Scientists have identified the gene that keeps females female. An international team found that the action of a single gene is all that stops females from developing male physical traits, including testes and facial hair.

When this gene was artificially "switched off" in adult female mice their ovaries began to turn into testes and they started to produce a level of testosterone found in healthy male mice.

The discovery could eventually revolutionize gender reassignment therapy and improve treatments for babies who are born with a mixed gender.

The research, published today in the journal Cell, challenges a common perception that gender is determined purely by the X-chromosomes and Y-chromosomes. The gene that was switched off, known as FOXL2, lies on a non-sex chromosome that is shared by males and females.

We take it for granted that we maintain the sex we are born with, including whether we have testes or ovaries. But this work shows that the activity of a single gene, FOXL2, is all that prevents adult ovary cells turning into cells found in testes," said Robin Lovell-Badge, from the National Institute for Medical Research, a co-author of the paper.

The gene appears to have a "see-saw" relationship with another gene, SOX9, which is normally active only in males. When one is on, the other is automatically off. In the first few days of male development SOX9 is turned on, and this stops FOXL2 from becoming active for the rest of the man’s life. The reverse occurs in females, with FOXL2 being switched on first.

The discovery that gender depends, at least in part, on the competing action of genes that are shared by both sexes suggests that gender can be more easily manipulated than previously thought.

FOXL2 was already known to be important for the growth of ovaries during development and for their maintenance during a woman’s life. However, scientists did not anticipate that egg-producing cells in the ovary could be co-opted by a competing male gene to carry out the male reproductive functions.

"We expected the mice to stop producing eggs, but what happened was much more dramatic," said Mathias Treier, of the European Molecular Biology Laboratory, who led the study.

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