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Q: How is sex determined in chickens and what does it mean?
A: In birds, the sex of the embryo is determined by the egg, not the sperm (as it is in humans). Avian sex differentiation is not determined by XY chromosomes like it is in humans and most mammals (or even the XO system used by some insects). In those systems the sex is determined by the sperm, by what the male contributes to the offspring. By contrast, birds have a ZW system of sex determination (shared with butterflies, moths, fish and reptiles). In that system, sex is determined by the female within her ova, and the sex of offspring is not affected at all by the sperm which the male contributes