The dmrt3 gene: why some Standardbreds canter more easily than others

16 Nov 2015

Off the track Standardbreds are in large supply and are popular horses for pleasure riding in countries such as Australia and Sweden. Not all Standardbreds pace, and the gene that causes them to do so was identified by scientists a couple of years ago. This gene is called the DMRT3 gene or ‘gait keeper’ gene and gaited horses contain a mutation in this gene that enables them to have ‘ambling’ or lateral gait. The mutation occurs when the DNA base cytosine (C) is replaced with DNA base adenine (A). Thus, horses of AA genotype are pacers and horses of CA or CC genotype are trotters. Pacers tend to go faster than trotters and there are separate races for trotters and pacers.

Recently researchers from Sweden conducted a study to see if the DMRT3 gene affected the ability of Standardbreds to canter. They collected DNA hair samples of 155 ridden Standardbreds to assess their genotypes. They also gave questionnaires to the riders of these animals and asked them to score their horses on ability to walk, trot and canter under saddle including balance and rhythm within the gait, and ability to transition in and out of gaits. The researchers found that horses with an AA genotype, were scored significantly lower in canter than horses with a CA genotype. Horses with a CA genotype also ranked better for jumping ability. The AA horses had ok rhythm in canter, but they were generally not as balanced and had more trouble transitioning into canter. There were no CC genotypes in the study and the prevalence of this genotype is less common in Standardbreds.

This information could help people have a better understanding of how to best retrain and rehome Standardbreds as riding horses. This is not to say that horses with a AA genotype cannot canter or jump, they just are less likely to be good at it than a horse with a CA genotype. Standardbreds are known for their great temperament and there are many people out there dedicated to rehoming them to life after racing. Potentially some AA horses could be trained to perform more rideable ambling gaits. The Adenine base mutation at the DMRT3 gene is also associated with the ambling gaited breeds such as the Icelandic and Tennessee Walking Horse. The gaits of these horses are just the opposite of pace, extremely comfortable for riding.

So, if you are looking at purchasing a Standardbred, how do you know what its genotype is? AA genotype horses tend to naturally favour pace over trot so if you watch them in run-around the paddock or lunge them then you will soon see that they are inclined pace.

The study also went on to look at the effect of the DMRT3 genotype in two other gaited breeds, Icelandic Ponies and Nordic trotters.

Interestingly the DMRT3 gene was not found to have an effect on canter quality in Nordic trotters but it did affect ability to perform an ambling gait.

In Icelandic ponies an AA genotype was strongly related to ability to tolt (tolt is the Icelandic ridden gait) and also to ability to canter.

Reference

Andersson, L. S., Larhammar, M., Memic, F., Wootz, H., Schwochow, D., Rubin, C. J., … & Kullander, K. (2012). Mutations in DMRT3 affect locomotion in horses and spinal circuit function in mice. Nature, 488(7413), 642-646.

Jäderkvist, K., Holm, N., Imsland, F., Árnason, T., Andersson, L., Andersson, L. S., & Lindgren, G. (2015). The importance of the DMRT3 ‘Gait keeper’mutation on riding traits and gaits in Standardbred and Icelandic horses. Livestock Science, 176, 33-39.

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