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The Strange Case of the Y Chromosome Vanishing

Humans and other mammal babies’ genders are decided by the Y chromosome. However, the Y chromosome seems to be degenerating and could disappear in a few million years.

The bad news is that this means the possible end of humanity as we know it. The good news is that there are two branches of rodents that have already lost the Y chromosome and still survive.

The spiny rat has evolved a new male-determining gene that is not Y.

Usually, in humans, females have two X chromosomes, and males have a single X and Y chromosome. Names do not have anything to do with the shape, as X stands for “unknown.”

X has around 900 genes that do jobs unrelated to sex, but Y contains around 55 genes and many non-coding DNA which are repetitive and don’t seem to do anything except for the gene that kick-starts the male development in the embryo at around 12 weeks after conception. This regulates the development of a testis, and the embryonic testis makes male hormones (testosterone and derivatives) ensuring that the baby develops as a boy.

This master sex gene was identified as SRY (sex region on the Y) in 1990. It works by triggering a genetic pathway starting with a gene called SOX9 which is key for male determination in all vertebrates, although it does not lie on sex chromosomes. The problem occurs as X has more genes than Y with SRY plus a few others.

Australia’s platypus has completely different sex chromosomes, which are more like those of birds, and in platypus, the XY pair is just an ordinary chromosome, with two equal members. This suggests that X and Y were an ordinary pair of chromosomes not that long ago.

For whatever reason, the Y chromosome lost significant active genes over the 166 million years that humans and platypus have been evolving separately, which is a loss of about five genes per million years. The other 55 genes will be gone in about 11 million years. The estimated expected lifetime of the Y chromosome is under dispute.

Thankfully, we know two rodent lineages that have lost the Y chromosome and are still surviving. The mole voles of Eastern Europe and the spiny rats of Japan also boast some species in which the Y chromosomes have completely disappeared. Researchers from Hokkaido University biologist Asato Kuroiwa has had more luck with the spiny rat, finding the genes on other chromosomes but found no sign of SRY or the gene that substitutes for it. Instead, what they found was a small duplication near sex gene SOX9, on chromosome 3.

When the researchers introduced a duplicated DNA containing the switch turning the SOX9 on, it boosted the SOX9 activity, so that SOX9 could work without SRY.

What this implication holds for humans is yet unclear. While lizards and snakes can make eggs out of their genes, this is not possible in humans as at least 30 crucial “imprinted” genes work only if they come from the father via sperm.

Thus for humans to continue to survive, men are needed, and the end of the Y chromosome could lead to the extinction of the human race; however, hope is not lost, as humans can evolve a new sex determining gene, with unforeseen risks.

We may not be humans in 11 million years, at least not as we are now.

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