Sex-determination system
A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most common sex-determination systems in animals involve a genetic mechanism based on the chromosomes of that organism. However, other systems can involve other variables such as temperature. The details of some sex-determination systems are not yet fully understood.
Mammals
The most familiar sex-determination system is the XY sex-determination system found in human beings and other mammals.
Other gene-based systems
The WZ sex-determination system is found in birds and some insects and other organisms. 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).
The haploid-diploid system is found in Hymenoptera. Males are haploid; females are diploid. Thus, if a queen bee mates with one drone, her daughters share 3/4 of their genes with each other, not 1/2 as in the XY and WZ systems. This is believed to be significant for the development of eusociality, as it increases the significance of kin selection.
Other sex-determination systems
Many other exotic sex-determination systems exist. In some species of reptiles, including alligators, sex is determined by the temperature at which the egg is incubated. Other species, such as some snails, practice sex change: adults start out male, then become female.
Some species have no sex-determination system. Earthworms and some snails are hermaphrodites; a few species of lizard, fish, and insect are all female and reproduce by parthenogenesis.
In some arthropods, sex is determined by infection. Bacteria of the genus Wolbachia alter their sexuality; some species consist entirely of ZZ individuals, with sex determined by the presence of Wolbachia.
Other unusual systems (need confirmation of these):
- Swordtail fish?
- The Chironomus midge species
- etc
See also
