A study being called the most comprehensive genetic analysis of North American Thoroughbreds found no genetic ties related to health concerns or durability and detected a slight rise in inbreeding that researchers concluded "isn't inherently problematic."
Researchers from the University of Kentucky Martin-Gatton College of Agriculture, Food and Environment in collaboration with the University of Nebraska-Lincoln, University of Minnesota, and University of California-Davis all contributed to the study that was published in Scientific Reports.
Using whole-genome sequencing, called WGS in shorthand, the research team analyzed the DNA of 185 Thoroughbreds born between 1965 and 2020. The study's goals included generating data that could be used to address concerns about inbreeding and genetic diversity within the breed, while also offering breeders valuable insights to guide future decisions.
"The impetus for this study was to quantify the extent of genetic variation and inbreeding in Thoroughbreds," said Ernest Bailey, professor in the Maxwell H. Gluck Equine Research Center and the study's lead author. "By identifying trends, we're equipping breeders with the data needed to make informed choices that preserve the breed's health and performance."
The study compared two groups of Thoroughbreds: 82 born between 1965 and 1986, and 103 born between 2000 and 2020. From more than 14 million genetic variations identified, researchers found that the horses from the older generations carried slightly more genetic diversity, while horses in the younger generations showed modest increases in inbreeding.
"Selective breeding focuses on enhancing desirable traits like speed and stamina, but it can also increase the risk of amplifying harmful genetic variants," said Jessica Petersen, associate professor at the University of Nebraska-Lincoln Department of Animal Science. "Our data gives breeders a clearer roadmap and tools to navigate these challenges."
2011 Study RAiseD Inbreeding ConcerNs
Previous studies have raised concerns about inbreeding trends in Thoroughbreds.
In a study published in 2011, the genotyping of 467 Thoroughbreds born between 1961-2006 showed an increase in the average inbreeding coefficient. More significantly, the study notes, the majority of the increase occurred during 1996-06, when the number of North American stallions breeding 100 or more mares in a given season rose from 14 to 128.
This study's conclusions also noted that the loss of genetic diversity observed is not excessive but still found the sharp rise over the 10-year period a concern. A decline in average starts per runner, at 11.31 in 1960 to 5.87 in 2023, has raised questions about whether this trend has roots in genetics or whether it is related more to changes in how racehorses are managed.
The latest study headed by the University of Kentucky found no evidence of genetic issues related to durability or other health issues. By analyzing "runs of homozygosity"—stretches of identical DNA inherited from both parents—the researchers detected a slight rise in inbreeding in horses in the younger group but stressed that it isn't inherently problematic.
"Inbreeding can help solidify positive traits, like speed or endurance," Petersen said. "However, it's also important to monitor and avoid harmful genetic combinations. With tools like WGS, scientists can proactively identify and manage risks before they impact the breed."
Occurence of 'Speed Gene' On The Rise
One notable discovery was a 10% increase in the frequency of a genetic variant linked to the "speed gene," which is the gene related to myostatin—a protein related to muscle growth and the type of muscle fiber development. This gene has been associated with a racehorse's optimal distance performance. Research has shown 83% of racehorses with the C:C variation of this gene performed best in races of a mile or less, while those with the T:T variation performed best at distances greater than a mile.
The trend in North America, according to study by the University of Kentucky et al., is a shift in breeding priorities toward shorter distances.
"DNA doesn't lie," said Ted Kalbfleisch, professor in the University of Kentucky's Department of Veterinary Science and study co-author. "This change in the genome aligns with the growing popularity of shorter, faster races and demonstrates how breeding decisions leave measurable imprints on the genetic makeup of the breed."
The study's practical application is the foundation it built for identifying potential genetic risks and, if necessary, pave the way for genetic tests that could flag harmful variants to guide mating decisions.
"The data shows that breeders have done a good job," Bailey said. "Genomic tools make this clear at the DNA level but are unlikely to replace a breeder's insights and intuition in the quest to create a better racehorse. However, these are precisely the tools we need to monitor and respond to hereditary problems that may arise."
