Virtually all 500,000 of the world’s thoroughbred racehorses are descended from 28 ancestors, born in the 18th and 19th centuries, according to a new genetic study. And up to 95% of male thoroughbreds can be traced back to just one stallion.

Thoroughbred horses were developed in 18th century in the UK. English mares were bred with Arabian and other stallions to create horses with great stamina for distance racing. Today, thoroughbreds are the most valuable of breeds, representing a multi-billion dollar annual industry, worldwide.

To assess the genetic diversity of modern racing horses, geneticist Patrick Cunningham of Trinity College in Dublin, Ireland, compared 13 microsatellite DNA loci – repeating sequences of DNA which vary in length – in 211 thoroughbreds and 117 other Shetland, Egyptian and Turkish horses. He also examined studbooks dating back to 1791.

He found the majority of the half million progeny alive today are descended from just 28 “founder” horses.


It was already known that just a handful of stallions (but many mares) were used to found the thoroughbred breed. But startlingly, the new research finds that, in 95% of modern racehorses, the Y-chromosome can be traced back to a single stallion – the Darley Arabian, born in 1700.

Speedy selections

Related work on sequencing the horse genome is also uncovering genes in thoroughbreds linked to speed and stamina. Screening for these traits could one day guide owners’ and breeders’ decisions when buying horses, which may sell for many millions of dollars.

“We hope to produce sounder, faster and better-performing horses,” says Cunningham. He and colleague Emmeline Hill at University College Dublin is also using the horse genome to uncover genes that explain why one animal runs faster than another.

“Horses are flight animals naturally selected for speed and stamina in the wild,” explains Hill. “With domestic selection, speed was further augmented in the thoroughbred.”

Thirty-five per cent of the difference in racing performance between horses can be explained by genetics alone, says Hill. She is cross-referencing up to 140 recently discovered human genes for fitness and performance in a bid to track down equine equivalents. These genes are involved in traits related to the cardio-respiratory system, muscle strength and metabolism, she says.

Bleeding lungs

However, the analysis of thoroughbred genetics is also revealing the other side of the coin, notes Matthew Binns of the Royal Veterinary College in London, UK. Many negative traits are associated with inbreeding in the diminutive gene pool, he says. “The selections we’ve made for fantastic beasts have had some detrimental consequences.”

One tenth of thoroughbreds suffer orthopaedic problems and fractures, 10% have low fertility, 5% have abnormally small hearts and the majority suffer bleeding in the lungs, says Binns.

But as well as allowing breeders to select for performance-related genes, elucidating the horse genome may allow researchers to breed out negative traits, he says.

“Now we have a good amount of the horse genome, there are interesting times ahead,” says Binns. “Over the next 10 years there will be some changes in this very traditional industry.”

Cunningham presented his findings on Monday at the British Association Festival of Science in Dublin.