Some animals have a sex determination system different from the XY system.
The X0 system is the sex determination system of many insects; in this system the females are XX and the males have only one X chromosome (a conditioned represented by X0).
In birds, in some fishes and in lepidopterae (butterflies) insects the sex determination is made by the ZW system; in this system females are ZW and males are ZZ.
In another system, the haploid-diploid sex determination system, one of the sexes is represented by the fertilized diploid individual and the individual of the opposite sex is formed by parthenogenesis, being haploid (it occurs in bees and other insects).
