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.
But the gynandomorphs show that something very different happens in birds. Birds have Z and W chromosomes; males are ZZ and females are ZW. Zhao and McBride used glow-in-the-dark molecules that stick to the two chromosomes to show that the gynandromorphs do indeed have a mix of ZZ and ZW cells. However, they aren't split neatly down the middle. Their entire bodies are suffused with a mix of both types, although the male half has more ZZ cells and the female half has more ZW ones.
Even though the three chickens were both male and female, one of them only had a testicle on one side, the second only had an ovary on one side, and the third had a strange hybrid organ that was part testis and part ovary. These malformed organs pumped the same soup of hormones throughout the birds' bodies but, clearly, each side responded differently.
Zhao and McBride started to suspect that each cell has its very own sexual identity, and that this individuality exists from the chicken's first days of embryonic life. They proved that by transplanting cells from embryonic sex organs from one animal to another. All the transplants produced a glowing green protein so Zhao and McBride could track their whereabouts, and those of their daughters.