The Thoroughbred horse is so inbred that every horse is more closely related to all other Thoroughbreds than first cousins.
Inbreeding of course, in any species tends to fix desirable qualities such as size, color, temperament, agility, strength, intelligence, speed, reaction time and behavior qualities. However, it also tends to perpetuate defects that may be anatomical, physiological, or behavioral. The people who originate breeds of domestic animals are usually unaware of the defects within the breed until many generations have gone by, and when it is too late or, at least, very difficult to eradicate the defect.
The Thoroughbred horse, unlike most horse breeds, is remarkably free of genetic defects. I’m not saying that some defects do not exist, but, compared to most breeds, there are relatively few.
Why is this so?
Because the Thoroughbred has not been bred primarily for its appearance, gaits, or performance except for racing.
If a Thoroughbred stallion didn’t race well, nobody wanted to breed to him. Moreover, if there were very serious physical or behavioral problems, he probably wouldn’t be a desirable stud.
So horseracing is one reason most Thoroughbreds are reasonably physically sound (at least before they begin to race) and reasonably sound mentally (at least before inept handling has screwed them up).
Secondly, the Thoroughbred Association’s rules require direct cover. That means the stallion must naturally breed the mare.
There is some disadvantage in direct cover. The mare must be transported to the stud farm. Often she has a very young foal at her side. Thus the foal (and the mare) may be exposed to stress, trauma, and communicable diseases.
What the direct cover requirement assures is that the stallion can produce a limited number of foals each year, so that if a genetic defect sneaks in, it will not pollute, potentially many hundreds of foals.
If, by contrast, artificial insemination, (A.I.), frozen semen, etc. are allowed, it is possible (indeed probable) that genetic defects can threaten a huge number of horses within the breed.
For example, The American Quarter Horse, a recent registry (The A.Q.H.A. was formed after World War II) is now threatened by a dozen serious genetic defects.
This great breed, whose origins are very mixed and go back to colonial days, is now plagued with too many physical problems, mostly spread by stallions who were show or track winners, but whose genes included susceptibility to various skin, muscle and other defects, or increased propensity for ailments due to performance trauma.
The A.Q.H.A. is now requiring genetic studies hoping to eliminate these inheritable problems, but it will require great diligence, a long, long time, and strict enforcement to eliminate these problems from the gene pool.
Unfortunately, if a horse has won a lot of money at the racetrack, or in the show arena, many people will want to breed to him, even if they know about the problems.
Evolution encourages survival of the fittest. That’s Natural Selection. Artificial selection is often the result of human ego and/or greed.
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