Inbreeding Demystified

There are few topics in the world of pedigree theory—and for that matter in the Thoroughbred community as a whole—that are more emotive than the subject of inbreeding. On one hand, inbreeding, and what is perceived as a consequential decline in genetic variety, is blamed for all manner of ills including a supposed decline in soundness. On the other, it has been an article of faith for some of the world’s great breeders.

There are two key aspects in regards to inbreeding, and one relates to the other. The first is the practice of selecting an ancestor (or ancestors) in the pedigree and creating a mating to inbreed to that ancestor. The second aspect, and a result of the first, is the concept of the overall inbreeding percentage of the horse to the nth generation (i.e. the number of unique individuals in what is a closed Thoroughbred breeding pool).

Firstly let us state that it would be both a false criticism and disingenuous to suggest that we are in some way "anti-inbreeding," or that critiquing inbreeding practice serves a purpose of promoting nicking (as if the two are mutually exclusive, which in some cases they are not). Our position at this moment is that correctly utilized with other considerations, inbreeding and linebreeding are good ways to reliably produce high-class runners, but that the most useful forms of inbreeding and linebreeding, like parallel patterns, are far too complex to submit themselves to genuine statistical analysis.

As far as the incidence of inbreeding to a single ancestor (mare or stallion) within any specific similar population, generally there is very little difference between a group of stakes winners and a group of non-stakes winners. This includes the likes of sex-balanced (well and truly put to bed by Roger Lyons 15 years ago, but it still abounds as a theory!), Formula One and Delta pattern inbreeding, and the often cited Rasmussen Factor. Most studies we have seen published are flawed in the way they've been set up to start with. If you do a study on the basis of "good" vs. "bad" horses, like the studies we have seen from the proponents of inbreeding, it gets skewed as the “commercial population” that produces a higher percentage of the good horses has a smaller gene pool than the dross. That is, the success of the commercial population has more to do with the quality of the sire and dam rather than the potency of inbreeding per se.

Two of the better studies that have been published (at least on the internet, rather than a peer reviewed journal) were those completed by noted pedigree statisticians David Dink and Adrian Parry. Both came at the issue in slightly different ways but ended up with the same result: inbreeding to individual horses, mares in particular, conveys no particular advantage. (For those that are interested, Dink has done separate inbreeding studies on the so-called "superior mares" like Somethingroyal, Almahmoud, Flower Bowl, Rough Shod, La Troienne, Gold Digger, and Natalma.)

In regards to the concept of inbreeding co-efficients (that is, the level of inbreeding that the whole pedigree has), we are of the belief that the industry would be well served in creating and publishing inbreeding co-efficients using The Jockey Club database, something that is a relatively easy process to do (why it hasn’t yet been done is a mystery!). These statistical inbreeding co-efficients would give breeders at least some guide as to the level of inbreeding that a particular foal or racehorse has. While they used slightly different populations, both Parry and Dink came to similar conclusions, that despite conferring no particular benefit, inbreeding is on the rise in terms of the percentage of horses that were inbred, rising from 5% in populations bred in the 1990s towards 10% for those bred in the last decade. Additionally, in a more recent statistical study completed by Dink, he noted that there has been a distinct decrease in the number of out-crossed horses in a 20-year period (from 16.5% to 12% of the population).

While there is a difference between the statistical measurement of inbreeding co-efficients and the actual level of inbreeding at a genomic level, it is worth noting that there have been two genetic papers recently released that seem to concur with the statistical findings of Parry and Dink. The first was a paper that explained the creation of an equine SNP chip that also allowed the authors to show the level of inbreeding that occurred in each of the breeds they studied. Of the 14 breeds studied, the level of actual genetic inbreeding were highest in the Thoroughbred and lowest in the Mongolian and the Quarter Horse. A further paper used the same SNP chip to discuss the concept of inbreeding as it relates to the Thoroughbred itself. The paper, based on approximately 450 horses, noted that the level of inbreeding in Thoroughbreds as measured at a genomic level has increased over the past 40 years, and more rapidly in the last 15 years, which coincides (it could be associative, not causal) with larger stallion books.

One thing is clear: neither paper associated the level of inbreeding with apparent lack of soundness in the breed as a whole. There are a number of people in the industry that believe that the rise in inbreeding has equated to unsoundness, but there is just no evidence to suggest this at this time.

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