Roth, I.T., Schielke, B., Rensing, M., and Bernau, M. 2021. Comparison of American Quarter Horses Competing in Western Pleasure, Hunter under Saddle, and Reining Using Linear Traits. Animals. 11: 2861. DOI: https://doi.org/10.3390/ ani11102861

Quarter Horses are known as one of the most versatile breeds in the world for their multi-discipline uses in both Western and English-style riding. Linear traits such as length, circumference, and angles were used to determine performance in each of the disciplines studied.  A total of 24 traits were examined from horses from Reining, Western Pleasure, and Hunter Under Saddle disciplines. Different genes were more prevalent in certain disciplines than others based on the desired genes for that type of performance. Western Pleasure horses displayed slow and gathered movements, meanwhile, the horses bred for Hunter Under Saddle tend to have a much longer and faster stride to cover more ground. Reining horses showed evidence of their physical appearance for maneuverability and acceleration. Equine phenotypes are determined by their disciplines in the Quarter Horse. 

Petersen, J.L., et al. 2013. Genome-Wide Analysis Reveals Selection for Important Traits in Domestic Horse Breeds. PLOS Genetics. 9(1): e1003211. DOI: https://doi.org/10.1371/journal.pgen.1003211

The researchers looked at several breed-defining traits in 33 different horse breeds, and were able to identify regions of the genome that provide significant differences between each breed. They found out that the MSTN gene is the most commonly found signature selection gene in the American Quarter Horse. This gene encodes for hind-end muscular fibers that aid in sprinting ability. The MSTN gene can indicate within equine selective breeding, it can help us predict the preferred phenotype of the animal for certain disciplines within the equestrian community. The discovery of this gene is important when it comes to selective breeding in these horses, as it is key evidence in the selection of gait and size when it comes to breed standards. 

Meira, C.T., et al. 2014. Speed Index in the Racing Quarter Horse: A Genome-Wide Association Study. Journal of Equine Veterinary Science. 34(11-12): 1263-1268. DOI: https://doi.org/10.1016/j.jevs.2014.09.003

The racing line of Quarter Horses uses a selective attribute known as the speed index. This speed index is used by breeders to determine the performance of the animal under varying conditions, such as distance. There were a few stand-out genes found to contribute to the speed index, one example is the MSTN gene or the myostatin gene. There were a few other genes of interest that contributed to the animals’ speed; GRM8, GRIK2, NEB, ANK1, and KAT6A. The MSTN gene varies between both prime racing breeds, the Thoroughbred and the Quarter Horse, based on their physical appearances.