The Y chromosome, which contains the male-determining gene in humans and other mammals, is degenerating in the human species and may cease to exist after a few million years. This would lead to extinction unless humans evolve a new sex gene.
How the Y chromosome, the “Male Gene,” works
Female humans, along with other mammals, have two X chromosomes, while males have just one. This is paired with a much smaller Y chromosome in men. The X chromosome contains around nine hundred genes, which carry out lots of tasks unrelated to sex, and the Y contains far fewer genes (55) along with a great deal of non-coding DNA, such as simple, repetitive DNA that appears not to have any particular function.
The Y chromosome is important because it contains the gene that initiates male development in the embryo, which switches on at around twelve weeks into pregnancy. This male-inducing gene switches on others that regulate the development of a testis, and the embryonic testis produces male hormones (testosterone, among others), which leads to the development of a baby boy.
This master sex gene was identified as SRY (sex region on the Y) in 1990 and works by triggering a genetic pathway beginning with a gene called SOX9, which is crucial for male determination in all vertebrates.
The Y is disappearing
The majority of mammals have an X and Y chromosome similar to those of humans. The X carried lots of genes, while the Y contains SRY and a few others. This biological system has innate difficulties, due to the uneven amount of X chromosome genes in males and females.
In discovering the evolution of this system, it is useful to look at the duck-billed platypus, which has entirely different sex chromosomes. In platypuses, the XY pair is simply an ordinary chromosome with two equal members. This suggests the mammal X and Y were an ordinary pair of chromosomes not too far back in evolutionary history.
This further suggests that the Y chromosome has lost around almost nine hundred active genes over the 166 million years that humans and platypuses have been evolving separately. Consequently, this amounts to a loss of around five genes per million years, meaning the last fifty-five genes will have disappeared within the next eleven million years.
The case of the Y chromosome in rodents
There are two species of rodents that have already lost their Y chromosome and continue to survive. This includes the mole voles of eastern Europe and the spiny rats of Japan. Their X chromosome remains in a single or double form in both sexes.
It is unknown how the mole voles determine sex without the SRY gene, but a team led by Hokkaido University biologist Asato Kuroiwa has made headway with the spiny rat. It is a group of three species on different Japanese islands. Kuroiwa’s team discovered that most of the genes on the Y of spiny rats had been relocated to other chromosomes, but the SRY was not found, nor was the gene that substitutes for it.
In 2022, the research team published their study in the academic journal PNAS, having eventually discovered sequences that were in the genomes of males but not females. They refined them and then tested for the sequence on every individual rat. The team discovered a very minute difference near the key sex gene SOX9. It was on chromosome three of the spiny rat. A small duplication—just seventeen thousand base chromosome pairs out of more than three billion—was present in all males and no females.
The researchers posited that this tiny bit of duplicated DNA contains the switch that would usually turn on SOX9 in response to SRY. When they introduced this duplication into mice, they found that it boosts SOX9 activity, meaning the change could allow SOX9 to work without SRY.
The future of the Y chromosome?
Much speculation has been made about the eventual disappearance of the human Y chromosome, particularly with regard to the future of the human race. Several lizards and snake species are female-only and can produce eggs from their own genes via parthenogenesis. However, this is not possible in humans and other mammals because they have at least thirty crucial “imprinted” genes that only work if they come from the father via sperm.
In order for humans to reproduce, sperm is needed. By association, males are hence necessary. This means the end of the Y chromosome could spell the extinction of the human race unless, as the new findings suggest, humans can evolve a new sex-determining gene.
