Genetics of Swayback in Saddlebred Horses Examined

The gene responsible for causing the swaybacked appearance of many American Saddlebred horses might be playing an advanced game of "hide and go seek," but genetic researchers at the University of Kentucky's Gluck Equine Research Center are one step closer to uncovering this gene and, thus, helping breeders one day avoid perpetuating the trait.

Swayback, also referred to as lordosis, lowback, or softback, is the excessive curvature of the spine.

According to Gluck scientist Ernest Bailey, PhD, who studies immunogenetics and genomics and is a co-author on the study, a former doctoral student in his program laid the groundwork for the recent study in 2003 , demonstrating that swayback had a hereditary basis in Saddlebreds.

Researchers have shown that extreme lordosis is associated with pathology (physical damage to the spinal cord and associated tendons, ligaments, and other anatomic structures).

"If the gene or genes responsible for extreme lordosis in the American Saddlebred could be determined, then breeders would have a better understanding of the condition and be able to use this information in their breeding programs," noted Bailey.

To help elucidate the genetic causes of swayback in Saddlebreds, Bailey's graduate student Deborah Cook analyzed entire genomes of 20 affected and 20 unaffected horses. She noted one genetic marker was significantly associated with the presence of swayback, suggesting a region on Chromosome 20 possessed a gene that could cause inheritance; 17 of the 20 affected horses had this genetic marker.

"These results suggest that a major gene is involved and that the condition is passed on to offspring via a recessive mode of inheritance as earlier hypothesized," said Bailey. This means that two copies of the gene responsible for the swayback condition are needed for a horse to be affected, so both the sire and the dam must possess the recessive genes to result in swaybacked offspring.

The study was expanded to include 33 swaybacked horses and 287 unaffected horses. Among the 33 swaybacked horses, 80% had two copies of this chromosome sequence in their DNA (meaning both parents contributed a recessive gene) while only 15% of the unaffected horses had two copies of this sequence.

"Since we did not find a genetic marker unique to the swayback horses (but, rather, a significant association) ... we have not yet identified the mutation responsible for the trait," relayed Bailey. "But it means that we know where to look on Chromosome 20."

Additional work is necessary to identify the gene responsible.

This research is not only important for the Saddlebred industry but it also can serve as a model for investigating congenital skeletal deformities in horses and other species, Bailey concluded.

The study, "Genetics of swayback in American Saddlebred horses," was published in a special edition of Animal Genetics. The entire journal supplement, funded by the Dorothy Russell Havemeyer Foundation, is available free online

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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