The sex of human and other mammal babies is decided by a male-determining gene on the Y chromosome. But the human Y chromosome is degenerating and may disappear in a few million years, leading to our extinction unless we evolve a new sex gene.
The good news is two branches of rodents have already lost their Y chromosome and have lived to tell the tale. A 2022 paper in Proceedings of the National Academy of Science shows how the spiny rat has evolved a new male-determining gene.
How the Y chromosome determines human sex
In humans, as in other mammals, females have two X chromosomes and males have a single X and a puny little chromosome called Y. The names have nothing to do with their shape; the X stood for 'unknown'.
The X contains about 900 genes that do all sorts of jobs unrelated to sex. But the Y contains few genes (about 55) and a lot of non-coding DNA – simple repetitive DNA that doesn't seem to do anything. But the Y chromosome packs a punch because it contains an all-important gene that kick-starts male development in the embryo.
At about 12 weeks after conception, this master gene switches on others that regulate the development of a testis. The embryonic testis makes male hormones (testosterone and its derivatives), which ensures 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. READ MORE...
