The ZW sex-determination system is a system that birds, some fish, and some insects (including butterflies and moths) use to determine the sex of their offspring. The ovum determines the sex of the offspring in this system, in contrast to the XY sex-determination system and the X0 sex-determination system. The letters Z and W are used to distinguish this system from XY system. Males are the homogametic sex (ZZ), while females are heterogametic (ZW). The Z chromosome is larger and has more genes, like the X chromosome in the XY system.
