The names Darley Arabian, Godolphin Arabian and Byerley Turk are writ large in the annals of horseflesh for those in the know. The three horses are responsible for almost all of the paternal bloodlines in thoroughbred horses around the world.
One in particular, the Darley Arabian, born in 1700, can lay claim to 95 per cent of the paternal lineages, according to Patrick Cunningham, professor of animal genetics at Trinity College, Dublin. "It was as if 95 per cent of all the people in the country were called Murphy and all were related to a single Murphy," he says. And the Byerley Turk was thought to have been ridden at the Battle of the Boyne in 1690 by a Cpt Byerley, who fought with the Williamites.
Cunningham's analysis with colleagues of the gene pool found in modern thoroughbreds is published in the current edition of the journal Animal Genetics. It shows how the bloodlines used to breed thoroughbreds have been put through a "bottleneck" that has left them short of genetic diversity.
That there should be such a high level of relatedness in these horses is not surprising, given the history of the industry, says Cunningham. This is because the bible of the industry is The General Stud Book, and only horses entered in it are considered thoroughbreds.
"The original stud book that records their breeding was set up in the 1780s by James Weatherby. His descendants are still running the firm," says Cunningham.
Weatherby chose fine animals from Britain and Ireland and traced their ancestry back as far as possible until a male with an unknown sire or a female with an unknown dam were found, the "terminal" historic points of the Studbook.
Since then, upwards of a million horses have been entered in The General Stud Book, all descended from these original animals. Some were favoured over others, however, which led to some males and females having a greater impact on the genetic make-up of today's thoroughbreds, says Cunningham.
The research group started with a computer analysis of the stud-book entries to establish the key "foundation" animals, then used samples from 211 thoroughbreds at Coolmore Stud, at Fethard, Co Tipperary. "We used DNA analysis to check the genetic origins of the British and Irish thoroughbreds which are the source of the 500,000 thoroughbreds in the world."
The findings were startling. The top 10 "founders", for example, were responsible for 45 per cent of the genetic make-up in modern animals. The top 20 contributed 65 per cent of the total genes and the top 30 founders 78 per cent of the genes found in today's animals.
"The most striking thing is that we were able to confirm the dominance of the three most important founder stallions, but what is new is that one of them is responsible for 95 per cent of all the male lineage," explains Cunningham.
"The percentage of paternal lineages attributable to the Darley Arabian line has been increasing for nearly 175 years," he writes.
The DNA analysis involved a search for segments of DNA known as microsatellites. These are sections of DNA that are not influenced by natural or human- orchestrated selection and so remain unchanged from one generation to the next. The team searched for 13 microsatellites that helped confirm the "closed population" of thoroughbreds today. "The stud book prevented anything else from getting in."
The researchers at Trinity also studied DNA from Egyptian and Turkish horses, confirming these as central to the origins of the stud-book foundation horses.
A reason for looking at the genetic make-up of today's very valuable animals is a "continuing concern that the narrowed genetic base of the thoroughbred may be limiting genetic progress in performance and contributing to an increased frequency of heritable disease," writes Cunningham.
His current view, however, is that "there is sufficient genetic diversity" for the characteristics of importance to the thoroughbred industry.