Heterogametic Sex Hypothesis

In mammals, including humans, sex is generally determined by the X and Y chromosomes. If a baby has a pair of X chromosomes, she’s a girl. If the baby inherits an X chromosome and a Y chromosome, he’s a boy. In birds, however, the situation is reversed. Female birds have a pair of unlike sex chromosomes while males have the like pair. (In this sort of sex-determination system, scientists use the letters W and Z, and talk about a ZW pair for female birds and a ZZ pair for males.) The “...