But let’s be clear: A vanishing Y does not mean vanishing males. Instead, Graves suggests that a new sex-determining system could evolve, as it did for the spiny rats. Instead of XY or XX, male and female rats are both XO, meaning they have a single unpaired X chromosome; an unknown spot in the genetic code now determines the gender. The same could happen to humans, with some novel genetic mutation usurping the Y chromosome’s role.
Not everyone agrees, of course. “I am convinced that the Y genes are too important to be lost, and the situation in spiny rats is probably just an isolated situation,” says biologist Jennifer Hughes of the Whitehead Institute at MIT. She and others note that the Y has been able to keep its important genes through “purifying selection,” in which deleterious changes are removed over time.
Graves realizes our Y may not disappear tomorrow — or ever. Her point is that it could fade away, given its degraded history. Unless it starts to happen soon, we’ll just have to live in suspense.
[This article originally appeared in print as "Y Not?"]