The Thoroughbred Speed Gene and What It Actually Predicts

The "speed gene" breeders and bloodstock agents talk about is a single, specific variant in the myostatin gene (MSTN), a gene that controls muscle growth. A 2010 genome-wide study found one version of it, written g.66493737C>T, to be the strongest pedigree-based predictor yet found of which distance a Thoroughbred is built to run best at. It is a real, published, peer-reviewed finding, and it still only predicts a tendency across groups of horses, not a result for any one of them.

Every horse carries two copies of this gene, one from each parent, which is why it is reported as C/C, C/T or T/T: C/C horses, the study found, race best at shorter distances (roughly 1,000 to 1,600 meters, about 5 to 8 furlongs), C/T horses suit middle distances (roughly 1,400 to 2,400 meters), and T/T horses show more stamina and tend to suit longer races (above roughly 2,000 meters, about 10 furlongs, in the range of the Kentucky Derby's own 1 1/4 miles).

A commercial test such as Etalon Diagnostics' speed gene test runs a sample from a horse (or an unborn foal, from the parents) and reports which of the three genotypes it carries. Breeders and buyers use this the way they use a pedigree: as one input into whether a particular mating, or a particular yearling, looks suited to sprinting or going a distance. It does not test speed, soundness, temperament or heart; it reports one gene's distance association, nothing more, and plenty of fast C/C horses and sound T/T stayers exist on either side of what the test would predict.

It is worth separating the speed gene test from the DNA testing that is actually mandatory in American racing. The Jockey Club has required DNA testing of every Thoroughbred foal since the 2001 foal crop, and it is used for parentage verification, confirming a foal's registered sire and dam are its actual parents, not for predicting performance. A foal's DNA sample, along with photos and a written description of its markings, is required before The Jockey Club will issue its registration papers. No performance gene, myostatin included, is part of that mandatory process.

A horse's myostatin genotype is not printed in a program, is not part of its official past performances, and is not something a track or a handicapping service reports on race day. Even where it is known, within breeding and bloodstock circles, it describes a tendency shared by a large population of horses with the same genotype, not a fact about the one horse in front of you; T/T horses with no stamina and C/C horses that stretch out and win at a mile and a quarter both exist. For wagering purposes, the actual past performances, the class level, the trip and the trainer and jockey, covered in handicapping horses for beginners, tell you far more about how a specific horse is likely to run today than any gene test could. Thoroughbred bloodlines covers what a pedigree in general can and can't say about distance.