THE DIFFERENCE BETWEEN THE TRIPLE CROWN AND AN ON-TRACK BREAK DOWN
Much of the equine genome remains a mystery. There has been very little research done to identify specific genes that influence traits such as speed, endurance and other athletic traits. While the genetics of all Thoroughbred horses remains fairly limited because of the history of the breed, only one major “speed gene” has been identified. This gene influences muscle-growth. More genes have been discovered that lead to positive phenotypic athletic ability. The only research done to discover the genetics behind negative athletic traits has occurred for a tendon injury with a pattern of inheritance being discovered. A factor unrelated to muscle or tendons, also controlled unknown genes, influences heart-size, which can be contributed to the X-factor of amazing Thoroughbred racers.
All male Thoroughbreds are descended from 3 foundation sires all of Arabian descent: the Godolphin Arabian, the Darley Arabian and the Byerley Turk (Binns, Boehler, and Lambert 1). This is extremely limiting to the equine genome. According to the article “Identification of the myostatin locus (MSTN) as having a major effect on optimum racing distance in the Thoroughbred horse in the USA,” there are about 500,000 registered Thoroughbred horses worldwide (Binns, Boehler, and Lambert 1). Because of the limited foundation of horses, inbreeding is extremely common and is nearly unavoidable for the more recent horses. It has been estimated in the article “A New Look at Racehorse Genetics” that the modern Thoroughbred is “more inbred than the equivalent of a brother/ half-sister mating” (Anderson 1823). It is common practice to classify these racers into a specific distance which their bloodlines would suggest they would perform best at, including sprint, middle-distance runners and stayers (Binns, Boehler, and Lambert 1). The one identified gene can help determine which classification the horse should fall into, and this gene, commonly referred to as the “speed gene,” is the myostatin (MSTN) gene.
This gene contains a Single-Nucleotide Polymorphism (SNP) in the first intron of the MSTN gene, which is associated with muscle hypertrophy (Tozaki et al. 43). The article “A cohort study of racing performance in Japanese racehorses using genome information on ECA18” has explained that two alleles have been discovered: C and T. If the horse’s genotype is C/C, the horse should be successful as a sprinter. This horse is strong at running short-distances because of a lack of endurance (Tozaki et al. 43). If the genotype is C/T the horse is a middle-distance racer, having both endurance and speed (Tozaki et al. 43). If the horse is T/T at the locus, this horse will be classified as a stayer, with more endurance than speed, allowing them to excel in longer races, but fail during sprinting races, common races for 2-year-olds (Tozaki et al. 43). The reason these horses are specifically adapted to certain distances is due to the MSTN locus affecting individual muscle types, specifically Type IIB fibers, which are fast acting and their energy source is glycogen (Binns, Boehler, and Lmbert157). The MSTN gene is located on chromosome 18, and according to the article “A genome-wide SNP-association study confirms a sequence variant (g.66493737C>T) in the equine myostatin (MSTN) gene as the most powerful predictor of optimum racing distance for Thoroughbred racehorses” five of the top ten SNP’s are also located on this chromosome (Hill et al. 5) The article “Targets of selection in the Thoroughbred genome contain exercise-relevant gene SNPs associated with elite racecourse performance,” describes these other SNP’s. In fact, a total of 13 more SNP’s in 9 genes were discovered to influence phenotypic athletic abilities (Hill E. W. et al. 56). More research is required in order to determine which traits these genes code for, but the first steps to discovering genetic links to successful racehorses have been taken.
Opposed to these possible genes that lead to successful racers like those of the Triple Crown races, there has been another study to determine if an injury of the superficial digital flexor tendon (SDFT) is hereditary. The article “Estimation of heritability for superficial digital flexor tendon injury by Gibbs sampling in the Thoroughbred racehorse” concluded that while a specific gene or allele could not be discovered, appropriate breeding programs could impact the prevalence of the SDFT injury (Oki et al. 415). This could be a possible explanation for the breakdowns of horses during their races, especially for those who have questionable bloodlines or a large amount of inbreeding.
The X-Factor is a theory proposed to explain why some racehorses are simply incredible, including Secretariat and Eclipse. This X-Factor contributes to the heart size of the horse, increasing the size from an average of 8 pounds, to 14 pounds seen in Eclipse or 22 pounds seen in Secretariat (Kirsan). This mutation gene is supposedly located on the X-chromosome, hence its name, meaning the dam is more of a contributor than the sire. It is a common misconception to breed for the sire’s bloodline opposed to the dam’s bloodline, which may change in future breeding programs. Even today breeders focus on “stallions and often do not even bother recording the lineage of the mares” (Anderson 1823). If breeding for the X-Factor, this program is only successful if breeding from 4 major bloodlines including Princequillo, War Admiral, Blue Larkspur and Mahmoud (Kirsan). These horses are descendants of Eclipse, who appeared in Pocahontas’ bloodline 11 times (please see the link below to view her 7 generation bloodline). Pocahontas, a very influential broodmare was homozygous for the X-Factor. Her offspring would all have the X-Factor (Kirsan).
With more research and dedication soon the entire equine genome will be sequenced and understood. Breeding racehorses will be more scientific, resulting in more Triple Crown winners and less On-Track Breakdowns. The discovery of the myostatin locus, the inheritance pattern of injuries to the superficial digital flexor tendon and the sex-link X-Factor are all genetic breakthroughs in the horse racing industry, with future breakthroughs to come.
All male Thoroughbreds are descended from 3 foundation sires all of Arabian descent: the Godolphin Arabian, the Darley Arabian and the Byerley Turk (Binns, Boehler, and Lambert 1). This is extremely limiting to the equine genome. According to the article “Identification of the myostatin locus (MSTN) as having a major effect on optimum racing distance in the Thoroughbred horse in the USA,” there are about 500,000 registered Thoroughbred horses worldwide (Binns, Boehler, and Lambert 1). Because of the limited foundation of horses, inbreeding is extremely common and is nearly unavoidable for the more recent horses. It has been estimated in the article “A New Look at Racehorse Genetics” that the modern Thoroughbred is “more inbred than the equivalent of a brother/ half-sister mating” (Anderson 1823). It is common practice to classify these racers into a specific distance which their bloodlines would suggest they would perform best at, including sprint, middle-distance runners and stayers (Binns, Boehler, and Lambert 1). The one identified gene can help determine which classification the horse should fall into, and this gene, commonly referred to as the “speed gene,” is the myostatin (MSTN) gene.
This gene contains a Single-Nucleotide Polymorphism (SNP) in the first intron of the MSTN gene, which is associated with muscle hypertrophy (Tozaki et al. 43). The article “A cohort study of racing performance in Japanese racehorses using genome information on ECA18” has explained that two alleles have been discovered: C and T. If the horse’s genotype is C/C, the horse should be successful as a sprinter. This horse is strong at running short-distances because of a lack of endurance (Tozaki et al. 43). If the genotype is C/T the horse is a middle-distance racer, having both endurance and speed (Tozaki et al. 43). If the horse is T/T at the locus, this horse will be classified as a stayer, with more endurance than speed, allowing them to excel in longer races, but fail during sprinting races, common races for 2-year-olds (Tozaki et al. 43). The reason these horses are specifically adapted to certain distances is due to the MSTN locus affecting individual muscle types, specifically Type IIB fibers, which are fast acting and their energy source is glycogen (Binns, Boehler, and Lmbert157). The MSTN gene is located on chromosome 18, and according to the article “A genome-wide SNP-association study confirms a sequence variant (g.66493737C>T) in the equine myostatin (MSTN) gene as the most powerful predictor of optimum racing distance for Thoroughbred racehorses” five of the top ten SNP’s are also located on this chromosome (Hill et al. 5) The article “Targets of selection in the Thoroughbred genome contain exercise-relevant gene SNPs associated with elite racecourse performance,” describes these other SNP’s. In fact, a total of 13 more SNP’s in 9 genes were discovered to influence phenotypic athletic abilities (Hill E. W. et al. 56). More research is required in order to determine which traits these genes code for, but the first steps to discovering genetic links to successful racehorses have been taken.
Opposed to these possible genes that lead to successful racers like those of the Triple Crown races, there has been another study to determine if an injury of the superficial digital flexor tendon (SDFT) is hereditary. The article “Estimation of heritability for superficial digital flexor tendon injury by Gibbs sampling in the Thoroughbred racehorse” concluded that while a specific gene or allele could not be discovered, appropriate breeding programs could impact the prevalence of the SDFT injury (Oki et al. 415). This could be a possible explanation for the breakdowns of horses during their races, especially for those who have questionable bloodlines or a large amount of inbreeding.
The X-Factor is a theory proposed to explain why some racehorses are simply incredible, including Secretariat and Eclipse. This X-Factor contributes to the heart size of the horse, increasing the size from an average of 8 pounds, to 14 pounds seen in Eclipse or 22 pounds seen in Secretariat (Kirsan). This mutation gene is supposedly located on the X-chromosome, hence its name, meaning the dam is more of a contributor than the sire. It is a common misconception to breed for the sire’s bloodline opposed to the dam’s bloodline, which may change in future breeding programs. Even today breeders focus on “stallions and often do not even bother recording the lineage of the mares” (Anderson 1823). If breeding for the X-Factor, this program is only successful if breeding from 4 major bloodlines including Princequillo, War Admiral, Blue Larkspur and Mahmoud (Kirsan). These horses are descendants of Eclipse, who appeared in Pocahontas’ bloodline 11 times (please see the link below to view her 7 generation bloodline). Pocahontas, a very influential broodmare was homozygous for the X-Factor. Her offspring would all have the X-Factor (Kirsan).
With more research and dedication soon the entire equine genome will be sequenced and understood. Breeding racehorses will be more scientific, resulting in more Triple Crown winners and less On-Track Breakdowns. The discovery of the myostatin locus, the inheritance pattern of injuries to the superficial digital flexor tendon and the sex-link X-Factor are all genetic breakthroughs in the horse racing industry, with future breakthroughs to come.
Equinome Speed Gene Test is a Genetic Test that can be ordered for a specific horse to determine what distance that horse would be successful in running. This video explains which genotypes code for which distance the horse should run, and it also explains the probability of the offspring when crossing these different genotypes. The process is quiet simple, requiring only a blood sample and a submission form.