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[Male] [Sex] [Chromosome]

Scientists have discovered that the Y chromosome uses a neat [trick] to repair its most crucial genes, a strategy that apparently helps keep it from rotting away over evolutionary time. Instead of doubling up to protect its genetic cargo [like] other chromosomes, the lone Y safeguards its genes by having sex with itself, an international consortium has found. Proving perhaps that nature has a sense of [humor], scientists have discovered that the Y chromosome - the [one] that makes a man a man - has a remarkable ability to make do-it-yourself repairs. June 20, 2003 — The human male sex chromosome does have the ability to repair itself and [may] not be headed for extinction as had previously been thought, according to a surprising new study. A 40-strong team of researchers led by David Page of the Whitehead Institute at the [Massachusetts] Institute of Technology, report their findings in this week's issue of the journal Nature. As well as having a previously unknown and elaborate back-up system for self-repair, the Y chromosome also carries 78 genes. "The Y chromosome is a hall of mirrors," [said] Page, whose team has for the first time identified the full genetic sequence of a Y chromosome, from an [anonymous] donor. Both the male Y and [female] X chromosomes are thought to have originally been the same size, but after the Y took on the sex-determining role for maleness it apparently began to lose genes. At this time it also lost the ability to pair up exactly with its partner and to swap faulty genes for good ones, as the other [22] pairs of non-sex chromosomes do. Earlier studies had suggested that the Y chromosome carried only a few dozen genes, compared with more than 1,000 known on the X chromosome. A team of [Australian] researchers led by [Jenny] Graves, of the Research School of Biological Sciences at the Australian National University in Canberra, previously found that the Y chromosome had been losing five genes per million years. Graves had thus predicted that the chromosome might be heading for extinction within five to 10 million years. But Page said that the Y's full genome sequence has revealed that scientists generally had underestimated both its number of genes and its powers of self-preservation. The team believes the Y has developed an apparently [unique] way of pairing up with itself. They found that many of its 50 million DNA "letters" occur in sequences known as palindromes. [Like] their grammatical counterparts, these sequences of letters read the same forward as backward but are arranged in opposite directions — [like] a mirror image — on both strands of the DNA double helix. This means that a back-up copy of each of the genes they contain occurs at each end of the sequence. When the DNA divides during reproduction, the team believes, it opens an opportunity for genes to be shuffled or swapped and faulty copies to be deleted.

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The Y chromosome is one of the [two] sex chromosomes (X and Y). Only men [carry] the Y chromosome. The Y chromosome is male specific, passing from [father] to son. It is responsible for many male characteristics.

The path of [evolution] is usually so [quirky] and complex that scientists shy away from making predictions. But the future of the Y chromosome seems clear. Graves points out that, on average, three to six genes have disappeared from the Y every million years since the chromosome emerged. At that rate, the Y has only 10 million years left. It's an old chromosome, at death's [door]. The death of the Y doesn't [mean] the death of men. Men need only look to the [mole] vole for comfort. Burrowing through the soil of western Asia are two species of these rodents (Ellobius tancrei and E. lutescens) that have lost all the genes from their Y chromosome--in fact, they no longer have a Y chromosome at all. In one of these species, both males and females have been left with just the unpaired X; in the other, both sexes have two X's. No [one] knows how [mole] voles ended up being the first mammals to cross over into the Y-free future. But along the way, they must have evolved new genes--on other chromosomes--that are responsible for making males. One of those genes took over the [job] of SRY, and the chromosome on which it resides is probably on its way to becoming the new Y. If our species manages to survive for another 10 million years, our descendants [will] go on making men even after their Y chromosome vanishes. But the change [may] not be smooth. Graves speculates that several new systems for determining sex could emerge within the global human population. [People] conceived under [one] system might be genetically incompatible with those conceived under others. As a result, the human species could fragment into separate populations and, ultimately, separate species. Which of them [will] prefer football and which the ant nest, we'[ll] have to wait and see.

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