Archaeogenetics [creativecommons license] via Wikimedia Commons
Some time ago, the future of the human Y chromosome was called into question. After all, other species, such as mole voles and spiny rats, have lost their Y chromosomes, and sex-determination responsibilities have fallen to other chromosomes. In fact, the human Y did not start off as a sex-determining chromosome either. Human X and Y chromosomes were once a pair of autosomes, like the 22 others we carry, and sex was determined by environmental factors rather than genetics. The ancient X and Y swapped gene copies during crossing-over, just like the other autosomes, to maintain genetic diversity and eliminate potentially harmful mutations. About 300 million years ago, however, one part of the X stopped swapping with the Y, then another, a third, a fourth and—about 30 million years ago—a fifth. As a result, the corresponding portions of the Y chromosome decayed, and it eventually lost ~97% of the genes that it once shared with its partner, the X chromosome. It seemed that the Y was disappearing. Some said this was happening at an unsustainable rate, that it would be gone altogether within 10 million years. Prospects seemed dim for the withered Y.
Lab Anim. (NY) 41, 89 (2012).
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