Thoroughbred breeding and genetics are things that should go hand in hand, but surprisingly, the race horse industry has never fully embraced the concept of using the science of genetics. Perhaps it’s the tendency for the wealthy owners identifying with their Thoroughbreds as a sort of aristocracy, and applying mating practices more akin to dynastic marriages than livestock breeding. 

In the latter half of the 20th Century, certain genetic traits were being identified and studied, but few applied to the race horse specifically. Coat color genetics became important in other horse breeds, but since the Thoroughbred has no color standards, it was more of a curiosity when white, spotted and even a rare few palomino Thoroughbreds made news just by being born. Genetic disorders, such as HYPP (hyperkalemic periodic paralysis) in Quarter Horses or SCID (severe combined immune deficiency disease) in Arabians were important discoveries, but did not affect Thoroughbreds. 

A few heritability studies were made on specific traits, but the one with the most significance to racehorses were those indicating racing speed was influenced upwards of 33% by heredity. This served to point out that there are many other factors at work in addition to genetics in the outcome of any given race horse, including nutrition, training and the competence of the horsemen around it. 

In 2001, The Jockey Club in North America began using DNA samples to verify parentage, replacing the old standard of blood typing, but that’s about as far as using the new science went until 2007. That’s the year the Equine Genome Project completed the mapping of the horse’s genetic code.

This still left the Thoroughbred industry rather unchanged, content to breed horses as they had been since the late 1600s, “breeding the best to the best” with hit-or-miss results.  There were studies done comparing race times, which suggested that Thoroughbred weren’t getting any faster, that the breed had reached its peak evolution. It was never pointed out that very few mare owners had actually attempted serious livestock breeding techniques, like stallion testing, and genetic studies as they had in other breeds and species of livestock where dramatic improvement was achieved in a relatively short period of time.

That was the status quo until 2010, when Dr. Emmeline Hill and her team at Trinity College in Dublin, Ireland, announced their research on a gene (identified as MSTN) tied to the development of myostatin, a protein critical to the specific muscle tissue used in locomotion, like galloping.  This is the so-called “Speed Gene.”  Hill’s studies showed that combinations of alleles for this gene (identified as C:C, T:T or C:T) determined whether a horse was engineered to be a sprinter or a stayer, or something in-between.   The gene combination that indicated heavier muscling (C:C) also tested positive for earlier maturity but limited distance ability. The combination for longer and leaner muscling (T:T), also produced later maturing horses who got better as distances stretched beyond a mile.  The combination that was a mix of the two alleles (C:T) resulted in a horse with more versatility than either allele in its homozygous or pure form, both for maturity and best racing distance.

Many horseman replied with a shrug.  It had long been observed that sprinters and stayers were two different body types with different rates of maturity, and also that crossing the two often resulted in something in-between, but the predictability of the cross was uncertain. This wasn’t earth-shaking, or was it?  Dr. Hill’s findings were remarkable in the simplicity of the thing, that one single gene was responsible for all of this instead of several genes working in concert, as had been originally suspected. 

Dr. Hill pointed out that once you knew a horse’s genotype, you could better manage it as a young horse and avoid pushing it in the wrong direction too early. Furthermore, once you knew the genotype of a stallion or mare, you could mate them with more certainty.  A horse that was C:C crossed on another C:C would always result in a C:C baby, just as a T:T stallion with a T:T mare would always result in a T:T foal. The hard part came in crossing the mixed types. A horse that was C:C on a C:T would get a C:C 75% of the time, and a C:T 25% of the time.  Crossing a C:T on a C:T was even harder to predict, with possibilities of C:C 25%, C:T 50% and T:T 25%.  Knowing the genotype can help tilt the odds, or help a breeder decide to keep or cull. 

Several genetic testing services have put themselves on the market. All offer a variation of the myostatin test, with refinements, including predicting the level of performance (elite or not) and optimal racing distance. There are also specific genetic markers that indicate height at maturity, which can be helpful in evaluating a young horse’s development, and markers that suggest a horse’s favored racing surface, dirt or turf.

Studies in mitochondrial DNA, which is passed from a mother to her offspring, were initially useful in testing the female lines that had been recorded since the beginning of the General Stud Book. The results proved some female lines to be perfectly accurate, while others did not match up to their pedigrees, meaning there was some human error along the line. Dr. Stephen Harrison has shown that mitochondrial DNA, which is related to energy production within the cell, has a direct effect on the equine athlete’s energy production and stamina.  

Recent research has found a genetic link to an airway problem commonly known as “roaring,” or recurrent laryngeal neuropathy (RLN). Taller horses are more susceptible to RLN, as are their foals. Studies of “bleeders,” horses susceptible to exercise induced pulmonary hemorrhage (EIPH), also indicate a genetic component, although the actual genes have not been identified.

There is still much work to do in genetic research with race horses, but also much promise if breeders are willing to pay attention to the science.