Y chromosome is evolving faster than the X, primate study reveals

The Y chromosome in primates — including humans — is evolving much more rapidly than the X chromosome, new research on six primate species suggests.

For instance, humans and chimpanzees share upwards of 98% of their DNA across the whole of the genome, but just 14% to 27% of the DNA sequences on the human Y chromosome are shared with our closest living relatives. 

The finding surprised scientists, given that humans and chimpanzees diverged just 7 million years ago — a blip in evolutionary terms.

"I expect my genome to be very different to that of bacteria or insects because a lot of time has elapsed, evolutionarily speaking," study co-author Brandon Pickett, a postdoctoral fellow at the National Human Genome Research Institute (NHGRI) at the National Institutes of Health, told Live Science. "But from other primates, I expect it to be pretty similar."

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It's not clear exactly why the Y chromosome is evolving so rapidly. For starters there is only a single copy of the Y chromosome per cell — in primates, females carry two copies of the X chromosome, while males carry an X and a Y chromosome ––the Y chromosome plays a critical role in sperm production and fertility. Having only a single copy of the Y chromosome presents a vulnerability ––if changes happen to occur, there is no second chromosome to act as a backup. 

And changes are likely to occur due to something called mutation bias. The Y chromosome may be so prone to change because it generates many sperm. This requires lots of DNA replication. And every time DNA is copied, there's a chance for mistakes to creep in.