Platys (Xiphophorus maculatus) are fascinating animals. One of the most peculiar aspects of Xiphophorus is how sex is determined. In human beings, sex is determined by a pair of chromosomes (long strands of DNA). Females have two of the same kind; an XX pair, while males have two different kinds of chromosomes, a Y and an X.
"In the platyfish, X. maculatus, three different sex chromosomes: W, X, and Y occur. Females may have WY, WX or XX genotypes. The males are either YY or XY. Interestingly YY males are fully viable and fertile, indicating that no essential genes have been deleted during the evolution of the Y." [Source]
This is very interesting because this means there are two different kinds of stable strains. There are XX/XY strains, and WY/YY strain. When a breeder crosses two different strains, the resulting offspring can have some very skewed sex ratios. For example, when a breeder crosses a YY male to an XX female, the resulting offspring will all be XY males! Or if an XY male is crossed to a WX female, 75% of the offspring will be female, and 25% will be male!
YY x XX: 100% XY (males)
YY x WX: 50% XY (males), 50% WY (females)
YY x WY: 50% YY (males), 50% WY (females)
XY x XX: 50% XY (males), 50% XX (females)
XY x WX: 25% XY (males), 25% XX (females), 25% WX (females), 25% WY (females)
XY x WY: 25% YY (males), 25% XY (males), 25% WX (females), 25% WY (females)
This nifty little trick, if utilized correctly, can be used by a breeder to get more females than males!
"In the platyfish, X. maculatus, three different sex chromosomes: W, X, and Y occur. Females may have WY, WX or XX genotypes. The males are either YY or XY. Interestingly YY males are fully viable and fertile, indicating that no essential genes have been deleted during the evolution of the Y." [Source]
This is very interesting because this means there are two different kinds of stable strains. There are XX/XY strains, and WY/YY strain. When a breeder crosses two different strains, the resulting offspring can have some very skewed sex ratios. For example, when a breeder crosses a YY male to an XX female, the resulting offspring will all be XY males! Or if an XY male is crossed to a WX female, 75% of the offspring will be female, and 25% will be male!
YY x XX: 100% XY (males)
YY x WX: 50% XY (males), 50% WY (females)
YY x WY: 50% YY (males), 50% WY (females)
XY x XX: 50% XY (males), 50% XX (females)
XY x WX: 25% XY (males), 25% XX (females), 25% WX (females), 25% WY (females)
XY x WY: 25% YY (males), 25% XY (males), 25% WX (females), 25% WY (females)
This nifty little trick, if utilized correctly, can be used by a breeder to get more females than males!