We say ‘daughter’ rather than son because of ‘Haldane’s rule’. The polymath J. B. S. Haldane, whom we shall meet at several points in this book, pointed out that in interspecies hybrids, if one gender is infertile, it is invariably the one with two different sex chromosomes. Thus in mammals, where the male has X and Y sex chromosomes, hybrids such as male mules are always infertile, whereas female mules (XX) can sometimes have normal babies. In birds and butterflies this pattern is reversed, because males are defined by their two Z chromosomes, whereas a W and a Z define a female. This pattern
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