The animal on the right is no ordinary chicken. Its right half looks like a hen but its left half (with a larger wattle, bigger breast, whiter colour and leg spur) is that of a cockerel. The bird is a 'gynandromorph', a rare sexual chimera. Thanks to three of these oddities, Debiao Zhao and Derek McBride from the University of Edinburgh have discovered a truly amazing secret about these most familiar of birds - every single cell in a chicken's body is either male or female. Each one has its own sexual identity. It seems that becoming male or female is a very different process for birds than it is for mammals.
In mammals, it's a question of testicles, ovaries and the hormones they produce. Embryos live in sexual limbo until the sex organs (gonads) start to develop. This all depends on a sexual dictator called SRY, a gene found on the Y chromosome. If it's present, the indifferent gonads go down a male route; if not, they take a female one. The sex organs then secrete a flush of hormones that trigger changes in the rest of the body. The sex chromosomes are only relevant in the cells of the gonads.
