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 number of genes, our evidence becomes stronger. However, we have never found a gene which is characteristic of a breed. While horses of all breeds can have albumin allele A and albumin allele B, the frequency of the alleles in a population of horses provides a small bit of evidence for the breed of origin.
A single locus does not give us a lot of power to determine the breed of a population of horses. We need a lot of genetic markers to effectively distinguish between breeds. With the advent of DNA testing, we have millions of DNA variants to use in comparing horse populations. Another application of these genetic markers is to identify the genetic relationship among populations. The point is that closely related populations and breeds share more genetic characteristics than distantly related populations.
Through the efforts of collaborating scientists around the world, DNA markers were developed for the horse and have been used widely to compare horses. These studies have benefited greatly from the increased use of parentage testing by horse breed registries around the world and adoption of a common set of microsatellite DNA markers for routine testing.
The development of genetic markers based on single nucleotide polymorphisms (SNPs) of DNA (Chapters 4 and 6) provided greater resolution for investigations of relationships among horse breeds. Fig. 1.1 illustrates the results of analyses of genetic distance in 38 horse populations using 6028 SNP markers (Petersen et al., 2013).
Fig. 1.1. Majority rule, neighbor join tree created from 6028 SNP markers using Nei’s genetic distance and allele frequencies within each population. The percentage bootstrap support indicated on all branches was calculated from 1000 replicates. The approach creates a “family tree” of horse breeds where the length of the branches is proportional to the genetic distance between two populations. (Reprinted with permission from Petersen et al. 2013.)
Not all breeds or populations of horses appear on this tree. However, an effort was made to select horses from different parts of the world and with widely divergent breeding histories. Not surprising to students of horse breeds, the Arabian horse appears in the middle of the tree with the Thoroughbred breed at the opposite end from the pony breeds and draft horse breeds.
Summary
• Population, landrace, and breed are all terms used to designate related groups of horses and reflect their relationship to each other and the level of selection applied by breeders.
• From the time of domestication, diverse breeds of horses have been developed based on selective breeding.
• Genetic markers can be used to measure differences among these different populations.
References
Petersen, J.L., Mickelson, J.R., Cothran, E.G. et al. (2013) Genetic diversity in the modern horse illustrated from genome-wide SNP data. PLOS One 8: e54997.
Website
Oklahoma State University, Animal Science Department webpage on Breeds of Livestock: http://afs.okstate.edu/breeds/horses (accessed February 13, 2020).
2 Evolution and Domestication of Horses
The special relationship between horses and people began approximately 5500 years ago. However, the horse encountered at the dawn of domestication was already the product of more than 50 million years of evolution occurring across several continents (reviewed in MacFadden, 1992). Changes in climate, geography, and interactions with the natural flora and fauna influenced the genes and gene combinations that were successful in each generation. By the time that people encountered the horse, it was large, strong, fleet, and social. These traits facilitated the special relationship between horses and people.
Evolution and Migration of Early Equids
Earliest ancestor of horses
The earliest recognized ancestor of the modern horse belonged to a species in the clade hyracothere (an animal or fossil of the genus Hyracotherium). Hyracotheres were also referred to as Eohippus, for “dawn horse”, and made famous by Thomas Huxley’s 1876 humorous cartoon showing Eohomo (dawn man) riding Eohippus (Fig. 2.1).
Fig. 2.1. Thomas Huxley’s 1876 cartoon of Eohomo and Eohippus (courtesy of the Division of Vertebrate of Paleontology, Peabody Museum of Natural History, Yale University.)
Such an event as a man riding an Eohippus never occurred since Eohippus (aka Hyracothere) existed over 50 million years ago and our genus, Homo, only appeared in the evolutionary record within the last 2 million years. Hyracotheres were small, perhaps the size of a small dog, and browsed leafy vegetation across North America and Europe. The hyracotheres present in North America became isolated from the rest of the world when rising waters submerged the land bridges that existed between the American and other continents. The modern horse is descended from just one of the species
