One in every 10,000 chickens is born gynandromorphic: half male and half female. Legend has it that such birds were once tried as the spirit-partners of witches. Now developmental biologist Michael Clinton has an explanation that is a bit more scientific, if nearly as bizarre. We expected to find that the birds had abnormal cells, says Clinton, who works at the Roslin Institute in Edinburgh, Scotland. Instead he found healthy male and female cells. These cells keep their identity even when injected into an embryo of the opposite sex, indicating that their gender is innate.
The discovery that each cell in a chicken can be inherently male or female is a huge departure from biological dogma, which holds that hormones control sex characteristics in vertebrates. Gender-imprinted cells may exist in us, too. Male and female cells might respond slightly differently to hormonal signals, which may partially explain differences in male and female behavior and susceptibility to some diseases, Clinton says.