System	Mechanism		Heterogametic Sex	Organisms
XX-XO	Females XX	Males X	Male	Some grasshoppers and other insects
XX-XY	Females XX	Males XY	Male	Many insects, fish, amphibians, reptiles; mammals, including humans
ZZ-ZW	Females ZW	Males ZZ	Female	Butterflies, birds; some reptiles and amphibians
Genic sex determination	No distinct sex chromosomes Sex determined by genes on undifferentiated chromosomes		Varies	Some plants, fungi, protozoans, and fish
Environmental sex determination	Sex determined by environmental factors		None	Some invertebrates, turtles, alligators