The ZW sex-determination system is a system that determines the sex of offspring in birds, some fish and crustaceans such as the giant river prawn, some insects (including butterflies and moths), and some reptiles, including Komodo dragons. In the ZW system it is the ovum that determines the sex of the offspring, in contrast to the XY sex-determination system and the X0 sex-determination system, wherein it is the sperm that determines the sex. The letters Z and W are used to distinguish this system from the 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.