Unlike some reptiles, a chickens gender is determined at fertilization. Sex determination in this species is not temperature-dependent nor humidity-dependent. The sex of a chick is determined when the mother hen add her genes in the fertilization process. If she contributes a W chromosome, the chick will be female. If she adds a Z chromosome, it will be a male. Males are homogametic (ZZ), while females are heterogametic (ZW).