Horse Genetics. Ernest Bailey
to outside horses. Today, all registered Standardbreds must have parents that are registered. Horses are readily traded between these continents and they have a common genetic profile. That being said, there is a significant genetic difference between trotting-bred and pacing-bred horses, despite having a common foundation stock. In effect, they appear to be two separate, genetic populations under the name of one breed registry!
Arab breed. The Arabian horse is one of the most ancient horse breeds and originated more than 2000 years ago in the Middle East, especially parts of Saudi Arabia, Iraq, and Egypt. The Arabian horse plays a major role in the culture and mythology of the Middle East. The Bedouin people who developed the breed were nomads who travelled between ancient Persia and Egypt with these horses. Undoubtedly, the Arabian horse shares ancestry with modern breeds identified as Ahkal-Teke, Turkoman horses, Persian Arabians, and the Barb or Berber of North Africa. These horses were prized for their athletic qualities and can be found in the ancestry of many modern horse breeds. Today the Arabian horse is prized for beauty and endurance in long distance races.
Warmblood breeds. The term “Warmblood” is used to refer to a type of horse developed in Europe since the 17th century for use in jumping competitions and dressage. Warmblood breeds include Hanoverian, Oldenburg, Trakehner, Holstein, Selle-Franҫais, Swiss Warmblood, Swedish Warmblood, and many other breeds. These are typically tall, athletic horses descended from crosses of local breeds and landraces of heavy horses, Arabian horses, Thoroughbred horses and Warmblood horses from other regions.
American Quarter horse breed. The American Quarter horse is known as the iconic horse of the American cowboy and descended, in part, from wild mustangs. Their name is derived from their immense sprinting ability; the founders of this breed were selected because of their speed when racing a quarter of a mile. Wild caught mustangs certainly formed part of the breed, but significant contributions were made by other horses, especially the Thoroughbred. Indeed, even today, there are rules allowing Thoroughbred-Quarter horse crossbred horses into the registry. Quarter horses are used for several different purposes including pleasure-riding, working cattle, racing, halter shows, and competitions such as cutting cattle and reining. Indeed, by some measures the Quarter horse breed appears to comprise several genetically distinct subpopulations based on selective breeding for different purposes.
Shetland pony breed. Shetland ponies are diminutive horses identified with the subarctic islands north of Scotland. Their small size is probably the selective consequence of the harsh living conditions and scarce foodstuffs on the islands, favoring small, hardy horses. Shetland ponies have been known on the islands since the Bronze Age and probably originated with crosses of Norse ponies and Celtic ponies. They are known as hardy, long-lived horses with docile personalities and are suitable for children to ride. Shetland ponies have been used extensively in crosses to develop the modern Miniature horse breed.
Icelandic horse breed. This breed originated with horses brought to Iceland in the 9th Century by Norwegians and probably crossed with other pony breeds. In 982, the Icelandic parliament passed laws prohibiting the importation of horses to Iceland, so the population has been closed to crossbreeding for more than 1000 years. The breed is well known for pacing and for performance of an ambling gait called the tölt.
Draft horse breeds. Draft horse breeds were developed in many areas of Europe and include breeds such as the Percheron, Suffolk Punch, Frisian, Belgian, Clydesdale, and Shire. These animals were very important for pulling, ploughing, and work associated with farming and early industry. Most of them began as landraces before their distinctive characteristics were appreciated and registries formed to preserve their special qualities and prevent their extinction following the advent of the combustion engine and their replacement on farms by tractors. Draft breeds include many of the largest animals within the species and breeders can be found all over the world.
Of course, there are many other fine horse breeds, not included in these descriptions. They are no less interesting and no less important than the breeds listed above. Aficionados of the other breeds can take some comfort in being spared the cursory treatments accorded here. As noted earlier, the point of highlighting these few breeds is to provide a reference for our discussions of genetics and to illustrate the varied histories and origins characteristic of horses. There are as many interesting stories as there are populations of horses and people who love them.
Genetic Relationships Among Breeds
We cannot use a single genetic trait to identify the breed of a horse. There are no Arabian horse genes or Thoroughbred horse genes. We may think of dish-faces as an Arabian horse trait, but if we look at enough horses in any large population we will find a horse with a dish-face. However, if we look at a field of horses and see lots of gray horses with dish-faces, we are going to think Arabian horse. While individual genes in a single horse do not characterize a breed, their prevalence in a population is characteristic of a breed. This is true with any genetic variant.
Consider the gene for albumin. Studies in the 1960s led to the identification of two biochemical forms, A and B. We can test herds of horses of different breeds for albumin, then determine the frequency of the two forms. Table 1.1 shows the frequency of the two albumin alleles in 13 breeds of horse.
Table 1.1. Frequencies of the albumin gene alleles among 13 horse breeds.
| Breed | Frequency ALB-A | Frequency of ALB-B |
| Andalusian | 0.59 | 0.41 |
| Arabian | 0.43 | 0.57 |
| Belgian | 0.35 | 0.65 |
| Icelandic | 0.43 | 0.57 |
| Lipizzaner | 0.08 | 0.92 |
| Miniature | 0.27 | 0.73 |
| Norwegian Fjord | 0.34 | 0.66 |
| Paso Fino | 0.42 | 0.58 |
| Quarter horse | 0.28 | 0.72 |
| Shire | 0.45 | 0.55 |
| Standardbred | 0.59 | 0.41 |
| Tennessee Walking horse | 0.44 | 0.56 |
| Thoroughbred | 0.17 | 0.83 |
From this table you can see that both alleles of albumin are found in all breeds of horses. The presence of either or both alleles in an individual horse tells us nothing about the breed of the horse. However, if we were to find a pasture with 100 horses of the same breed, typed them for albumin, found a gene frequency for B of 0.45, we would reject the suggestion that the horses were Lipizzaner, Miniature horses or Thoroughbreds. The frequency of the B allele would be too high. Of course, we would also draw this conclusion from other observations, but the main point is that the gene frequencies for albumin alleles gives us evidence for and against the breed membership for a herd of horses. As we test a larger