- 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 two sexes. In many cases, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual morphology. In animals, this is often accompanied by chromosomal differences. In other cases, sex is determined by environmental variables (such as temperature) or social variables (the size of an organismrelative to other members of its population). The details of some sex-determination systems are not yet fully understood.
XX/XY sex chromosomes
The XX/XY sex-determination system is one of the most familiar sex-determination systems and is found in
humanbeings and most other mammals, although at least one monotreme, 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 the SRYgene.
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 gene SRYon 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 the autosomes, and are termed allosomes.
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
insects, including the grasshoppers and crickets of order Orthopteraand in cockroaches (order Blattodea).
nematode"C. elegans" is male with one sex chromosome (X0); with a pair of chromosomes (XX) it is a hermaphrodite.
ZW sex chromosomes
The ZW sex-determination system is found in
birds and some insects and other organisms. The ZW sex-determination system is reversed compared to the XY system: females have two different kinds of chromosomes(ZW), and males have two of the same kind of chromosomes(ZZ).
Haplodiploidy is found in
insectsbelonging to Hymenoptera, such as ants and bees. Unfertilized eggs develop into haploidindividuals, which are the males. Diploidindividuals are generally female but may be sterile males. Thus, if a queen beemates 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 of eusociality, as it increases the significance of kin 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
alligators, some turtles, and the tuatara, 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. In tropical clown fish, the dominant individual in a group becomes female while the other ones are male.
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.
arthropods, 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] :
* The Chironomus midge species
Platypuslacks the mammalian sex-determining gene SRY, 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]
Clarence Erwin McClungwho discovered the role of chromosomes in sex determination.
* For humans:
** Human sex determination and differentiation
Gender verification in sports
Sex organ, or primary sexual characteristic
Secondary sex characteristic
* (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.]
* [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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