Selecting a Thoroughbred: The Role of Genetics

Any owner will agree that he or she considers several factors when purchasing a horse. From conformation to show records and bloodlines to temperament, the number of aspects a potential horse owner evaluates can be endless.

But add to that list--at least in the Thoroughbred industry--molecular genetics. Researchers have made strides in deciphering what genes (when found or not found) could suggest about a horse's athletic potential. At the 2011 Thoroughbred Pedigree, Genetics, and Performance Symposium, held Sept. 7-8 in Lexington, Ky., Matthew Binns, PhD, adjunct professor in the University of Kentucky Department of Veterinary Science and consultant for The Genetic Edge in Midway, Ky., discussed a genetic perspective on traditional pedigree methods and the characteristics of genetically complex traits, such as athletic performance.

"Is genetics important to the horse industry?" Binns began. "A major determinant of sale prices is the pedigree. And pedigree can be thought of as a surrogate for genetics."

Binns described the Thoroughbred horse and its tradition for heavy pedigree keeping as a geneticist's playground: "It's a fascinating history. It's a group of animals that's perfect for genetics."

Inbreeding

Binns discussed inbreeding briefly, which is a common topic in the discussion of Thoroughbred breeding. While the inbreeding coefficient for Thoroughbreds, on average, is somewhere around 8%, some individuals could have higher or lower coefficients.

He explained that each horse has 64 chromosomes (half from the sire and half from the dam). He then, using 2010 Horse of the Year Zenyatta as an example, explained how some instances of inbreeding work.

"Zenyatta 'on average' would have gotten about 4 of her 64 chromosomes (or about 6% of her DNA) from Nashua, to whom she is inbred 5x5," he said. "She may have got more, she may have got less, she may even have got nothing. The chances of her getting the same chromosomal region down both sides, and thus recreating Nashua's genotype in that region are very small."

There are also some shared genetics between siblings, Binns explained. Two half siblings, for example, will share about 25% of their DNA. Likewise, broodmares by the same sire will pass about 12.5% of the sire's genetics on to their offspring.

"Proof of Principle"

Binns noted that some traits--such as bay or chestnut coat color--can be evaluated using simpler Mendelian genetics (i.e., the punnett square) as bay is dominant and chestnut is recessive. On the other hand some traits, such as athletic performance, are more difficult to analyze and are, thus, called complex traits.

Binns then described a study that examined the incidence of white leg markings in Thoroughbred horses--a "well-defined complex trait" in the same general category as athletic performance.

White markings in horses, he relayed, result from an absence of melanocytes (pigment-producing cells).

"Asymmetry in markings is common, though limbs have identical genotypes and similar intrauterine environments," Binns explained. "The strong correlation between white facial markings and white legs suggest some of the same genes are involved in both traits."

Interestingly, Binns noted that gender also seems to play a role in the development of white markings as males are observed to have more markings than do females; white markings are found more frequently and are more extensive on front legs than on the back legs; and more markings are seen on the left side of the body than the right.

To carry out his study, Binns evaluated photographs of 409 Thoroughbred horses that had previously been genotyped using a gene chip. He recorded the horses' sex, base coat color, and number of legs with white markings.

Then, he compared the gene profiles of the horses that had white legs to those that did not, searching for variations in the genotypes.

After extensive comparisons and calculations, Binns was able to target four loci (the specific location of a gene or DNA sequence on a chromosome) on three chromosomes that seemed to play a role in the presence or absence of white leg markings in Thoroughbreds: one gene on ECA2 (ECA is the abbreviation for "equine chromosome"), two genes on ECA3, and one gene on ECA16.

Binns identified one loci on ECA3 and the loci on ECA16 as good candidate genes, both of which were later validated by another group of researchers.

He noted that he would take the same general approach to studying Thoroughbreds' racing performance as he did white leg markings.

Genetics of Performance

Binns then discussed studies he's carried out examining the complex trait of equine genetics as it relates to performance.

"Race performance is like a rubix cube," he explained. "Scientific studies have estimated the heritability of racing performance in the Thoroughbred and concluded that genetics contributes 35-50% to performance."

In one study Binns was involved in, he reviewed information from a bank of approximately 4,500 DNA samples. About 1,150 had undergone full genome scans, more than 200 of the samples were taken from Grade 1 race winners, 18 of the samples came from individuals who won U.S. Triple Crown races, and many samples were from top Thoroughbred stallions of the past 50 years.

He paired the DNA samples with other information collected about each submission, including the individual's racing history (including Beyer figures where available), the distance at which they raced most successfully, the surface the animals were most successful running on, and the horse's height.

Binns then evaluated the samples more closely and identified a panel of SNP markers (single nucleotide polymorphisms, the smallest and most common type of genetic variant) associated with racing ability, established whether the SNPs were dominant or recessive traits, developed a scoring system giving "Genetic Grades A, B, C, and D" (i.e., ranked the horses' genetic potential), and established incidence of "Genetic Grades in Sales Population." He also identified markers that included a "female-specific" SNP.

Then Binns grouped the horses into four categories--A, B, C, and D (A being the most potential and D being the least)--which rated their genetic potential as athletes.

After extensive evaluation, Binns found that:

  • The SNPs that were associated with the horses' ideal race distance were located on ECA 18--the same chromosome linked with double muscle disorders in other species;
  • About half of the Kentucky Derby winners evaluated showed the same general genotypes; and
  • Overall, about 10% of the horses tested were grouped in the A category, 40% were rated B, 40% were C-rated, and 10% were classified as D (Binns noted this was important for potential owners to consider, as he recalled several Grade D horses that have sold for high dollar amounts at auction).

Take-Home Message

So what does all this mean for a potential buyer? Rather than just focusing on pedigrees, some genetic tests are available that could provide additional insight into Thoroughbreds' athletic potential.

Of course, genetics is still a developing science and researchers are making new advancements daily. The results of any genetic test--many of which can now be obtained through a variety of laboratories--should be interpreted with caution and considered alongside traditional methods of horse selection, such as conformation.

"It's complicated," Binns concluded. "Every aspect of the business is complicated."

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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