The ZW sex-determination system is a system that birds, some fishes, 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). For other meanings of bird, see bird (disambiguation). ... Atlantic herring, Clupea harengus, the most abundant fish species in the world, Photo by Uwe Kils This page is about the animals which live in water. ... Orders Subclass Apterygota Symphypleona - globular springtails Subclass Archaeognatha (jumping bristletails) Subclass Dicondylia Monura - extinct Thysanura (common bristletails) Subclass Pterygota Diaphanopteroidea - extinct Palaeodictyoptera - extinct Megasecoptera - extinct Archodonata - extinct Ephemeroptera (mayflies) Odonata (dragonflies and damselflies) Infraclass Neoptera Blattodea (cockroaches) Mantodea (mantids) Isoptera (termites) Zoraptera Grylloblattodea Dermaptera (earwigs) Plecoptera (stoneflies) Orthoptera (grasshoppers, crickets... For other uses of the term butterfly, see butterfly (disambiguation). ... Lepidopteran on a flower. ... A human ovum Sperm cells attempting to fertilize an ovum An ovum (plural ova) is a haploid female reproductive cell or gamete. ... The XY sex-determination system is a well-known sex-determination system. ... The X0 sex-determination system is a system that grasshoppers, crickets, roaches, and some other insects use to determine the sex of their offspring. ...
In the WZ sex-determination system, the situation is reversed: females have two different kinds of chromosomes (WZ), and males have two of the same kind of chromosomes (ZZ).
It is unknown whether the presence of the Wchromosome induces female features or the duplication of the Z chromosome induces male ones; Unlike mammals, no birds with a double Wchromosome (WWZ) or a single Z (Z0) have been discovered.
Chromosomes in the WZ region in birds are autosomal in mammals, and vice-versa; therefore, it's theorized that the WZ and XY couples come from different chromosomes of the common ancestor.
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