Man's secret ingredient

Sydney Morning Herald
June 19, 2003

By Deborah Smith

Science Writer

The thing that makes men men - their Y chromosome - is not as puny as scientists thought. New research shows it is endowed with 78 genes, rather than the expected handful. And in a surprise find, the chromosome has the ability to repair most of these genes, including those important to sperm production.


But it's not all good news. The study confirmed that part of the Y chromosome is rotting away, with about a third of its genes expected to decay to nothing in the next few million years. It took a team of 40 researchers, led by David Page of the Whitehead Institute in Massachusetts, to work out the order of the 24 million "letters" of DNA on the Y chromosome of one anonymous man - a feat they likened to piecing together a jigsaw puzzle of a small boat on a blue ocean with lots of blue sky.

The findings, which will improve understanding of male infertility and sex-linked diseases, are published today in the journal Nature.

Women have two X chromosomes and men have one X and one Y. About 300 million years ago, the Y was identical to the X and had about 1000 genes. But after developing a mutation that led to maleness - a gene known as SRY - the Y has been shrinking ever since.

This is because it does not have a partner chromosome with which to swap genes to repair damaging mutations.

The study identified 27 genes still surviving on the Y chromosome that were relics from the time when both X and Y were identical.

"Whereas the gene on the X is a functional working copy, in many cases the corresponding gene on the Y is a rotted out hulk that no longer does any business," Dr Page said.
But the researchers also found another 49 genes on the Y that were only switched on in the testes, leading them to believe they were important in sperm production.

These genes were able to avoid degradation because they occurred in multiple copies. A gene could repair mutations by swapping DNA with its unmutated pair.

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

The Australian researcher who discovered the SRY gene more than a decade ago, Associate Professor Andrew Sinclair, of the Murdoch Children's Research Institute in Melbourne, said the find was very exciting and might shed light on other chromosomes.

 

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