DNA of the male-determining Y chromosome has been completely sequenced end-to-end, and it’s just as weird as we expected. Will we finally be able to understand how it works?
The findings provide a solid base to explore how genes for sex and sperm work, how the Y chromosome evolved, and whether – as predicted – it will disappear in a few million years.
We have known for about 60 years that specialised chromosomes determine birth sex in humans and other mammals. Females have a pair of X chromosomes, whereas males have a single X and a much smaller Y chromosome.
The Y chromosome is very different from X and the 22 other chromosomes of the human genome. It is smaller and bears few genes (only 27 compared to about 1,000 on the X).
We have a lot of evidence that 150 million years ago the X and Y were just a pair of ordinary chromosomes (they still are in birds and platypuses). There were two copies – one from each parent – as there are for all chromosomes.
The proto-Y degenerated fast, losing about 10 active genes per million years, reducing the number from its original 1,000 to just 27. A small “pseudoautosomal” region at one end retains its original form and is identical to its erstwhile partner, the X.
There has been great debate about whether this degradation continues, because at this rate the whole human Y would disappear in a few million years (as it already has in some rodents).
The Y is the last human chromosome to have been sequenced end-to-end, or T2T (telomere-to-telomere).