Genetic testing of Thoroughbred horses has uncovered a very common ancestor in the breed, a stallion named Eclipse who lived in the late 18th century. Eclipse was undefeated in his 26 races and was a great-great-grandson of the Darley Barb and is the genetic ancestor of 95 percent of the Y chromosomes alive today in Thoroughbred horses. A full 80 percent of all living Thoroughbreds are part of Eclipse's bloodline [source: Casselman].
The Thoroughbred Breed
The three foundation sires created a new breed of horse, the Thoroughbred. The Oriental group horses, particularly the Godolphin, brought quickness and agility to the breed. Arabians tend to be small horses, light and quick on their feet; Arabians have lighter bones than other breeds. Bred with the larger, slower English mares, the foundation sires produced a new breed that was both quick and muscular. The result is a breed of horse that averages 16 hands (a hand equals four inches, or about 10 centimeters, measured from the ground to the withers, a ridge just above the shoulders) in height and weighs an average of 1,000 pounds (454 kilograms) [source: Horse Hats]. It has an average lifespan of 25 years [source: WGBH].
Thoroughbreds have two unusual characteristics that make them particularly well suited for racing. Their long necks move in unison with their front legs, which propels them further forward. Second, the hind legs in the breed bend and straighten in a nearly vertical line, which produces a spring-like effect and propels the horse forward more efficiently with each step [source: Jockey Club].
The pattern of movement any animal produces with its legs is known as its gait. The Thoroughbred's gait in a full gallop is something spectacular. The average stride length of a Thoroughbred tends to be more than 20 feet (6.1 meters) [source: Horse Hats]. This means that the distance between the point where one hoof makes contact with the ground and the point where that same hoof next makes contact with the ground is as long as two average stretch limousines parked bumper to bumper. In fact, the legendary Thoroughbred Man O' War, who raced in 1919 and 1920, had a stride length of 28 feet (8.5 meters) in full gallop [source: Bouyea].
With Thoroughbred horses capable of taking 150 strides a minute, the horses can reach speeds of more than 40 miles per hour (almost 65 kilometers per hour) [sources: Horse Hats, Jockey Club]. The horses' ability to endure such speeds during races that are commonly more than a mile long is due to the efficient oxygen delivery system the breed has developed, thanks in no small part to its large heart and efficient spleen. Horses can only breathe through their noses, and they breathe only in rhythm with their stride. They breathe in when extending and exhale when the legs come together, acting effectively as a bellows [source: APS].
The Thoroughbred's heart can circulate up to 75 gallons (284 liters) of blood each minute, and when running, the spleen introduces a perfusion of oxygen-rich red blood cells into the horse's bloodstream, increasing red blood cells from 35 to 65 percent of the total blood volume [source: APS]. The result is the horse's entire system is efficiently fed the oxygen it needs during races.