Why, O Y? (Image: Alfred Pasieka/Getty)
The Y chromosome, which makes men male, has been shrinking for 180 million years. But there’s more to this rotting husk than anyone suspected
THERE’S nothing very macho about the Y chromosome. Even though it’s what makes men male, the human Y, like its counterparts in almost all mammals, is tiny compared with its partner, the X chromosome. It’s lost hundreds of genes – and if the Y continues to lose them, it could someday wink out of existence entirely.
Claims of its impending demise are starting to look premature, however. Far from being a rotting husk, the modern Y, tiny though it is, is turning out to be a highly evolved and surprisingly important part of men’s wider genetic endowment, responsible for far more than just maleness.
It is easy to see why some biologists thought the Y was destined for oblivion: it is all on its own. There are two copies of all other chromosomes, which are basically containers for holding genes. Each copy acts as a backup for the other. The pairs line up and swap bits when organisms reproduce. Some offspring get landed with chromosomes full of damaged genes and are eliminated by natural selection, whereas others inherit undamaged copies and survive to reproduce.
Way back in the evolutionary past, there was no Y, just a regular pair of chromosomes. Sex was determined by environmental factors such as temperature. But then a gene on a single chromosome mutated in a way that made any individual that inherited it male. At first this proto-Y could still swap genes with its partner, the…
