sex chromosome

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 organism relative to other members of its population). 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 the most familiar sex-determination systems, as it is found in human beings, most other mammals, as well as some insects. However, 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 also lacks the SRY gene, whereas some rodents, such as several Arvicolinae (voles and lemmings), are also noted for their unusual sex determination systems.

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 SRY 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 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 Orthoptera and in cockroaches (order Blattodea).

The 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). In the chicken, this was found to be dependent on the expression of DMRT1.^[1]

Haplodiploidy

Haplodiploidy is found in insects belonging to Hymenoptera, such as ants and bees. Unfertilized eggs develop into haploid individuals, which are the males. Diploid individuals are generally female but may be sterile males. Thus, if a queen bee 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 of eusociality, as it increases the significance of kin selection. This^{[clarification needed]} 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. Some 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, and blue wrasse fish are the reverse. In Bonellia viridis, larvae become males if they make physical contact with the female, and females if they end up on soil.

Some species, however, have no sex-determination system. Hermaphrodites include the common earthworm and certain species of snails. A few species of fish, reptiles, and insects reproduce by parthenogenesis and are female altogether.

In some 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 is still being researched):

• Swordtail fish
• The Chironomus midge species
• The Platypus lacks the mammalian sex-determining gene SRY, meaning that the process of sex determination in the Platypus remains unknown.^[2]

See 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

This article includes a list of references or external links, but its sources remain unclear because it has insufficient inline citations. Please help to improve this article by introducing more precise citations where appropriate. (April 2009)

• "Evolution of Sex Chromosomes: The Case of the White Campion". PLoS Biol 3 (1): e28. 2005. doi:10.1371/journal.pbio.0030028. http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0030028. 

Further reading

Sex chromosomes and sex determination in amniotes

• Vallender, J.; Lahn, T. (Nov 2006). "Multiple independent origins of sex chromosomes in amniotes" (Free full text). Proceedings of the National Academy of Sciences of the United States of America 103 (48): 18031–18032. doi:10.1073/pnas.0608879103. ISSN 0027-8424. PMID 17116892. PMC 1838700. http://www.pnas.org/cgi/pmidlookup?view=long&pmid=17116892.  edit
• Veyrunes, F.; Waters, D.; Miethke, P.; Rens, W.; Mcmillan, D.; Alsop, E.; Grützner, F.; Deakin, E. et al. (Jun 2008). "Bird-like sex chromosomes of platypus imply recent origin of mammal sex chromosomes" (Free full text). Genome research 18 (6): 965–973. doi:10.1101/gr.7101908. ISSN 1088-9051. PMID 18463302. PMC 2413164. http://www.genome.org/cgi/pmidlookup?view=long&pmid=18463302.  edit

Sex determination in insects

• Cho, S.; Huang, Y.; Zhang, J. (Nov 2007). "Sex-specific splicing of the honeybee doublesex gene reveals 300 million years of evolution at the bottom of the insect sex-determination pathway" (Free full text). Genetics 177 (3): 1733–1741. doi:10.1534/genetics.107.078980. ISSN 0016-6731. PMID 17947419. PMC 2147941. http://www.genetics.org/cgi/pmidlookup?view=long&pmid=17947419.  edit

Sex determination in plants

• Matsunaga, S (Aug 2006). "Sex chromosome-linked genes in plants" (Free full text). Genes & genetic systems 81 (4): 219–26. doi:10.1266/ggs.81.219. ISSN 1341-7568. PMID 17038793. http://joi.jlc.jst.go.jp/JST.JSTAGE/ggs/81.219?from=PubMed.  edit
• Matsunaga, S. (Jun 2009). "Junk DNA promotes sex chromosome evolution". Heredity 102 (6): 525–526. doi:10.1038/hdy.2009.36. ISSN 0018-067X. PMID 19337304.  edit

Book

• Bainbridge, David A. (2003). The X in sex: how the X chromosome controls our lives. Cambridge: Harvard University Press. ISBN 0-674-01028-0. 

External links

• The Unusual Sex Determination System of Chironomus
• The Enigma of Sex Determination in Reptiles
• Nature news article about duck-billed platypus sex determination
• Hens, cocks, and avian sex determination
• The Y chromosome as a battleground for sexual selection