Men, or at least male biologists, have long been alarmed that their tiny Y chromosome, once the same size as its buxom partner, the X, will continue to wither away until it simply vanishes. The male sex would then become extinct, they fear, leaving women to invent some virgin-birth method of reproduction and propagate a sexless species.

The fear is not without serious basis: The Y and X chromosomes once shared some 800 genes in common, but now, after shedding genes furiously, the Y carries just 19 of its ancestral genes, as well as the male-determining gene that is its raison d’être. So much DNA has been lost that the chromosome is a fraction of its original size.

But there are grounds for hope that the Y chromosome has reached a plateau of miniaturized perfection and will shrivel no more. Researchers led by Jennifer F. Hughes and David C. Page of the Whitehead Institute in Cambridge, Mass., have reconstructed the Y chromosome’s past and find that its gene-shedding days seem to be over. Men are not living on borrowed time after all, they reported on Wednesday in the journal Nature.

In people, sex is determined by a single gene that resides on the Y chromosome. Chromosomes come in pairs, with one set bequeathed by each parent, and the Y is paired with X such that men have an X-Y pair and women an X-X. When the male-determining gene first arose, some 320 million years ago, the X and Y were both full-length chromosomes, each bearing the same set of 1,000 or so genes.