“Heritability” is another word that requires some explanation. You do not even hear it or read it all that often outside the field of genetics. It is mentioned only twice in Racehorse Breeding Theories, for example. Considering that “heritability” affects ALL breeding theories, you would think that the word would pop up more often than that in the course of a fair-sized book on the subject of breeding theories, but it does not.
The dictionary lists “heritability” only as the noun form of the adjective “heritable,” which is defined thusly: “1. Capable of being inherited : HEREDITARY. 2. Capable of inheriting or of taking by inheritance.” Which is not very useful.
My last post generated lots of comments, more than any other post I have ever written, which is gratifying. It also generated some back and forth among readers, which is also good. I had to step in only once to clarify a point.
At this point I will quote a particular exchange between two readers. Bill wrote:
“So If only the sire and dam are important. Then breeding a champion to champion should = a champion.”
Ned Williams replied:
“No. Still very difficult to breed a champion. The randomness of genetics makes it very difficult. However, your chances are improved. By this I mean you have a greater chance than if the champion + champion is 2,3,4 generations back in the pedigree.”
The latter’s response to the former was absolutely correct. It captures the essence of “heritability” without using the word. You might say that “heritability” is the “randomness of genetics” that makes transmission of racing class so difficult and unpredictable other than in general terms of probability. It is more frequently referred to as regression toward the mean, the average, the norm, take your pick of the word for “middle.”
Now I will quote two paragraphs from Racehorse Breeding Theories, Chapter 2, “Genetics and Gene Mapping,” page 23. Dr. Gus Cothran is an animal geneticist with the University of Kentucky.
“A sensible approach and a reasoned practicality are necessary for any successful breeder, and Cothran threw in a comment that ought to make anyone think long about his operation. He said, ‘Most studies of racing ability find that its heritability is about 30 percent, and that’s not surprising, when you consider what it is.’
“Racing ability is not a genetic trait like coat color. So it does not transmit from parent to offspring in the same way that a purely genetic characteristic would do. Athletic ability in the Thoroughbred is the result of many traits, many factors, and many relationships among the horse and his caretakers, health, environment, and so forth.”
The statement that heritability in the Thoroughbred is only about 30% means that genes account for only about 30% of the final outcome in terms of racing ability or class. All other factors account for the remaining 70%. Some studies say that heritability is more like 40%. It is somewhere in that range of 30%-40% anyway. And it is definitely less than 50%.
Now I am going to try to explain this matter in my own words, using an analogy (as usual). Imagine that it were possible to measure racing class in the Thoroughbred on a scale from 0 to 150 or so, with 100 being average, 150 being a “superhorse,” and 50 being the lowest-class claimer (albeit a winner).
What happens when you mate a sire who is a 140 (the equivalent of a G1 winner) with a mare who is also a 140????? As Ned Williams explained to Bill, you should definitely NOT expect that the average progeny of such a mating to be 140 as well. If heritability is 30%, you should expect the average progeny of such a mating to be a 112. If heritability is 40%, you should expect the average progeny of such a mating to be a 116.
How do I arrive at those numbers???? If you breed a 140 to a 140, you should expect to get a foal who is also a 140 only if heritability is 100% (which it is not). If heritability is 30%, then you would expect to get a 112 on average because 12 is 30% of 40, and 40 is how much both parents were better than average (100). So you should expect an average result of 112 (100 plus 30% of 40, or 12).
If you mated a 140 sire to a 130 dam in this same scenario, use the average of the sire and dam and multiply by .30. The average of the sire and dam would be 135. Three-tenths of 35 is 10.5. So you would expect a 110.5 (100 plus 10.5) on average from such a mating.
All this math makes a certain amount of sense if you think about it. Say that 140 is a G1 winner, 135 is a G2 winner, 130 is a G3 winner, 125 is a decent stakes winner, and 120 is a cheap stakes winner. Therefore, 112-116 would be a decent allowance horse. And a decent allowance horse is about what you would expect on average from mating a 140 to a 140.
But as Ned Williams pointed out, you have a much better chance of getting a 140 from the mating of a 140 to a 140 than from the mating of a 130 to a 130 (or a 120 to a 120 or a 110 to a 110 or a 100 to a 100 or a 140 to a 100, etc.). And increasing your chances of getting that 140 (a G1 winner or better) is what breeding is all about.
I should probably stop at this point and give readers some time to assimilate these thoughts. My next post will provide some specific numbers on how “heritability” decreases “influence” using as an example Northern Dancer and some other sires.