gene activation that starts the embryo developing into an anatomically male critter. If that SRY gene isn't there, even if you have the Y chromosome, the critter, the human, develops into an anatomically female human with some modifications because they're actually missing their second X chromosome. But they've got one X chromosome and they don't have the stuff on the Y chromosome that actually turns on maleness. Come to Anatomy with me and you will learn all about the development of the reproductive system at that six-week mark in utero. All the embryos are the same regardless of their chromosomes. And then the anatomy that SRY gene turns things on and off so that different structures develop differently. For example, the scrotum in an anatomically male human is actually just labia majora, like zipped up. Testes are ovaries that have just been drawn down into that zipped up labia majora. It's actually super interesting and really cool and points to how vastly similar we are and how we really are on this lovely little spectrum. But there are things about heredity that come into play because XY humans only have one copy of the X chromosome, while as XX humans have two. So we're going to look at some of the sex-linked patterns that happen because of this difference in chromosome number and character.