What makes a man a man? Socially, that is a complicated question. Genetically, however, it is as simple as a single Y chromosome.
But guys, that chromosome is in trouble.
In a new study, researchers say there is a dramatic loss of genes from the human Y chromosome that eventually could lead to its complete disappearance -- in the next few millennia. While the Y chromosome's degeneration has been known to geneticists and evolutionary biologists for decades, the study sheds new light on some of the evolutionary processes that may have contributed to its demise and posits that, as the degeneration continues, the Y chromosome could disappear from our genetic repertoire entirely.
"It's certainly possible, but it's difficult to predict when it will happen," said Kateryna Makova, an associate professor of biology at Penn State University, who led the study, which was published Thursday in the journal PLoS Genetics.
Although geneticists and evolutionary biologists agree that the Y chromosome is degenerating -- and far more rapidly than its X counterpart -- they reject the idea of a world far in the future where men are obsolete.
"The idea that the Y chromosome has just bailed out of an airplane without a parachute simply doesn't fit the facts," said Dr. David Page, director of the Whitehead Institute for Biomedical Research in Cambridge, Mass., and a Y chromosome expert. "The evidence from studies on natural deletions of [genes on] the human Y chromosome shows there are consequences, especially for sperm production, that implies very strong natural selection against the loss of genes on the human Y chromosome."
Humans have 23 pairs of chromosomes packed with genes that dictate every aspect of our biological functioning. Of these pairs, the sex chromosomes are different; women have two X chromosomes and men have an X and a Y chromosome. The Y chromosome contains essential blueprints for the male reproductive system, in particular those for sperm development.
But the Y chromosome, which once contained as many genes as the X chromosome, has deteriorated over time and now contains less than 80 functional genes compared to its partner, which contains more than 1,000 genes. Geneticists and evolutionary biologists determined that the Y chromosome's deterioration is due to accumulated mutations, deletions and anomalies that have nowhere to go because the chromosome doesn't swap genes with the X chromosome like every other chromosomal pair in our cells do.
However, Melissa Wilson, lead author of the study and graduate research fellow at Penn State University, pointed out that if there is no difference between a male who has lost a particular gene and one who still retains it, especially if both are still fertile, then that gene must be nonessential.
"Because they can lose [a gene] ... we conclude that it's on its way to dying in humans," she said.
Yet the Y chromosome perseveres, despite its rapid rate of deterioration.
"The key flaw in the logic [of Y chromosome deterioration] is the assumption that the Y chromosome can only lose genes," Page said. "But the human Y chromosome has gained genes not even on the X chromosome. Men who lose those genes do not transmit their Y chromosome."