STANDARD SETS. “In vertebrates the most widespread situation is that a pair of sex chromosomes differs in males and females. Either females can be XX and produce only one type of gamete, being the “homogametic” sex, and then males are XY and thus heterogametic, like most mammals; or if females are heterogametic (in analogy: ZW), then males have two copies of the same sex chromosome and are ZZ, like in birds.”
http://www.pnas.org/content/112/34/10575.full