Defining a Breed

Genetics expert Phillip Sponenberg, DVM, PhD, addresses the differences between "breed" and "purebred." By the Editors of EQUUS magazine.
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Genetics expert Phillip Sponenberg, DVM, PhD, addresses the differences between "breed" and "purebred." By the Editors of EQUUS magazine.

Question: I am trying to find a good definition of the terms "breed" and "purebred" to be able to distinguish the difference between the two. Also, what is the rule on how many generations of breeding strictly within a breed it takes to be considered purebred?

Answer: I am afraid you are looking for clear-cut definitions and hard-and-fast rules that don't exist. When speaking of horses, more than other livestock species, several different concepts for "breed" emerge. Each of these has legitimacy, but each is very, very different.

The narrowest definition is that a breed is a group of animals distinct enough by appearance to be logically grouped together and that when intermated produce that same appearance. To the criterion of similar appearance could also be added "similar by descent (relationship) or geographic origin." Under this definition, the concept of breed is one of genetic consistency. This is the most useful definition if predictability is what is desired from a breed.

Most horse breeds with closed studbooks--meaning only foals with registered parents may be registered--do satisfy the requirements to be considered as breeds under this definition, but only in the narrowest genetic sense. Extremes include the Icelandic Horse, which has been geographically and, therefore, genetically, isolated for 1,000 years, and breeds such as the Thoroughbred, which has been more artificially isolated for a few hundred years.

A few horse associations add restrictions to the genetic breed concept so that not all horses that are born of registered parents are automatically registered. For these breeds, registered parents are essential for registration but only make foals eligible. Further evaluation or testing might be required, or the foal might have to meet certain other requirements, such as color. Many European breeds, in their homelands, require varying levels of testing for horses to be registered as eligible to reproduce. Other examples are Cleveland Bay and Friesian horses, in which color restrictions apply so that chestnut foals of either breed are not eligible for registration even though the parents may have been registered.

Other horse breeds, and many of the most popular breeds in this country, have "open" herdbooks, allowing outcrosses produced from matings with other designated breeds. Quarter Horses allow Thoroughbred outcrosses, Paints allow Thoroughbred or Quarter Horse outcrosses, and Appaloosas allow Thoroughbred, Quarter Horse or Arabian outcrosses. The result of having herdbooks open, but open in only a restricted direction, is that the core identity of the breed as genetically based is really not hampered much. These breeds form what could be called, in a larger sense, the "Western Stock Horse," which is heavily influenced by Thoroughbred breeding. Selection pressure keeps them somewhat distinct by favoring a different type. They are less consistent than a very tightly defined genetic breed (Friesian, for example) but are still consistent enough to be predictable and much more predictable than other more completely open registries. However, if breeders are more interested in the traditional, foundation type within these breeds, then they mourn the inclusion of any outcrossing. At the furthest extreme are registries that are one notch more closed than "wide open," such as many of the strictly color-based registries, including pinto, gray and a few others.

"What is a purebred horse?" then becomes a pretty tricky question. At one extreme the answer could well be "any registered horse," although the consistency arising from generations of mating within a defined genetic pool are clearly lacking with such a definition.

At the other extreme is some sort of insistence on predictability and genetic consistency, which can come from isolated matings within a specific restricted population. Thoroughbreds and most heavy draft breeds are good examples of such a strategy.

The issue of "When does a population become purebred?" likewise has no easy answer. If two very different and genetically distant breeds, Arabians and Belgians for example, are crossed, and then a herdbook established, it might well take seven or eight generations of breeding, with very intense selection, for genetic repeatability to be achieved. If old-style Oldenburgs and old-style Hanoverians, two breeds close in genetic relation and appearance, are crossed, the result could be achieved much more quickly, perhaps in two or three generations.

The key importance of pure breeds is that they represent real, repeatable, predictable choices for horse owners. To some extent modern horse breeding has converged on a very few popular international types. Most breeds are slowly (through selection) or quickly (through outcrossing) being transformed into those types. This trend means that future horse owners may not have as wide an array of choices in horse types as have we, or as have previous generations. Fortunately, the American Livestock Breeds Conservancy ( is working diligently with breeders of rare horse breeds to ensure that future generations do have a choice.

D. Phillip Sponenberg, DVM, PhD
Virginia-Maryland Regional College of Veterinary Medicine
Blacksburg, Virginia

This article first appeared in the March 2003 issue of EQUUS magazine