Is it a boy or a girl? Expecting parents may be accustomed to this question, but contrary to what they may think, the answer doesn't depend solely on their child's sex chromosomes.

Scientists at the European Molecular Biology Laboratory (EMBL) in Heidelberg, Germany and the UK's Medical Research Council's National Institute for Medical Research (NIMR) have discovered that if a specific gene located on a non-sex chromosome is turned off, cells in the ovaries of adult female mice turn into cells typically found in testes.

Their study, published in the journal Cell, challenges the long-held assumption that the development of female traits is a default pathway.

If it had a gene called Sry, which is located on the Y chromosome, it was thought that an embryo would develop into a male, if not, then a female. But in adult animals it is the male pathway that needs to be actively suppressed, the researchers discovered.

A gene called Foxl2, which is located on an autosome -- a chromosome other than the sex chromosomes -- and therefore present in both sexes, was known to play an important role in the female pathway, but its precise function remained elusive. The researchers turned this gene off in the ovaries of adult female mice.

"We were surprised by the results," says Treier, "We expected the mice to stop producing oocytes, but what happened was much more dramatic: somatic cells which support the developing egg took on the characteristics of the cells which usually support developing sperm, and the gender-specific hormone-producing cells also switched from a female to a male cell type."

Thus, the scientists discovered that Foxl2 plays a crucial role in keeping female mice female. Teaming up with the group of Robin Lovell-Badge at the NIMR, they were able to decipher together the underlying molecular mechanism.

These findings will have wide-ranging implications for reproductive medicine and may, for instance, help to treat sex differentiation disorders in children, for example where XY individuals develop as females or XX as males, and to understand the masculinising effects of menopause on some women.

This experiment clearly boosts the notion that gender is a malleable concept. Steve Connor, science editor of the Independent, commented: "The findings suggest that being male or female is not a permanently fixed state but something that has to be continually maintained in the adult body by the constant interaction of genes to keep the status quo – and the gender war – from slipping in favour of the opposite sex."

Professor Lovell Badge concurred: "it may eventually remove the need for surgery in gender-reassignment treatment… It's still very speculative, but it's possible that this approach could produce an alternative to surgery and the removal of gonads – ovaries and testes." ~ Science Daily, Dec 11; Independent, Dec 11