The genetic basis of speed in racehorses

A genetic test can determine the optimum racing distance for an individual horse..

For centuries, managing the careers of Thoroughbred racehorses has been more art than science. Now, however, trainers can use a genetic test to determine the optimum racing distance for an individual horse based on a variation of a specific muscle-growth gene.

A group of horses racing on turf
The optimum racing distance for any horse is determined, in part, by his muscle growth and devleopment.

“The test identifies which version of the myostatin gene [MSTN] a particular horse has,” explains Emmeline Hill, PhD, a genetics researcher at University College Dublin in Ireland and co-founder of the company Equinome. Myostatin is a protein that regulates muscle growth and development. “You can think of DNA as being made up of the letters G, A, T and C. The various combinations of those letters spell out a horse’s unique genetic code,” says Hill. “In the myostatin gene, a single change from a C to a T can lead to dramatic differences in when and how a horse’s muscle mass develops.”

When Hill and other Equinome researchers sequenced the MSTN gene from 24 unrelated Thoroughbred racehorses, they identified a variable spelling of the letters within the gene—at one particular position it could either be “C” or “T.” Because one copy of the variant is inherited from the sire and one from the dam, an individual will be either C:C, C:T or T:T. By comparing the racing performances at different distances of 179 Group-race winning racehorses to individual MSTN genotypes, the researchers were able to identify relationships between genetics and success over various distances.

“We found that horses with two copies of the C’ variant of the gene, which we call C:C, typically have greater muscle mass as 2-year-olds and excel in sprint races up to one mile in length,” says Hill. In contrast, horses with the T:T variant are best suited to races longer than one mile and generally do not perform optimally as 2-year-olds. “That doesn’t mean they won’t mature into great horses; it just means that?they take longer to mature,” she says. The final MSTN variant identified by Equinome is C:T, a mixture of speed and stamina, equally likely to win at both shorter and longer distances.

Identifying the MSTN variation a particular horse carries can help guide key training and racing decisions, says Hill. “If you know a horse is a T:T, you might choose to minimize racing during his 2-year-old year to let him mature, then enter him in longer races as a mature horse to maximize his chance of success.” And because a horse’s MSTN variation is genetically inherited from both his parents, a breeder can match mares and stallions based on the desired outcome. “If you have a mare that is a C:T and you’d like to get a sprinter out of her, you’ll have the best chances of getting one if you match her with a stallion that is a C:C.”

Hill stresses that the test cannot predict which horses will be winners; instead it simply suggests which race distances best suit them: “You could enter a C:C horse in appropriate races only to find that he’s still slower than the rest of the field. The real benefit of this test is that it allows us to treat horses as individuals—to manage them as they need to be managed, not how we think they should be or how we’ve always done it in the past.”

A blood sample is needed for the gene test, which costs approximately $1,250. Equinome is continuing its research. “We are also working on tests for Standardbreds and investigating whether or not muscle variations influence the power and ability of show jumpers,” says Hill.

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