On the surface, it looks as if our identity as male or female is determined in the womb. The decision seems final – a genetic switch flicks towards either setting, and locks into place for the rest of our lives.

This tidy image is wrong. Two recent studies in mice have shown that the switch isn’t locked – it’s held under constant tension by two rival genes – DMRT1 and FOXL2. It’s a tug-of-war fought over sexual fate, which goes on throughout our lives. Take away either contestant, and its adversary pulls the switch to the opposite setting. Ovaries can transform into testes and vice versa, even in adults.

By default, mammal embryos develop as females. A structure called the gonadal ridge