Why the Y chromosome is vanishing and what this means for the future

The biological sex of a human being is determined by which chromosomes make up that baby's genetic material. Females usually have two X chromosomes, while males tend to have one X and one Y chromosome. But research suggests that the Y chromosome has been rapidly degenerating over the course of our evolution. In fact, the gene may one day disappear altogether.

Why is the Y chromosome disappearing?

The Y chromosome has seen significant changes over the course of human evolution. Within the past 166 million years, "the human Y lost most of its 1,600-odd genes, a rate of nearly 10 per million years," Darren Kent, a professor at the University of Kent and Peter Ellis, a lecturer at the University of Kent, said in The Conversation. The X chromosome contains approximately 900 genes with multiple functions, whereas the Y has approximately 55 genes with only 27 of them being male-specific. In addition, "many are present in multiple copies, most of them inactive, lying in giant loops of DNA. Most of the Y is made of repetitive 'junk DNA.'" With such an unstable composition, the Y chromosome is at risk of completely disappearing over the course of multiple generations.

"The early 'proto-Y' chromosome was originally the same size as the X chromosome and contained all the same genes," Kent and Ellis said. However, because males only have one copy of the chromosome, it does not have the opportunity to go through genetic recombination, which is the "shuffling of genes that occurs in each generation which helps to eliminate damaging gene mutations." Without recombination, Y chromosomal genes degenerate over time. "This significant gene loss over the centuries has lead scientists to predict that the Y chromosome might vanish entirely in about 11 million years," said Earth.com.

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What are the implications?

The most important part of the Y chromosome is the master sex gene called SRY, which triggers a fetus' development as a male. "The Y makes no sense in terms of function, but is easy to understand in terms of evolution," Jenny Graves, a sex chromosome geneticist at La Trobe University in Melbourne, said to Newsweek. "The X and Y were once upon a time just an ordinary pair of chromosomes. Then one partner acquired a variant gene (SRY) that determines maleness." So what happens if the Y disappears? "When humans run out of Y chromosome, they might become extinct (if we haven't already extincted ourselves long since), or they might evolve a new sex gene that defines new sex chromosomes," Graves said. Rest assured, this type of evolution will take millions of years.

Some clues may be found in rats. "The good news is we know of two rodent lineages that have already lost their Y chromosome — and are still surviving," said Graves at The Conversation. A 2022 study published in the journal PNAS found that the important Y chromosome genes could be capable of relocating to other genes, as observed in mice. The researchers found a "tiny difference near the key sex gene SOX9, on chromosome 3 of the spiny rat," said Graves. "A small duplication (only 17,000 base pairs out of more than 3 billion) was present in all males and no females," and "this small bit of duplicated DNA contains the switch that normally turns on SOX9 in response to SRY."

There is also the possibility that humans develop a whole new sex gene. "A 'war' of the sex genes could lead to the separation of new species, which is exactly what has happened with mole voles and spiny rats," Graves said. "If someone visited Earth in 11 million years, they might find no humans — or several different human species, kept apart by their different sex determination systems."

Still, human males are not going away any time soon. Some experts firmly believe that the Y chromosome will not disappear at all, claiming that it has several defense mechanisms in place. A 2017 study published in the journal PLOS Genetics said the chromosome's evolution of palindromes — DNA sequences that read the same forward as backward — and repetitive copies of genes could keep it from degrading faster by allowing "damaged genes to be repaired using an undamaged back-up copy as a template." On a positive note, "by embracing ongoing research and understanding the complexities of genetics, we can find reassurance in the resilience of our species," said Earth.com.