The Amazing Missing Chromosome

Reprinted from the A.N.U. Reporter, Winter 2004

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A new version of the human male is a distinct possibility, given the genetic erosion of the "junk" sex-determining chromosome that makes men male.

The Y-chromosome, which carries an essential gene known as S.R.Y. that induces embryonic testis development (and thus the release of male hormones) is self-destructing, having shed ninety-seven per cent of its other genes in the 300 million years since it evolved.

The Y-chromosome is running out of time, according to geneticist Professor Jenny Graves, the head of the Comparative Genomics Group in the A.N.U. Research School of Biological Sciences.

"The Y-chromosome has lost 1,393 of the 1,438 genes it began with 300 million years ago. At this rate it would lose the last forty-five in just ten million years," Professor Graves said at a lecture in May, 2004.

"It has saved itself from extinction only by adding bits from another chromosome. Most genes on the human Y have partners on the X from which they evolved.

"Even the sex determining gene S.R.Y. has a partner on the X, the brain-expressed SOX3, from which it derived. This leads to many questions about how a brain-determining gene could take on a new role as a testis-determining gene."

Although it may seem the decline of the Y-chromosome and the essential S.R.Y. gene would lead to the end of the human race, Professor Graves said nature has proved this is not necessarily the case.

"S.R.Y. has been lost in at least two groups of rodents. The mole voles of eastern Europe and the country rats of Japan have no Y-chromosome, and therefore no S.R.Y.

"Somewhere else in their genome, a new sex-determining gene must have taken over the function of S.R.Y. Which gene or genes took over this task, and how they work, are questions we will be investigating in future."

Professor Graves predicts that as the human Y-chromosome deteriorates, one or more sex-determining genes will develop, possibly within different human populations.

"What would happen if different new sex-determining arose in different human populations? Could mole vole man breed with country rat woman? Probably not, so the two populations would ultimately become different hominid species."

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