The Genetic Origin and History of Speed in the Thoroughbred Racehorse

By Jennifer Coates, DVM on Feb. 8, 2012

I seem to have genetics on the brain lately. I’ve written a couple of posts about the evolution of the domestic dog, and now I just have to tell you about a paper outlining the genetic origins of Thoroughbred race horses that I came across today.

First, a little background as to why I’m so excited about this topic: I was the typical horse-crazy young girl. My parents couldn’t indulge my fantasy of having my own horse (I had to wait until I was 30 to be able to satisfy that dream on my own), but they did pay for riding lessons, horse camps, and as many books about horses as I could read. Through my "research," I came to understand that all Thoroughbreds alive today originated from one of three foundation stallions. To quote from Racing Through the Century by Mary Simon:

In the time of Charles II, England’s cold-blooded native horses were bred primarily for work and war, an increasingly unsatisfactory situation in a sporting sense. British horsemen attempted to resolve this by importing stallions of exceptional beauty from the deserts of the Middle East to cross with local mares. The result of this selective breeding program over time was a refined, fleet-footed equine possessed of strength, speed, stamina, and competitive fire — all the ingredients one could possibly want in a race horse.

Three importations made following the reign of Charles II deserve particular mention. In 1688, Captain Robert Byerly captured an elegant black stallion from a Turkish officer at the Hungarian siege of Buda and brought him home as a spoil of war. Sixteen years later, British consul Thomas Darley smuggled a handsome Arabian colt out of the Syrian Desert and into the county of Yorkshire. And around 1729 a mysterious horse of romantically obscure Eastern lineage appeared at the Earl of Godolphin’s stud near Cambridge. These, of course, were the Byerly Turk, the Darley Arabian, and the Godolphin Barb, foundation sires of the modern Thoroughbred racehorse.

Ahh, such romance and intrigue … what a great origin story. Except it turns out that it’s not the whole truth.

A team of researchers analyzed the pedigrees and genetic makeup of "593 horses from 22 Eurasian and North-American horse populations, museum specimens from 12 historically important Thoroughbred stallions (b.1764–1930), 330 elite-performing modern Thoroughbreds and 42 samples from three other equid species" and published their results in Nature Communications. Their work revealed that a genetic mutation (the C-variant) in the myostatin gene is responsible for a Thoroughbreds’ speed at relatively short distances.

According to lead researcher Emmeline Hill of University College Dublin, "The results show that the 'speed gene' entered the Thoroughbred from a single founder, which was most likely a British mare about 300 years ago, when local British horse types were the pre-eminent racing horses prior to the formal foundation of the Thoroughbred racehorse."

So, it turns out that a mare is at least as responsible for the Thoroughbred racehorse’s speed as are the three dashing "gentlemen" from the desert. Hold your heads high, fillies!