[SciShow intro plays]

Michael: What makes boys and girls different? It can be a really complicated question, but when it comes down to biological sex in humans, the difference is in the Y chromosome. We know that the Y chromosome is important but scientists also know that it's been shrinking in size over millions of years, and some even think that it might eventually disappear. But recent studies on the Y chromosome in a lot of mammals suggest that it has more important genes, besides the ones that contribute to "male-ness."

Chromosomes are bundles of DNA and other stuff that are stored in the nucleus of your cells. Most people have 23 pairs of chromosomes, one set from mom, and one set from dad. 22 of these pairs look the same in all humans, but the last pair is known as the sex chromosomes: X and Y.

Generally, biological females have two X chromosomes, and biological males have one X and one Y. The Y chromosome is less than half the size of the X, and it contains way fewer functional genes. But it wasn't always this way. Hundreds of millions of years ago, the X and Y chromosomes of our human ancestors were the same size and they could swap genetic information with each other through a process called "genetic recombination." Recombination helps us evolve over time and protects against potentially dangerous mutations in our genes. But at some point, the X and Y chromosomes became so different that they lost most of their ability to recombine. So any mutations that changed or deleted part of the Y chromosome couldn't really be fixed, and it kept shrinking over millions of generations. Today, in humans, the Y chromosome that men are carrying around with them has only 3% of the functional genes it once did.

But fear not, Y-havers! Many researchers think that this decay seems to have leveled off in most mammals including humans, over the past 25 million years or so. For example, two studies in 2014, one by MIT researchers and one by Swiss scientists, analyzed Y chromosomes across 8-15 mammalian species. As expected, they found some of the same sex-determining genes, like the SRY gene, which helps develop all those male reproductive bits, but they also found genes conserved across Y chromosomes in many species that didn't have to do with maleness.

These are regulatory genes, genes that regulate the expression of other genes by basically turning them on and off. While most genes on sex chromosomes only need one copy to function normally, scientists think some important regulatory genes require pairs. So in females, each X chromosome has one half of a pair of regulatory genes and in males, there's one gene on the X and the paired one is on the Y. So the scientists think that a lot of Y chromosome genes work with the genes on the X chromosome to help regulate other genes that shape healthy human development.

But lots more research needs to be done to figure out which pairs of genes affect which parts of development. No matter what though, it seems that the Y chromosome isn't just about creating "male-ness" after all and it's probably not going anywhere anytime soon.

Thanks for asking, and thanks to all of our Patreon patrons who keep these answers coming. If you'd like to submit a question to be answered, just go to patreon.com/scishow and don't forget to youtube.com/scishow and subscribe!