W chromosome

Barcellos LF, Thomson G, Carrington M, Schafer J, Begovich AB, Lin P, Xu XH, Min BQ, Marti D, Klitz W. Chromosome 19 single-locus and multilocus haplotype associations with multiple sclerosis evidence of a new susceptibility locus in Caucasian and Chinese patients. JAMA 1997 278 1256-1261. 

Heteromorphic sex chromosomes were hypothesized to have evolved from a homologous pair of chromosomes by the suppression of crossing-over between the ancestral pair of sex chromosomes and subsequent loss of gene function on the ancestral Y (or W) chromosome.2 One prediction of this hypothesis is that recombination between the X and Y chromosomes (or between the Z and W chromosomes) will be reduced or nonexistent. This prediction is well supported for eutherian mammals with rare exception, recombination between the X and Y chromosomes is reduced to a small region referred to as the pseudoautosomal region. As a consequence, most of the mammalian Y chromosome is isolated in the genome and is thus clonally inherited from father to son. 

In birds, the female possesses the heteromorphic pair of chromosomes, having the Z and W sex chromosomes, whereas the male has a pair of Z chromosomes. The W chromosome is much smaller than the Z chromosome, being the size of a microchromosome. In most species examined, the W chromosome also has a high proportion of heterochromatin, usually GC-rich (Fritschi Stranzinger, 1985 Auer et al., 1987 Mayr et al., 1989 1990). Chandra (1994) suggests that the W sex chromosome shows the characteristics of an inactive chromosome in somatic cells and could be regarded as facultative helerochromatin. 

In the domestic fowl, the W chromosome comprises 1.4% of the DNA present in the nucleus, compared with 10% for the Z chromosome (Tone el al, 1982). Being small, and only present in the one sex, its molecular structure has been easier to study than that of any of the other chromosomes, although at present no genetic loci have been definitely assigned to it. There have been suggestions that a sex-deteimining antigea HW, may be associated with the W chromosome (McCarrey el al, 1981), but this... 

Fig. 8.7. The sequence of the Xhol himily from the W chromosome of the domestic fowl (Kodama et al, 1987). There are 34 tandem repeat sequences. Each sequence is between 19-22 nucleotides long, giving a total of 715 nucleotides. The consensus sequence, consisting of clusters of (A) and (T) , is shown at the bottom.

Chandra, H.S. (1994). Proposed role of W chromosome inactivation and the absence of dosage compensation in avian sex determination. Proc. R. Soc. Land., 258B, 79-82. 

Kodama, R, Saitoh, R, Tone, M Kuhara, S., Sakaki, Y. Mizuno, S. (1987). Nucleotide sequeiKes and unusual electrophoretic behaviour of the W chromosome-specific repeating DNA units of the domestic fowl. Callus gallus domesticus. Ckromosoma, 96, 18—25. 

Saitoh, H. Mizuno, S. (1992). Distribution of Xhol and EcoRI family repetitive DNA sequences into separate domains in the chicken W chromosome. Chromosoma, 101, 474-7. 

Tone, M., Nakano, N., Takao, E., Narisawa, S. Mizuno, S. (1982). Demonstration of W chromosome-spedfic repetitive DNA sequences in the domestic fowl Gallus gallus domesticus. Chromosoma, 86, 551-69. 

Tone, M., Sakaki, Y., Hashiguchi, T., Mizuno, S. (1984). Genus spedfidty and extensive methylation of the W chromosome-specific repetitive DNA sequences from the domestic fowl, Gallus gallus domesticus. Chromosoma, 89, 228-37. 

Curtis, D. Horobin, R. W. Chromosome banding specification of structural features of dyes giving rise to G-banding. Histochem. J. 1982, 14, 911-928. 

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