Chromosomes have sex!?!?
I’m guessing most people are aware that boy-humans and girl-humans have slightly different chromosomes. One pair, out of the 23 pairs that make up (in almost all cases) our genomes, are known as sex chromosomes; girls get two X’s and boys get an X and a Y.
So a Y chromosome makes you a guy? Yes and no. In fact, there is a single gene that flips the switch between male and female in developing humans (and most other mammals), known as SRY (sex-determining region Y). This puppy happens to be located on the short arm of the Y chromosome.
What’s the big deal? Why do we need a whole special chromosome if it all comes down to one gene? And why is it so much smaller than the X chromosome anyway?
The theory goes that once a sex-determining gene exists, the male form will tend to become associated with other genes that are more beneficial to males, and/or the female form will become associated with genes that are more beneficial to females.
Over time, the human (mammalian) Y chromosome has accumulated enough mutations that parts of it no longer match the X chromosome well enough to crossover and undergo recombination. This is where matching pairs of chromosomes swap bits of themselves around before being put into into eggs or sperm.
When chromosomes can’t recombine, all the alleles (that’s versions of a gene) on that chromosome are stuck with each other – they can’t make new combinations in the next generation.
The thing about genes? They mutate. And if the selection pressure’s off? They mutate all over the place. You *have* to have males right? (Actually… well, that’s next blog post.) Therefore, that male-version SRY *has* to be passed on. And all the other junk that goes with it gets passed on too. Because without recombination, you have no choice.
So the other genes on the Y-chromosome that SRY is stuck with? They let themseleves go a bit. They get messed up, or inactivated. Sometimes whole bits of chromosome get put in the wrong way round, or just plain deleted. Important genes? Pshaw! The X chromsome will take care of those.
(Sometimes it doesn’t, of course. That’s how you get X-linked diseases that only, or primarily, affect males, such as haemophilia and red-green colour-blindness.)
The upshot is that the human Y chromosome now only has 70-some genes, compared with over 1,000 on the X chromosome, and is only about a third the size. I’ll leave you to think about that…