Y chromosome sequence unveiled 18/6/03. By the Whitehead Institute for Biomedical Research The DNA sequence of the human Y chromosome - the 'maleness' chromosome - has been published.

In the biological battle between the sexes, the Y chromosome has suffered defeat after defeat. The male-determining chromosome has seen its gene supply shrink from more than 1000 genes when sex chromosomes first evolved, to what scientists once thought was only a handful of genes, a downward trend predicted to continue until the Y disappeared altogether.

But two studies presented on 18 June 2003 and published in the journal Nature suggest that the rumours of the Y's demise have been greatly exaggerated. Researchers from Whitehead Institute for Biomedical Research in Cambridge, Mass., and Washington University School of Medicine in St Louis found that not only does the Y contain far more genes than scientists thought – the team found about 78 genes – it also includes a large number of genes arranged in pairs along this single chromosome in ways that may allow the Y to mimic the paired chromosome structure of the rest of the genome.

The researchers suggest that this arrangement may help the Y chromosome repair injured genes without the benefit of sexual recombination – the method of gene repair used by all other chromosomes. It's an elegant system that would debunk the theory of a 'rotting Y' – the widely held notion that the male chromosome and its dead or dying genes will continue to rot away over the next 5 million years until there's nothing left.

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"We have a new way of understanding how the rotting tendencies of the Y are counteracted," said lead researcher David Page.

All chromosomes in the nucleus come in pairs – except the Y. Each member of a chromosomal pair draws on its mate for genetic repair through sexual recombination. When one half suffers a genetic injury, as is the case with many diseases, it can discard the mutated gene and replace it with a normal copy drawn from the other member of the pair. But the Y has no sexual 'partner' with which to swap out defective genes.

"Genes constantly are being bombarded with little injuries – mutations. Mutations can either be beneficial or detrimental, but they are far more often detrimental," said Page. "On the Y, detrimental mutations cannot be discarded."

There's no question that this inability to discard has cost the Y hundreds of genes over time. Many of the chromosome's genes either have weakened or died out altogether. Sexual recombination is a card game the Y just can't win. But this new research suggests it doesn't always need to. For critical genes, it swaps with itself.

"This study shows that the Y chromosome has become very efficient at preserving its important genes," said co-lead investigator Richard K Wilson, director of the Genome Sequencing Center at Washington University School of Medicine in St Louis. "It's found different ways to do the things chromosomes must do to evolve, survive and thrive."

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The next steps in Y chromosome research will involve a closer scrutiny of this proposed self-repair mechanism, as well as plans to sequence the Y chromosome in mice.

Adapted from a news release by the Whitehead Institute for Biomedical Research .

Further reading

Skaletsky H, et al. The male-specific region of the human Y chromosome is a mosaic of discrete sequence classes. Nature 2003 423, 825–837A. Abstract

Rozen S, et al. Abundant gene conversion between arms of palindromes in human and ape Y chromosomes. Nature 2003 423, 873 – 876. Abstract