RMH Genetic Markers

An Interview with Dr. Gus Cothran
August 26, 1998 (Revised December 15, 2000)
Steve Autry

It has been over 14 years since the establishment of Rocky Mountain Horse Association. During that time many new members have joined the RMHA. Interest in understanding the history of the Rocky Mountain Horse has been rekindled. One of the frequent queries pertains to how it was ascertained that the Rocky Mountain Horse was a distinct equine breed. Dr. Gus Cothran, a longtime consultant to the Association was an active participant in that determination. Dr. Cothran has the following definition of what constitutes a breed posted in his office: A BREED IS A GROUP OF ANIMALS THAT HAS BEEN SELECTED BY MAN TO POSSESS A UNIFORM APPEARANCE THAT IS INHERETABLE AND DISTINGUISHES IT FROM OTHER GROUPS OF ANIMALS WITHIN THE SAME SPECIES.
From the above statement it is abundantly clear that the presence of common genetic markers is not a necessary prerequisite for a group of horses to achieve breed status. Still, Dr. Cothran feels that genetic evaluations can help confirm that there are distinguishing characteristics in a breed. There are seventeen marker systems that are customarily used to identify horse breeds. Within these marker systems there are one hundred and twenty-five possible variations. Dr. Cothran examined seventy-five Rocky Mountain Horses in 1989 to see if there were any distinguishing patterns in this group of horses. (He examined more Rocky Mountain Horses than are typically examined in similar studies of other breeds.) He found that there are markers in three of the systems that, while not exclusive, are seen more frequently in RMH’s than in the general horse population. The markers that are most distinctive in the RMH’s are:

D-dekÉ. This is a D Blood Group marker where the dek is an alphabetic designation of antigen sites. This variant is often found in horses of Spanish origin (Also seen in some Saddlebreds).

GPI variant FÉ. (Glucose Phosphate Isomerase) GPI-F variant is found in most breeds of the North American Gaited breeds group. These are the breeds most closely related to the Rocky. Its presence in these breeds probably is a reflection of common ancestry. The variant also occurs in Iberian breeds and in cold blood horses. The source of the variant in the North American breeds is probably from the Iberian breeds.

TF (F3)É(Transferrin) This marker is also more prevalent in Spanish breeds.

It should be emphasized that the presence of common genetic markers is not necessary in order to define a breed. The Thoroughbred horse has only one marker that is seen in just thirty percent of the breed. The Saddlebred has only one distinguishing marker (Protease Inhibitor J). Every horse in a breed does not carry the same markers even though they may carry most of the same traits (conformation, temperament, gait, color, etc.). Additionally, there is not any governing body that certifies breed status for a group of animals. The process is more informal than many people would suspect. Markers that are unusual or that are seen in higher frequency lend credence to the common heritage of a group of horses. Future advances in genetics may someday be sufficient to revisit this issue once the horse genome is more completely mapped. Some day there may be a certifying body, but for now the main course for legitimizing a breed is through public acceptance.

In summary, a breed is born when a group of horses meets the above breed definition and efforts are made to sustain those characteristics by regulating (selecting) future breeding policies to preserve those desirable characteristics that distinguish one group of animals from others in the same species. The establishment of a breed takes not only the presence of animals with common characteristics, but also the active process of selection and preservation of those traits by people. It is through these means that the Rocky Mountain Horse became a breed.

Steve Autry