Ireland is not a big country. It covers only 32,599 square miles, making it about the same size as Indiana. But Ireland plays a large and important role in the Thoroughbred industry. Some of the best runners in the world are raised on its limestone-enriched pastures. Two Irish-bred standouts-High Chaparral and Domedriver-won races at the 2002 Breeders’ Cup World Thoroughbred Championships, and another, Rock of Gibraltar, was Europe’s 2002 Horse of the Year.

Ireland is also a hotbed of genetic research involving the Thoroughbred. Patrick Cunningham, PhD, and his team of scientists at Trinity College in Dublin have been looking at the role heredity plays in the development of the breed for more than 20 years.

“The first study we did was on inbreeding in Thoroughbreds,” Cunningham said. “As you know, Thoroughbreds are notoriously infertile. It takes about three mares to produce two foals each year if you look at statistics, and that’s very low. If you look at populations of single offspring species like cattle and sheep-or horses in the wild-they’ll normally produce a foal crop of about 85% in natural conditions. We decided to look at infertility in the Thoroughbred, thinking it might well be due to inbreeding because in all species, as you increase inbreeding, fertility is one of the things that declines.

“What we showed is that inbreeding is about 13-14% in Thoroughbreds going back to the base of the population,” he continued. “But the variation in inbreeding was not related to variation in fertility of mares. How do you explain that? Our explanation essentially was that this inbreeding has been accumulating very slowly over 30 generations and fertility is subject to natural selection. We calculated the amount of natural selection that would offset any expected decline from inbreeding, and we concluded that if you eliminated about 8% of mares in any one generation for reasons of fertility, that would be enough to sustain fertility. So at any rate, we concluded that inbreeding was not something we had to worry about in the Thoroughbred population at that time because the population was quite big. We still don’t think we need to worry about it today