Z chromosome

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The Z chromosome is a sex chromosome found in birds and some reptiles , fish and insects . In contrast to mammals , males have the homozygous karyotype ZZ, while females have the hemizygous karyotype WZ.

Layout and function

Analogous to the XY chromosome ratio, the Z chromosome is much larger than the W chromosome. It has been best researched so far in the domestic chicken , where a length of approx. 74,600,000 base pairs is described, which contain 793 genes, of which the function of 668 is known so far (as of August 2010). In contrast, the W chromosome is only 260,000 bp long and contains only two genes.

It is not exactly known how the homozygous Z chromosome leads to the development of the male phenotype . A homolog to the SRY region of the Y chromosome has not yet been found in the WZ system. A dose-dependent effect of one or more genes on the Z chromosome may lead to the expression of the gender phenotype. The DMRT1 gene on human chromosome 9 , which has an ortholog on the Z chromosome, comes into question .

evolution

Based on DNA analyzes, it is assumed that, analogous to the XY system, the evolution of genetic sex determination came about through the gradual elimination of recombination between the Z and W chromosomes.

Consequences

Since male animals have two copies of the Z chromosome, while females only have one copy, the inheritance of sex-linked traits is the reverse of the inheritance that is known from XY species : Recessive alleles on the Z chromosome are phenotypically more common in females than in males Animals trained. A general Z-inactivation in male animals (analogous to X-inactivation in female mammals) does not seem to take place. It is unclear whether and how a dose compensation of the double Z chromosome occurs in males.

Individual evidence

  1. Ensembl Chicken Map View - Z-Chromosome , accessed September 1, 2010
  2. Ensembl Chicken Map View - W chromosomes
  3. S. Mizuno et al .: Z and W chromosomes of chickens: studies on their gene functions in sex determination and sex differentiation . In: Cytogenetics and Genome Research . 2002, No. 99, 2002, pp. 236-244. doi : 10.1159 / 000071599 . PMID 12900570 .
  4. I. Nanda et al .: Conserved synteny between the chicken Z sex chromosome and human chromosome 9 includes the male regulatory gene DMRT1: a comparative (re) view on avian sex determination . In: Cytogenetics and Genome Research . 89, 2000, pp. 67-78. doi : 10.1159 / 000015567 . PMID 10894941 .
  5. Jump up ↑ Evolutionary Strata on the Chicken Z Chromosome: Implications for Sex Chromosome Evolution
  6. H. Ellgren: Dosage compensation: Do birds do it as well? . In: Trends in Genetics . 18, No. 1, 2002, pp. 25-28. doi : 10.1016 / S0168-9525 (01) 02553-7 . PMID 11750697 .
  7. ^ A. Kuroiwa et al .: Biallelic expression of Z-linked genes in male chickens . In: Cytogenetics and Genome Research . 2002, No. 99, 2002, pp. 310-314. doi : 10.1159 / 000071609 . PMID 12900580 .
  8. H. Satoshi et al. (2009): XY and ZW: Is Meiotic Sex Chromosome Inactivation the Rule in Evolution? In: PLoS Genetics 5 (5): e1000493; PMID 19461890 , PMC 2679206 (free full text)