In birds, the male sex is the homogametic sex, having two Z chromosomes, while the female sex is the heterogametic sex, with one Z and one W chromosome. In platypuses, the male sex is heterogametic while the female sex is the homogametic sex.
In birds, males have two Z chromosomes (homogametic), while females have one Z and one W chromosome (heterogametic). In platypuses, the male sex is heterogametic, while the female sex is homogametic.