The bottom line is, in the racing world, a âniceâ horse is not as important as a âwinningâ horse. Below: Mountain Horses must be certified for both gait and temperment before breeding, to ensure the best traits get passed on to the next generation.
Exceptions to these generalisations happen in the cases of XX males or XY females, or other syndromes. This system is in contrast with the ZW sex-determination system found in birds, some insects, many reptiles, and other animals, in which the heterogametic sex is female.
In this case, females are XX, while males are determined by a single X chromosome (and thus the O, read as nothing, because there is no chromosome analogous to the Y). So some of the gametes produced by males have an X, while others carry no sex chromosome at all.