Читать книгу Horse Genetics - Ernest Bailey - Страница 63

Two of a horse’s 64 chromosomes are called the sex chromosomes. The gender of a horse is the result of action by genes on the sex chromosomes. The sex chromosomes are designated X and Y. Females inherit two copies of the X chromosome. Males inherit an X chromosome and a Y chromosome. In mares, the two X chromosomes pair during meosis, undergo genetic recombination and each egg possesses one of the X chromosomes. During meiosis in the male, the X and Y chromosome pair and each sperm inherits either an X or Y chromosome. The pairing of the X and Y chromo some during meiosis seems exceptional because the X and Y chromosomes are quite different in size and gene content. However, they share DNA sequences for a very short region, called the pseudoautosomal region (PAR) and this is the basis for them to pair during meiosis. The genomics and cytogenetics of X and Y chromosomes are discussed further in Chapters 6and 17.

The consequences of sexual reproduction are that the gender of a foal is determined by the contribution from the sire, not the dam. The dam always contributes an X chromosome to her offspring. The sire contributes sperm with either X or Y. As a consequence, the sex of the offspring is determined by the contribution of the sire—specifically, whether he contributes an X or Y chromosome.