Carried only by males, Y chromosomes are by far the smallest of all the human chromosomes. In fact, they carry fewer than 200 genes. In contrast, the X chromosome, which is carried by both sexes, encodes about 2000 genes. And it’s not even the largest chromosome. That honor is held by human chromosome 1, thought to encode over 4200 genes.
The Y chromosome wasn’t always so diminutive. 300 million years ago, the X and Y were a matched pair containing the exact same genes, just like all the other autosomes (non-sex chromosomes). In the case of the autosomes, that identicalness is maintained by exchanging genetic material (crossing over) between the pairs. 300 million years ago, a section of the Y stopped crossing over with the X. That unpaired section of the Y no longer had any evolutionary constraints on it, and many genes were lost from that region. In the intervening years, four more sections of the Y stopped matching up with the X, most recently 30 million years ago, with each event resulting in the loss of genes. Many scientists believed that the Y was still shedding genes and that in the far future, human males would no longer have a Y chromosome at all.
Not so, according to a new study led by Jennifer Hughes and David Page of MIT. They sequenced the Y chromosomes of both humans and rhesus macaque monkeys, a creature with which we share a common ancestor 25 million years ago. Since that divergence point, rhesus Y chromosomes have not lost a single gene, and human Y chromosomes have lost only one gene. More significantly, the lost gene was from the most recent region of the Y to have stopped crossing over with the X. In other words, the parts of the Y that haven’t matched up with the X for hundreds of millions of years were completely stable.
This strongly suggests that the Y is here to stay, which is good news for men and genealogists.