Most racehorses traced back to single ancestor

A genetic analysis of more than 200 thoroughbreds and a computer check of a million race-horse pedigrees has confirmed the highly inbred status of the modern racing champion.

Professor Patrick Cunningham of Trinity College, Dublin, said a combination of modern molecular genetics and a trawl through 300 years of pedigree stud books has identified the key ancestors of the half-million thoroughbreds in the world today.

One stallion, a bay colt called the Darley Arabian, stands out. The study found that his Y chromosome, which carries the male genes, is found in 95 per cent of modern thoroughbred stallions.

The Darley Arabian was born in 1700 and brought to England from Syria in 1704 by the British consul of the day, Thomas Darley, who made the stallion into a champion stud.

A mare called Tregonwell's Natural Barb contributed the largest proportion - 14 per cent - of the genes passed down the female line of racehorses, Professor Cunningham told the British Association's Festival of Science in Dublin.

By analysing the pedigree status of all horses registered in the General Stud Book over the past three centuries, Professor Cunningham found that a small number of stallions began to emerge as the founders of the male line.

"In the early days there was some jockeying for position between the early stallions but after about 70 years, some dropped away and three survived," he said.

The three stallions found to contribute most of the genes to today's stallions were the Darley Arabian, the Byerley Turk and the Godolphin Barb, "and eventually the Darley Arabian raced to supremacy," Professor Cunningham told the conference.

The findings came out of a wider scientific investigation into the genetics of the thoroughbred racehorse, the origin of which began when about 80 horses from the Middle East and North Africa were imported into England for breeding.

An analysis of the pedigree status and the DNA of about 211 racehorses showed that most of the genes of this original group of 80 have been lost over time, Professor Cunningham said.

"The effective number of founders was about 28 - there were about 28 horses contributing pretty well all the genes in today's population, that's despite the fact that there were about 80 horses recorded in the first stud book," he said.

"We've been able to measure the contribution of these founders to today's population and the top 10 contribute about 80 per cent of the genes," he said.

"We've measured the inbreeding over that time and there's about a 15 per cent accumulation of inbreeding. For every two horses in our sample we were able to get a molecular measure of their relationship. It turned out that on average that was 47 per cent, so any two horses share 47 per cent of their genes," he said.