Battle between the sexes began with a mutant gene

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The Independent Online

The sexual revolution, which is still having repercussions in the battle of the sexes today, began with a single genetic mutation 300 million years ago.

The sexual revolution, which is still having repercussions in the battle of the sexes today, began with a single genetic mutation 300 million years ago.

A study of the key events leading to the evolution of the two sex chromosomes of humans has revealed they started with the birth of a gene that decides the sexual fate of an embryo.

Scientists have found that all the differences between X chromosomes, found in both men and women, and the Y chromosome found only in men, stem from the sudden arrival of a gene that turns female embryos into males. The X and Y chromosomes determine the physical and emotional characteristics of the sexes.

Once the gene - known as the SRY gene - came into existence it meant that the sex of an embryo would henceforth be determined by the chromosomes and not, as previously occurred, by other factors such as the temperature of the environment.

David Page, a geneticist at the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, and Bruce Lahn, of Chicago University, have discovered how and when the sex-determining region of the Y chromosome -- the SRY gene -- came about.

The key event happened between 240 million and 320 million years ago, the scientists report in the journal Science. Before then sex was determined by environmental influences, which still determine the sex of reptiles today.

Women have two X chromosomes and men have one X and one Y chromosome. After the sudden creation of the SRY gene, the Y chromosome became genetically isolated and was driven down an evolutionary path of separate development from its natural companion, the X chromosome.

Dr Page said there are just 19 genes on the Y chromosome that are shared with the X chromosome, compared with thousands of genes shared between two X chromosomes. Studying these genes enabled the scientists to determine when the Y chromosome separated from the X.

"The farther back in time we look, the more similar X and Y appear, boosting the theory that they arose from a pair of identical autosomes [non-sex chromosomes]," Dr Page said.

The creation of the SRY gene led to the "hijacking" of one of the X chromosomes, he said. "In humans the ramifications of the hijacking are still being played out.'

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