For many years, bird breeders have known that occasionally, a ‘half-sider’ will arise naturally. These odd looking birds, also called gynandromorphs, are dramatically asymmetric in terms of plumage, size and other characters — because one side of the body appears male and the other female. The mammalian model of sex determination, in which an embryo is neutral sexually until a genetic switch initiates the development of hormone-producing gonads, was generally assumed to apply to all vertebrates. Now a study of three half-sider chickens suggests that in birds at least, sex determination is not dependent on the gonads alone, but is a property of cells across the whole body. Half-siders are found to be genuine male/female chimaeras, in which individual cells respond differently to the same cocktail of gonadal hormones. This raises questions about sex-determination and sexual development in general that, until now, had seemed more or less settled.
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