Sex determination and the restriction of sex-linked pigment patterns to the X and Y chromosomes of a poeciliid fish, Xiphophorus maculatusfrom the Belize and Sibun rivers of British Honduras. In these species, sexual development is decided from a major sex-determining region [ 1 ], which triggers a cascade of sex-specific genes that control development into a male or female [ 23 ].
Dosage compensation is a common phenomenon taxonomically, but varies in its extent; it is almost complete in mammals, but is partial in birds and some snakes [ 12 ]. Early empirical studies found evidence of population variation in sex chromosomes [ 4253 ], but a meta-population approach has rarely been applied to models of sex chromosome evolution humans have two sex chromosomes in Darlington 94 ].
Proc Biol Sci. Westergaard M.
Barbara Susman. McClung CE. Steinemann S, Steinemann M. Reversal of an ancient sex chromosome to an autosome in Drosophila. Turnover of sex chromosomes and speciation in fishes. Hum Genet —
It is the most common cause of intersexuality in females with 46XX, where untreated girls develop an outwardly male appearance. See also X trisomy ; XYY-trisomy. Humans have 23 pairs of chromosomes, one half of each pair inherited from each parent. Main article: ZW sex-determination system.
The old Y is lost. Tentative assignment by deletion mapping to Xp This is particularly interesting in the context of sex chromosome evolution, since the model did not assume any epistasis or cost of adaptation to a specific environment i.
If the populations stay separate, the locally adapted loci will be linked to male and female sex-determining regions equally often. Lassi Alvesalo, Evelyn M. Steps in the evolution of heteromorphic sex chromosomes.