The first edition of this book was published when DNA studies were first being applied to the horse. The second edition followed completion of the whole genome sequence of the horse. Now, seven years later, over 2000 horses have had their whole genomes sequenced, DNA tests using ancient horse bones have revolutionized our understanding of horse domestication, and the studies on regulation of gene expression have become the cutting edge of genetics research for the horse.
We do not need to be molecular geneticists to use this information. We can buy commercial DNA tests for coat color traits and disease susceptibility genes. Therefore, the purpose of this book is to provide basic information on genetics in an easy-to-understand format. As before, the book includes introductory chapters on basic genetics that review terminology and fundamental concepts. At the same time, the text contains very specific information about the DNA variants associated with traits, scientific speculation about how those traits came to be, and advice on how to apply the wide variation occurring among horses. It is not a book to read through at one sitting. The reader may want to simply peruse the table of contents and select sections pertaining to their interests. References are included to the original work if the reader wants details; however, these detailed studies are not necessary for a useful understanding of this new information.
The book covers a wide range of topics but not everything. This is an introductory text and we hope the lessons learned from the material in the book will make it possible to follow and understand the many exciting discoveries that did not fit or that are currently emerging areas of interest.
Finally, this edition is dedicated to the memory of two colleagues who shared a passion for horses and horse genetics, and made some of the discoveries described in this book:
Ann Bowling, PhD; June 1, 1943—December 8, 2000
Teri L. Lear, PhD; October 10, 1951—May 16, 2016
Respectfully,
Ernest Bailey, PhD
Samantha A. Brooks, PhD
2019
The horse captures our imagination because of its beauty, power and, most of all, its personality. Today, we encounter a wide array of horse breeds, developed for diverse purposes. Much of this diversity did not exist at the time of domestication of the horse, 5500 years ago (Chapter 2). Modern breeds were developed through genetic selection and based on the variety of uses of horses during the advance of civilization.
Domestication of the horse revolutionized civilization. A rider could go farther and faster than people had ever gone before. Horses provided power to till more land and move heavier loads. Any sort of horse could provide these benefits, as long as it could be domesticated. However, over time people became more discerning about the characteristics of their horses. The intuitive and genetic principle that “like begat like” led people to choose the best horses as breeding stock. At the same time, people in different parts of the world used different criteria when selecting horses. The horses were raised in different climates, fed different rations, exposed to different infectious diseases, and asked to do different types of work. Genetic differences could and did have a large impact on these traits. Over time, selection led to the creation of diverse types of horse around the world.
We use a variety of terms to describe the genetic diversity among groups of animals, both to distinguish horses from other animals and examine differences among the different types of horses. Those terms include genus, species, population, landrace, and breed. The domestic horse belongs to the genus “Equus,” a taxonomic category that includes Przewalski horses, donkeys, zebras, and Asiatic asses (Chapter 3). The species designation “caballus” is the taxonomic term used to delineate horses within the genus Equus and signifies that the members of this species have a common, recent evolutionary origin and are fully fertile when bred to other members of the species. All modern domestic horses, from tiny ponies to large draft horses, belong to the species, Equus caballus. Population is a term typically used to describe a group of animals that has been interbreeding for some time and shares hereditary characteristics. The people who domesticated horses probably encountered a single population of horses that lived nearby and had interbred for generations. In this way, the members of a population of horses share many genes. Before domestication, many populations probably existed, each with distinctive characteristics derived from which of their ancestors produced most offspring. Once humans began capturing and breeding horses, they certainly made choices about which individuals were most interesting to keep in their breeding herd. Over time, the characteristics of domestic horses will have diverged from the original genetic prototype and populations will have distinct characteristics.
Horse populations were initially constrained by geography and named for their region of origin. For example, Arabian, Shire, Exmoor, Norwegian Fjord, Andalusian, Sorraia, Hanoverian, and Ahkal-Teke are all names of modern horse breeds that originated in and were identified with specific regions of the world. Populations developed and selected in concert with regional influences are called “landraces.” Landraces can be quite distinctive and even form the basis for subsequent designation as a breed. The designation of breed is really a social one, and begins when pedigrees are recorded, quality standards identified, and breeders formally cooperate to develop the population. In the present day we value record-keeping, therefore “breed societies” are formed to administer breeding programs, promote the breed, and ensure the integrity of the breeding stock. The breed may have a distinctive geographical origin and the name of the breed may reflect that. Alternatively, the name may reflect distinctive phenotypic characteristics, as in the cases for Thoroughbred, Standardbred, Quarter horse, Appaloosa, Paint, and Warmblood horses. The major advantages of breed formation for horse owners is confidence in the unique characteristics of each breed. When you obtain a horse registered to a particular breed, you can have a pretty good idea about the temperament, personality, and abilities of that horse.
The differences between and within populations, landraces, and breeds are largely genetic. Parentage testing (Chapter 14) works for horses because of the extensive genetic variation found among horses belonging to the same population or breed. At the same time, horses within a population or breed will be more similar to each other than they will be to horses in other populations. Some of the genes determine coat colors, athletic abilities, conformation, and health. For example, Lipizzan are well known for having white coats, caused by a high prevalence of the gene for loss of hair pigmentation (gray) (Chapter 10). Arabian horses are well known for athletic endurance, so it is no surprise that a gene for endurance has a very high prevalence among them but a low prevalence among a breed renowned for sprinting ability, namely Quarter horses (Chapter 18). As a result, the lessons of horse genetics are often most clearly demonstrated with reference to well-established horse breeds. It falls outside the context of this book to fully describe all breeds; therefore, we encourage you to read literature that fanciers of each breed have produced, fully describing the history, breeding, and characteristics of their breed that endear them to their owners. You may also find a short description of most of the known horse breeds at the Oklahoma State University, Animal Science Department website on “Breeds of Livestock” (See Website section for URL). However, we conclude this chapter and begin the book with a brief description of a few of the breeds that we will mention in subsequent chapters.
Thoroughbred breed. This breed originated in England during the late 1600s and was used throughout
