Whether a mammal is a male or a female usually comes down to its set of sex chromosomes, either XX if you're female, or XY, male. But studies have shown the human Y chromosome has degraded over time, losing up to 1,600 genes in about 200 million years, by some estimates. More strikingly, some mammals, such as the Japanese spiny rat, have shed their Y chromosomes altogether. "So what is to stop the human Y from disappearing?" says evolutionary geneticist and molecular biologist Jennifer Marshall Graves of La Trobe University. She contends that the Y chromosome already starts at a disadvantage: Unlike the X, it has no partner for genetic recombination (the random exchange of segments between chromosome pairs), so it can accumulate harmful mutations, leaving it perennially vulnerable. She says the human Y should disappear in about 4.6 million years if its degradation continues at the rate it's happened so far - and there's no reason it has to take that long. Sudden changes have sparked previous losses in the Y, so we always could lose our Y much sooner.
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