If you ask what is the most influential horse in Thoroughbred racing, you’ll likely to hear names such as Secretariat, Seabiscuit, Man o’ War or War Admiral. The distinction, though, rests with a horse that isn’t exactly a household name.

Darley Arabian, a stallion born in 1700, has a profound influence on Thoroughbred racing. That’s because almost 95% of male Thoroughbreds today can trace their Y chromosomes back to his.

According to National Geographic, a new study found that selective breeding among horses for certain traits has been going on since the seventh century. But some of those traits, specifically the speed for which racehorses are known, have only been a recent focus. 

A senior author of the study, Ludovic Orlando, tells National Geographic that breeding horses for short-distance speed have only become a priority in the last 1,000 years. And even more recently, around 200 years ago to be exact, the emphasis on speed intensified, as breeders use only a small number of stallions. 

Despite horses getting faster, the breeding technique does have its drawbacks. Genetic diversity has reduced by about 15 percent, says Antoine Fages, the study leader. Low diversity presents the risk of harmful genetic defects. National Geographic reports that there are already illnesses like night blindness and myopathy are already affecting horses.