- Sex-determination system
A sex-determination system is a biological system that determines the development of sexual characteristics in an
organism . Most sexual organisms have twosex es. In many cases, sex determination is genetic:male s andfemale s have differentallele s or even differentgene s that specify their sexual morphology. Inanimal s, this is often accompanied by chromosomal differences. In other cases, sex is determined by environmental variables (such astemperature ) or social variables (the size of anorganism relative to other members of itspopulation ). The details of some sex-determination systems are not yet fully understood.Chromosomal determination
XX/XY sex chromosomes
The XX/XY sex-determination system is one of the most familiar sex-determination systems and is found in
human beings and most othermammal s, although at least onemonotreme , the platypus, presents a particular sex determination scheme that in some ways resembles that of the ZW sex chromosomes of birds, and it also lacks theSRY gene.In the XY sex-determination system, females have two of the same kind of sex
chromosome (XX), while males have two distinct sex chromosomes (XY). Some species (including humans) have a geneSRY on the Y chromosome that determines maleness; others (such as the fruit fly) use the presence of two X chromosomes to determine femaleness. The XY sex chromosomes are different in shape and size from each other unlike theautosomes , and are termedallosome s.XX/X0 sex determination
In this variant of the XY system, females have two copies of the sex chromosome (XX) but males have only one (X0). The "0" denotes the absence of a second sex chromosome. This system is observed in a number of
insect s, including the grasshoppers and crickets of orderOrthoptera and in cockroaches (order Blattodea).The
nematode "C. elegans" is male with one sex chromosome (X0); with a pair of chromosomes (XX) it is ahermaphrodite .ZW sex chromosomes
The ZW sex-determination system is found in
bird s and someinsect s and otherorganism s. The ZW sex-determination system is reversed compared to the XY system: females have two different kinds ofchromosomes (ZW), and males have two of the same kind ofchromosomes (ZZ).Haplodiploidy
Haplodiploidy is found in
insects belonging toHymenoptera , such asant s andbee s. Unfertilized eggs develop intohaploid individuals, which are the males.Diploid individuals are generally female but may be sterile males. Thus, if a queenbee mates with one drone, her daughters share ¾ of their genes with each other, not ½ as in the XY and ZW systems. This is believed to be significant for the development ofeusociality , as it increases the significance ofkin selection .This is common also in wasps that are parasitic and in the male greenflies.Non-genetic sex-determination systems
Many other sex-determination systems exist. In some species of reptiles, including
alligator s, someturtle s, and thetuatara , sex is determined by the temperature at which the egg is incubated. Other species, such as somesnail s, practice sex change: adults start out male, then become female. In tropicalclown fish , the dominant individual in a group becomes female while the other ones are male.Some species have no sex-determination system.
Earthworm s and some snails arehermaphrodite s; a few species of lizard, fish, and insect are all female and reproduce byparthenogenesis .In some
arthropod s, sex is determined by infection, as when 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] :
*Swordtail fish ?
* The Chironomus midge species
* ThePlatypus lacks the mammalian sex-determining geneSRY , meaning that the process of sex determination in the Platypus remains unknown. [cite web|url=http://pre.ensembl.org/Ornithorhynchus_anatinus/index.html|title=Explore the Platypus genome|publisher=Ensembl|date=2006-11|accessdate=19 January|accessyear=2007]ee also
*
Clarence Erwin McClung who discovered the role of chromosomes in sex determination.
*Testis-determining factor
*Barr body
*Protandry
*Protogyny
* For humans:
** Human sex determination and differentiation
**Gender verification in sports
**Sex organ , or primary sexual characteristic
**Secondary sex characteristic
**Shettles Method
**XYY syndrome References
* (2004) [http://dx.doi.org/10.1371/journal.pbio.0030028 Evolution of Sex Chromosomes: The Case of the White Campion.]
* (2006) [http://intl.pnas.org/cgi/content/full/103/48/18031 Multiple independent origins of sex chromosomes in amniotes.]External links
* [http://www.genetics.unimelb.edu.au/Martin/sexdt.html The Unusual Sex Determination System of Chironomus]
* [http://coloherp.org/cb-news/cbn-0009/TurtleSex.html The Enigma of Sex Determination in Reptiles]
* [http://www.nature.com/news/2004/041025/full/041025-1.html "Nature" news article about duck-billed platypus sex determination]
* [http://www.nature.com/embor/journal/v2/n3/full/embor459.html Hens, cocks, and avian sex determination]
* [http://cas.bellarmine.edu/tietjen/Ecology/y_chromosome_as_a_battle_ground_.htm The Y chromosome as a battleground for sexual selection]
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