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The ZW sex-determination system is a system that determines the sex of offspring in birds, some fish and crustaceans such as the giant river prawn , and some insects (including butterflies and moths), and some reptiles, including Komodo dragons. In the ZW system it is the ovum that determines the sex of the offspring, in contrast to the XY sex-determination system and the X0 sex-determination system, where it is the sperm which determines the sex. The letters Z and W are used to distinguish this system from the XY system. Males are the homogametic sex (ZZ), while females are heterogametic (ZW). The Z chromosome is larger and has more genes, like the X chromosome in the XY system.

It is unknown whether the presence of the W chromosome induces female features or the duplication of the Z chromosome induces male ones; unlike mammals, no birds with a double W chromosome (ZWW) or a single Z (Z0) have been discovered. It is possible that either condition causes embryonic death, or that both chromosomes are responsible for gender selection.[1]

In Lepidoptera (moths and butterflies), examples of Z0, ZZW and ZZWW females can be found. This suggests that the W chromosome is essential in female determination in some species (ZZW), but not in others (Z0). In Bombyx mori (the commercial silkworm), the W chromosome carries the female-determining genes.

No genes are shared between the avian ZW and mammal XY chromosomes,[2] and from a comparison between chicken and human, the Z chromosome appeared similar to the autosomal chromosome 9 in human, rather than X or Y, leading researchers to believe the ZW and XY sex determination systems do not share an origin, but that the sex chromosomes are derived from autosomal chromosomes of the common ancestor. A paper from 2004 compared the chicken Z chromosome with platypus X chromosomes and suggested that the two systems are related.[3] The platypus has 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 ZW-equivalent chromosomes at the other end according to the paper.

See also


  1. ^ Smith CA, Roeszler KN, Hudson QJ, Sinclair AH (2007). "Avian sex determination: what, when and where?". Cytogenet. Genome Res. 117 (1-4): 165–73. doi:10.1159/000103177. PMID 17675857. 
  2. ^ Stiglec R, Ezaz T, Graves JA (2007). "A new look at the evolution of avian sex chromosomes". Cytogenet. Genome Res. 117 (1-4): 103–9. doi:10.1159/000103170. PMID 17675850. 
  3. ^ Grützner, F.; Rens, W., Tsend-Ayush, E., El-Mogharbel, N., O'Brien, P.C.M., Jones, R.C., Ferguson-Smith, M.A. and Marshall, J.A. (2004). "In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes". Nature 432: 913–917. doi:10.1038/nature03021. 

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