Why Inbreeding

Why Inbreeding?

by Anne Peters ©

Anyone who reads the literature about Thoroughbred breeding has got to wonder why all this fuss about inbreeding? Many of the articles here on the Pedigree Post site draw attention to inbreeding and linebreeding with a flourish. Can that be all there is to pedigree study?

Well, clearly not. There are many factors, genetic and environmental, that come into play when breeding livestock, especially race horses. But the reason why many pedigree students concentrate on inbreeding is because of the very definition of what it is and what it does on a genetic level. To put it simply, inbreeding is a long-term breeding strategy that is not for everyone, certainly not for the impatient breeder wanting immediate results.

By definition, "inbreeding" is the mating of two closely related individuals. The degree of relationship varies. "Closebreeding" refers to the mating of very close relatives such as sibling to sibling or parent to offspring matings. In the Thoroughbred, it is generally accepted that repeating any ancestor within four generations falls under their outer limits of "inbreeding," while any duplications beyond that four generation limit are described as "linebreeding."

Geneticists and livestock breeders use the term "linebreeding" to refer to any mating program (even closer than four generations) that repeats lines to a specific, presumably superior, ancestor. This is done in the hopes of increasing the chances of that ancestor's superior genes coming down to the resulting offspring. This is the purpose of linebreeding, to increase the potential for inheriting superior gene combinations. These same experts know that using this breeding method with closely related individuals (i.e., inbreeding and closebreeding) is a very risky business because while it may increase the possibility of inheriting superior genes, it also increases the possibility of inheriting any hidden, possibly negative recessive characteristics that may go along with this ancestor's genotype.

Let's use a prominent example. One of the best older horses in training this year is Juddmonte Farm's Skimming, who is inbred 2x3 to Northern Dancer, since his sire is Nureyev (by Northern Dancer) and his dam is a daughter of Lyphard (by Northern Dancer). Anyone who looks at Skimming can see that this was a highly successful experiment. Not only is he a truly superior runner, a multiple grade one winner, he is also exactly what one would expect from an inbreeding experiment to the genetic dynamo Northern Dancer. Northern Dancer was a small, muscle bound, fiery bay stallion with great speed and determination. Skimming is very much the image of his inbred ancestor, from size and body type to color, temperament, and running style.

Skimming is one successful example, in fact is one of the most dramatic examples of inbreeding to Northern Dancer, a trend that is very popular right now (and very successful, we should also add), but there are far more failed examples than we'd like to list here. Many horses that are the result of such close inbreeding to Northern Dancer have been nothing at all like Skimming. Not even close. Inbreeding can produce marvels, but it can also produce the worst of the worst. Ugly. Unsound. Slow. Bad tempered. Unhealthy. Infertile.

So why do many pedigree enthusiasts pursue inbreeding patterns in their matings? Because, as livestock breeders and scientists have known for hundreds of years, one of the quickest ways to "fix" desired characteristics into a bloodline of any species is through the use of inbreeding to individuals carrying those characteristics. If you want a strain of animals that breeds true to a certain rare color, you use only individuals with that color and ones that are known to breed true to that color. If you want to breed a strain of mice prone to produce tumors (for laboratory study), you inbreed using individuals from that strain.

By the same token, if you want to improve any bloodline of any species, one of the quickest ways is to inbreed to the most superior individuals. If you want to breed super cows, you inbreed to cows that were superior milkers, or to bulls known to produce superior milkers. If you want to breed super horses, you inbreed to superior horses.

If that's the case, then why aren't super horses all extremely closely inbred, because they most certainly aren't. Studies indicate that inbreeding 4x4 or closer appears in only about 25 percent of stakes winners, leaving 75 percent without this inbreeding factor.

The reasons for this also lie in the study of genetics. Closebreeding (of siblings or parent to offspring) is one of the surest way to reveal recessive traits, and these come out in close matings as often as the desirable traits. So at least half the time, usually more (depending on the quality of the parents) closebred matings result in animals that are effectively culls. The only way to eliminate the undesirable recessive traits entirely is by no longer using breeding animals known to carry those recessive or undesirable traits.

Continued, unrelenting inbreeding over a series of generations also has a negative impact that is quite real and is referred to as "inbreeding depression." The lines may become dominant for certain characteristics, but they also tend to become weaker, less vigorous individuals than animals that are not as inbred. They are also often physically smaller and "runty." Even if you inbreed for large size, extremely inbred animals from these lines may be large, but they will also tend to be less healthy, higher maintenance animals with more physical problems.

But the upside of this situation is that these animals will tend to breed true, because they can do little else but pass on the characteristics programmed into their genetics. When crossed with harmonious outside lines that need their strengths, or add strengths where there are weaknesses, miracles can, and do happen. These outcrosses, with one or both parents being inbred (but to different strains) often result in a condition known as "heterosis" or hybrid vigor. Heterosis is just the opposite of inbreeding depression, since it tends to produce an individual that is bigger and stronger than either parent. This is often the formula for producing superior performance horses.

So we've moved from close inbreeding and it's dangers, to outcrossing and hybrid vigor. What about the in-between?

Inbreeding within the fourth and sixth generations of the offspring avoids many of the problems of closebreeding, but also pushes the desired ancestor(s) influence further back. Since with each generation, a horse's genetic impact is theoretically reduced by half (Galton's Law of Ancestral Contribution), something has to be done to compensate. This is accomplished by using prepotent horses as targets of the inbreeding, that is, individuals known to pass on their characteristics in a highly predictable manner. So one's chances of doubling up on a dominant, positive characteristic are increased via linebreeding to a prepotent ancestor. And especially effective via "typical" individuals of that prepotent ancestors, as in the case of Skimming, who has the "typical Northern Dancer sons" Nureyev and Lyphard in his pedigree.

In fact, in many cases, inbreeding to a super runner may not be as effective as inbreeding to a superior breeding stallion because the great runner may not be "prepotent" or dominant in its genetics, while a top stallion almost always is. Many "super horses" (like the outcrossed Secretariat) are believed to be the highly vigorous result of crossing unrelated strains and getting the ultra-hybrid result. This is also why many of these super horses severely disappoint at stud, because being hybrids with heterozygous gene combinations (versus homozygous dominant combinations found in prepotent horses), they are can not, by definition, be prepotent.

So, let's put things in perspective. Inbreeding focuses specific genes. Inbred animals are likely conduits for certain specific characteristics coming from their inbred ancestor. Too much inbreeding is not good and rarely produces the superior runner. But inbred animals frequently make outstanding breeding stock because when outcrossed, superior hybrid individuals often result.

In other words, inbreeding, done intelligently, and pursued using good judgement, good horsemanship, and good animals, has the potential to create superior breeding stock. This breeding stock has the potential, with clever outcrossed matings that invoke some linebreeding to propotent ancestors, to create some outstanding runners based on the concept of hybrid vigor.

Inbreeding is not for everyone and not for every mating. There is a difference between breeding to get a top runner and breeding to get a future broodmare or stallion. More importantly, a mating is not good just because it has inbreeding to any given ancestor. First, the ancestor needs to be a superior, prepotent one. Second, there should be a pattern and a building toward a goal. This is what you see in the matings of the best horses by the best breeders.