X and Y no longer appear to be the only factors that determine gender. In fact, according to a study released Thursday in the journal Cell, it looks like your sex isn't even something set at birth.
"It was thought that you were born either female or male and then your body forgot about it," Dr. Robin Lovell-Badge told The Telegraph. Lovell-Badge was a co-author of the study and is a researcher at Britain's Medical Research Council's National Institute of Medical Research.
The scientists involved in the study had noticed a gene not found on either the X or Y chromosomes, known as FoxL2, kept showing up in findings related to gender. In this experiment they decided to shut it down.
When FoxL2 was out of the picture, cells in the ovaries in female mice began to change into testicular cells. Further research showed FoxL2 suppresses the expression of another gene, Sox9, which promotes the production of testes. The researchers say although the experiment worked with mice, the findings could be applicable to humans.
So it seems for females, a functional FoxL2 is needed to tamp down Sox9, to keep from becoming male. This contradicts the long-held biological belief that female is the default setting and males emerge by suppressing this tendency.
The findings of this study could have implications for reproductive medicine, including sexual differentiation disorders, where people develop as a sex contrary to their sex chromosomes. It could also help scientists understand post-menopausal changes in women.