Principles of Genetics and Breeding

4.1.7.3 WXY - system

• This system is reported in platy (Xiphophorus maculatus).
• Y- chromosome produces males except when it is paired with a W chromosome. The W chromosome is a modified X chromosome that blocks the male determining function of the Y chromosome.
• There are three sex chromosomes in X. maculatus, W, X, and Y.
• Three combinations of sex chromosomes, WY,WX and XX cause female differentiation, and two combinations, XY and YY give rise to males.
• In wild population, six kinds of matings occur (Table 6.1) of which four result in an even sex ratio, one gives rise to a ratio of 3 females to 1 male, and one yields all male progeny.

Table 6.1. Sex ratio of the Platy fish, Xiphophorusmaculatus

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Mating Sexratio Genotypes of offspring

♀ x ♂ ♀♀ : ♂♂ ♀♀ : ♂♂

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WY XY 1 1 WX,WY XY, YY

WY YY 1 1 WY YY

WX XY 3 1 WX,XY XX XY

WX YY 1 1 WY XY

XX XY 1 1 XX XY

XX YY 0 1 ---- XY

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