“Chicken sex doesn’t work like ours. No, not that sex — but the process by which an embryo becomes a recognizably male or female animal. Unlike mammals, it’s not hormones that dictate a chicken’s sex. It’s a fundamental property of the cells themselves. But this only became apparent when biologists investigated several odd chickens that were half male and half female, as if a line were drawn down the center of their bodies. ‘We assumed this was caused by one side of the body having some kind of sex chromosome anomaly,’ said Michael Clinton, a University of Edinburgh developmental biologist and co-author of the study, described March 10 in Nature. ‘But when we looked at them closely, they were composed of entirely normal cells. We realized that birds don’t follow the mammalian model.’ In mammals, there are two types of sex-determining chromosomes, X and Y. Each cell in an embryo has a pair of chromosomes, either XX or XY, but the cells are otherwise identical. Then, early in development, in response to some environmental cue, a group of cells that will someday become ovaries or testes start to produce hormones that cause other cells to develop in male- or female-specific ways. It’s the hormones that matter: Exposed to lots of testosterone and deprived of estrogen, cells with female chromosomes will form masculine tissues, and vice versa.”
Who — or what — really controls your mind?