Equine Traits Associated With the X Factor

Horses, like the majority of animals on our planet, have two chromosomes. These chromosomes govern every facet of the horse's being and represent a genetic map of the animal and its traits. The X chromosome is larger than the Y chromosome and can therefore carry more information or markers. It is these markers that control the attributes of the animal. A number of traits rely on markers from both the X and the Y chromosomes, but, according to an article by R. Soutar, heart size appears to be a function of the X chromosome alone.

  1. Heart size

    • Thoroughbred horses typically have a heart that weighs about seven pounds. Certain outstanding runners have had much larger hearts, though, often weighing as much as 14 pounds, and, in the case of the famous Secretariat, a phenomenal 25 pounds. Of the many factors that govern the performance of a racehorse, the size of the animal's heart is the most significant.

      Marianna Haun, a spokesperson for a group of geneticists and cardiologists, proposes that the genetic coding for a large heart is derived from the X chromosome, and she refers to this condition, as the X factor.

      Female animals have two X chromosomes, whereas males have a single X and a single Y chromosome. A mare or female horse therefore has twice the opportunity to receive a marker for a large heart. Because one of the X chromosomes in the mare is normally dominant, it is possible for her to be a carrier of the large heart trait, without having a large heart herself. This female's offspring however, will likely have large hearts.

    Disposition

    • According to Andrea Laycock Mattson, the disposition of a horse is likely influenced by the X-chromosome. She argues that trainability and overall intelligence is influenced by this chromosome alone. Through domestication, horses serve man in numerous ways. They are used as draft animals that pull carts, ploughs or other heavy loads and to drive cattle and herd other animals. Horses are used in sports such as polo and racing, and they entertain us in circus acts. The ability of horses to respond to training and to work willingly is crucial for any of the many functions they perform for man. Because these traits may be governed by the X chromosome, breeders and trainers of horses are interested in learning how the animal's disposition is inherited genetically.

    Hemophilia A

    • This disease of the vascular system, through which the horse hemorrhages easily, is inherited through the X chromosome by a number of breeds, including thoroughbreds, quarter horses and Arabs. The symptoms are noticeable after heavy exertion, such as a horse race, when the animal begins to bleed freely from the nostrils. Hemophilia A in equines is considered to be one of the severe inherited coagulopathies.