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Sex-determination system |
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Sex-determination systemA 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.MammalsThe most familiar sex-determination system is the XY sex-determination system found in human beings and most other mammals. See the XY sex-determination system article for a detailed discussion of this system. Other gene-based systemsThe 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). Until recently, it was thought that the WZ system was unrelated to the XY system common to most mammals. A paper published in 2004 (Frank Grützner et al, Nature; doi:10.1038/nature03021) suggests that the two systems may be related. It would seem that platypuses have a ten-chromosome–based system, where the chromosomes form a multivalent chain in male meiosis, segregating into XXXXX-sperm and YYYYY-sperm, with XY-equivalent chromosomes at one end of this chain and the WZ-equivalent chromosomes at the other end. Haplodiploidy 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 systemsMany 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 [this section still being researched]: See alsoExternal links
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