By Adam Piore and Katie Bo Williams

Any astute trainer could have sensed that Trading Leather was exceptional even before he charged past the competition to secure victory at the esteemed Irish Derby last June. Observers only needed to admire the impressive musculature along his hindquarters, the length and structure of his legs, and his distinctive gracefulness in movement.

However, Jim Bolger, the renowned trainer behind Trading Leather, had additional insight regarding his colt's suitability for the mid-distance race held at the historic County Kildare track. Bolger had evaluated Trading Leather for the so-called "speed gene." Just like a parent eager to learn their unborn child's sex, Bolger had a clear understanding of the optimal race distances for Trading Leather and the approximate age he would be prepared to compete.

Genetic testing has now made its way into the realm of thoroughbred racing. Bolger, whose name is nearly synonymous with success in this unpredictable sport, has referred to this development as “the most significant advancement in breeding since its inception over three centuries ago.” The speed gene has become a pivotal consideration in Bolger's annual decisions regarding which of his approximately 100 thoroughbreds to mate and when to initiate training for his most promising young horses. He believes that combining specific gene types from sires and mares can yield new lines of lucrative champions. His confidence in the science behind the speed gene led him to establish a company that markets speed gene tests to other breeders and trainers.

The sports sector has long harbored aspirations for genetic testing. Following the mapping of the human genome in 2000, sports scientists have been in pursuit of identifying genes that contribute to athletic prowess. Australia saw the launch of the first test claiming to assess human athletic potential in 2004, which eventually made its way to the U.S. in 2008, following a study that linked a particular gene to muscle fibers critical for high-energy sports like sprinting and powerlifting. Earlier this year, Uzbekistan became the first country to announce plans to utilize genetic tests to scout potential Olympians, involving evaluations of children as young as 10, supervised by geneticists who have been analyzing the DNA of the nation’s top athletes for two years.

Each announcement regarding a "sports gene" invariably ignites discussions surrounding science and culture, as well as nature versus nurture. Can genetics fully account for athletic performance? Do they hold any weight against expert training?

In the traditional world of thoroughbred racing, some experienced trainers argue that they can identify a top horse without the aid of genetic tests; they believe that no genetic analysis can fully capture the intangible qualities that define a champion. According to these trainers, the essence of a great horse lies in its courage, heart, grit, and determination. Heart is exemplified by Seabiscuit as he fought to stay ahead of War Admiral, and by Rachel Alexandra, who valiantly resisted her male competitors at Saratoga despite her fatigue.

Despite the emphasis on tradition, the emerging scientific inquiry into thoroughbreds marks a significant breakthrough in both the racing industry and genetics. Research has pinpointed the importance of a single gene and illustrated how slight variations in that gene can affect athletic performance. The narrative began with Emmeline Hill, a young scientist from a notable Irish racing lineage.

Growing up on a horse farm in County Wexford, just outside Dublin, Hill often spent time with her grandmother, Charmian Hill, known as “The Galloping Granny.” The first woman to compete against men in Ireland, she rode horses in competitions into her 60s and is forever linked with Dawn Run, a legendary figure in Irish racing. At 12, Hill was excused from biology class to witness Dawn Run make history at the 1986 Gold Cup. By 19, she was working in the stables of John Oxx, who produced Sea The Stars, one of the decade’s most successful racehorses. After focusing her studies on genetics at Trinity College Dublin, Hill delved into human population genetics and research on sleeping sickness in African cattle. However, it wasn't long before she sought to merge her dual passions. In 2004, she received a grant from Science Foundation Ireland to investigate the genetics of racehorse performance, collecting DNA samples from thoroughbreds across Ireland. This was a propitious moment in genetics, as the human genome had been sequenced in 2000 and the horse genome was nearing completion. Geneticists worldwide were making groundbreaking discoveries reminiscent of Watson and Crick's identification of DNA's double helix structure.

Every horse (and human) possesses a genetic blueprint within each cell, comprising approximately 20,000 genes made up of 250 pairs to 2.4 million pairs of DNA’s core components known as nucleotides. These nucleotides contain one of four bases: cytosine, guanine, adenine, and thymine. The arrangement of these base pairs—totaling around 2.4 billion in a horse—provides the instructions for the structure of every protein produced by the body. These proteins, in turn, influence everything from a thoroughbred's height to coat color.

Hill recognized that many traits in animals arise from the interaction of various genes and did not anticipate finding a single genetic determinant for speed in racehorses. Nevertheless, she knew where to look for hints: within the gene that codes for the protein known as "myostatin."

Myostatin acts to inhibit the growth of muscle fibers by restricting the proliferation of muscle cell precursors, or myoblasts. Since larger muscles require greater caloric intake, some theorize that this protein offers an evolutionary advantage by keeping muscle size aligned with an animal's activity level.

In 1997, researchers at Johns Hopkins University discovered that mice with reduced myostatin levels exhibited significantly larger muscle mass. Subsequent experiments revealed that this mechanism was applicable across various species, including a notably muscular breed of cattle known as “Belgian Blue,” which also displayed genetic mutations that hindered myostatin production.

“Holy cow, this can't be correct!” Hill recalls her initial reaction. “But then I thought, no, this could actually hold true. It began to dawn on me that this might be something significant.”

A compelling case regarding the advantages of a myostatin deficiency was presented in a 2007 PLOS Genetics study on racing dogs. Researchers discovered that “bully whippets” possessing two copies of a gene that disrupted myostatin production were so muscular that it hindered their speed. Conversely, whippets with just one copy of the defective gene—resulting in muscle mass between the two extremes—were significantly faster than both those with normal myostatin levels and those with two copies of the mutation. This marked the first instance where researchers linked a myostatin mutation to enhanced athletic performance.

“The idea from a speed perspective is that muscle mass equates to power,” explains H. Lee Sweeney, a physiology professor at the University of Pennsylvania, who gained attention for creating muscle-bound “Schwarzenegger Mice” through genetic manipulation of myostatin levels. Although these mice didn’t exhibit extraordinary capabilities, their massive muscles effectively illustrated the impact of the protein. “Certainly, having no myostatin results in a substantial muscle mass that doesn’t contribute to athleticism,” Sweeney asserts. “However, even a slight reduction in myostatin can yield significant power.”

Research on myostatin in animals provided Hill with the assurance she was pursuing speed in the right domain. “We knew about this in various species, including sheep, pigs, humans, and fish,” she states. “So I inquired, ‘What about horses?’ A review of the literature showed that no one had published anything.”

Hill understood that thoroughbred horses, like racing dogs, had been selectively bred for speed over generations. The term “thoroughbred” refers to a registered racehorse with ancestry tracing back to three original stallions and approximately 30 foundational mares listed in The General Studbook established in 1791. Driven by a passion for the sport, humans have crafted a unique horse breed over 300 years, harnessing selective breeding to enhance speed.

To Hill, it seemed evident that traits breeders selected for must have genetic correlates. Industry professionals had long made assessments based on pedigree, physical attributes, and the racing records of parents and siblings to estimate the potential performance of a planned mating. Yet, Hill was unprepared for the straightforward results that emerged from her analysis of DNA collected from 148 thoroughbreds. Variants in the myostatin gene were particularly notable. While most sequences of the gene were identical, she discovered that horses had either two Cs, two Ts, or one of each letter inherited from their parents. These three genotypes are specific to horses and can be classified as CC, CT, or TT.

Hill aimed to determine if one of these genetic types was more prevalent among winning horses. She accessed race results for 142 of the horses she tested from racing databases. Although the genetic type didn’t predict winning or losing outcomes, Hill recognized that champion horses competing in different distances often exhibited distinct physical traits. Sprinters tended to be shorter, stockier, and more muscular, while long-distance competitors were leaner. Thus, she examined correlations between race distances and genetic types.

When the results appeared on her computer screen late one night, Hill assumed there had been an error. A statistically significant “P-value,” which indicates the likelihood of random correlation, is typically accepted as below 0.05. Hill's analysis resulted in a P-value with 19 decimal places—a remarkably strong correlation.

“Holy cow, this can't be right!” Hill recalls her astonishment. “But then I thought, no, this could actually hold true. It started to dawn on me that this might be something really significant.”

This breakthrough led Hill to identify three types of horses based on the genetic types of the myostatin gene inherited from each parent. She determined that CC horses excel in sprint races ranging from 1,000 to 1,600 meters, typically achieving their first win by age 2. CT horses (like Trading Leather) are best suited for races between 1,400 and 2,400 meters, also winning around the same age. TTs tend to perform best in races over 2,000 meters but generally peak in performance about seven months later than the other two genetic types.

In 2010, Hill published her findings in the journal PLOS One, demonstrating the correlation between a horse's genetic type and optimal race distance. “Initially, no one believed that a single gene could predict this,” Hill recalls. “But the more research we conduct, the more validation we find.”

Ernest Bailey, a professor of veterinary science at the University of Kentucky and a leading figure in equine genetics, expressed admiration for Hill's research. “One remarkable aspect was discovering a single gene with such a significant effect,” he noted. “Most human genes are identified only after examining hundreds or thousands of individuals due to their minimal effects.” Sweeney of the University of Pennsylvania also acknowledged Hill's research, stating that her “correlations appear robust and convincing.”

While it may seem astonishing that single-letter variations in one gene can lead to different traits, there are numerous other examples. A 2007 study published in Science compared the DNA of small dog breeds—such as Chihuahuas and Pomeranians—to larger breeds like Great Danes and St. Bernards, finding that a variation in a single gene responsible for growth factor was strongly linked to the small dogs’ size.

Similar selective pressures apply to domesticated horse breeds. In a 2013 study published in PLOS Genetics, researchers at the University of Minnesota, including James R. Mickelson, who leads a canine and equine genetics lab, examined 33 breeds, including those bred for sprint races like quarter and paint horses. The team, which included Hill, identified single genes associated with desirable traits, such as coat color, and also noted that the genomic region selected for sprinting traits aligned with myostatin, the speed gene.

Despite this progress, questions linger regarding whether the speed gene genuinely enhances racing speed. Muscle mass, dictated by the gene, is certainly not the sole determinant of speed; the right balance of body type, bone structure, and muscle size is critical. Mickelson explains that numerous factors contribute to a horse's athletic performance, such as muscle oxygen absorption efficiency, bone thickness, and heart size (Secretariat was renowned for his heart size).

Additionally, the precise method by which variations in the speed gene affect myostatin production and ultimately speed is still unclear. In the 2013 PLOS Genetics study, University of Minnesota researchers sought to unravel this mystery. They took muscle biopsies from horses and analyzed the composition of muscle fibers—crucial for athletic performance—associated with each genetic variant. Muscle fibers fall into two main categories: "slow-twitch" and "fast-twitch." These fibers utilize different proteins to produce contractions that enable horses to propel themselves. Fast-twitch fibers contract more rapidly than slow-twitch fibers, generating greater force over shorter periods. However, fast-twitch fibers rely on a metabolism that provides energy for brief bursts, while slow-twitch fibers convert glucose and fatty acids into energy more efficiently and fatigue less quickly.

An athlete's ratio of slow-twitch to fast-twitch fibers, predominantly determined by genetics, can indicate predisposition towards either sprinting or endurance sports. Fast-twitch fibers dominate in a cheetah's leg muscles, while slow-twitch fibers are prevalent in a turkey's legs. Mickelson highlights that the speed gene mutation “seems to induce a shift in fiber type without altering fiber size.” The correlation between fiber types and performance is evident in horses, with quarter horses—CCs suited for sprinting—exhibiting up to 10 percent more fast-twitch fibers compared to TTs, which excel in distance races. Mickelson believes the evidence is compelling: although the mechanisms of the speed gene are not fully understood, its impact on a horse's athletic performance is undeniable. Furthermore, Hill's work on horses suggests that myostatin presents a valuable and intricate target for future research. Some geneticists have already demonstrated that slight variations in the human myostatin gene can subtly influence "muscle power," impacting athletic performance.

In 2009, Hill partnered with Bolger to establish Equinome, a biotech firm that offers speed gene testing for $750. To date, Equinome has sold more than 5,000 tests. In the last two years, Hill and her team have developed an additional test to evaluate a horse's performance, based on a set of 80 gene variants linked to heart function and muscle contraction, which Hill believes are critical for success. Equinome is not alone in this venture; at least four other companies provide tests for various genetic traits supposedly associated with racing performance. Among these is Performance Genetics, based in Lexington, Kentucky, which utilized their genetic tests on Verrazano before acquiring the horse—once a favorite for the 2013 Kentucky Derby.

Nevertheless, many professionals in the industry remain skeptical about the tests. “I believe I can evaluate a horse's capabilities just by reviewing the catalog and its characteristics, achieving accuracy 98 percent of the time without incurring DNA testing costs,” states Jon Freyer, Bloodstock Manager at Arrowfield Stud, one of Australia's leading breeding farms. Freyer insists one can identify a “sprinter” or a “stayer” through the racing records of the horse's lineage. “You shouldn't need a $1,000 test to confirm something that's generally evident. Identifying if a horse is a CC or a TT should be straightforward,” he asserts.

Mike Trombetta, owner and trainer at Escape Stables in Maryland, who has prepared horses for the Kentucky Derby and the Preakness, expresses doubt about the utility of genetic testing. Even if he were aware of a horse's genetic type, it would not replace the necessity of assessing the horse in action. “As a trainer, I don’t abandon any horse until we've tested it over various distances and surfaces,” he states. “What will you do if the test doesn't yield the expected results? Abandon the horse without giving it a chance?”

Trombetta's question resonates with David Epstein, author of the 2013 book The Sports Gene, which argues that science has much ground to cover before genetic testing can significantly influence sports. Epstein points out that a more straightforward method for evaluating athletic potential would simply be to observe a horse or human in motion. “If you know what to watch for, why not just use a stopwatch?” he questions.

Hill, who also leads the Equine Exercise Genomics research group at University College Dublin, acknowledges that while the test serves as a tool, it can never replace the expertise of seasoned industry professionals who rely on their horsemanship skills. She admits that training and upbringing can account for anywhere from 30 to 65 percent of overall performance.

“Nevertheless, what if the test doesn’t yield the expected outcome? Abandon the horse without testing its capabilities?”

Hill argues that genetic tests can provide insights that trainers might overlook, such as the likelihood that TTs will reach their peak speeds later than CCs. She surveyed horses at Irish racing yards and discovered that trainers were entering the two genetic types into sprint and longer races at identical ages. “This indicates that trainers may not be fully aware of the genetic distinctions,” she observes. “They are still entering them in the same races at the same age.”

On a rainy January Tuesday at his farm in Coolcullen, County Kilkenny, Ireland, Bolger reflected on the transformative impact of the speed gene test on his breeding practices. He credited it with reducing the number of TT colts—horses he traditionally had less success training and racing. “I have a firm understanding of the types of mares and stallions,” he notes. “With that knowledge, we have decreased our TT numbers from over 25 percent to around 3 to 5 percent.”

That said, Bolger isn't ready to completely forgo his experience in favor of genetics. Observing Trading Leather as he ambled onto a patch of grass, snorting and bobbing his head, Bolger grew contemplative. “Breeding horses is somewhat akin to tossing pebbles at a wall and hoping some will adhere,” he mused. “Full siblings can differ vastly.” He gestured towards Trading Leather. “His full brother from an earlier generation is a TT, and he was ineffective.”

When evaluating a horse, Bolger admitted, “I don’t always know exactly what I’m observing. Sometimes, it’s just an intuition.” He suggested that genetics has more progress to make before it can pinpoint the trait he most desires to identify: “Is he a champion?” Acknowledging that recognizing such a quality will likely always require the discerning eye of seasoned professionals, along with a bit of luck, Bolger concluded, “But even for a top trainer, having scientific support is invaluable. I don’t have to rely on guesswork as much anymore; I have the science.”

Adam Piore is a freelance writer based in New York. @adampiore

Katie Bo Williams is a freelance writer based in Washington, D.C., and has covered thoroughbred horse racing for the Saratoga Special and the Thoroughbred Daily News.

Originally published at Nautilus on August 4, 2016.