More than 95% of Thoroughbreds share the same paternal lineage, and almost half of the genetic makeup of the breed can be attributed to just 10 foundation horses. These facts are just a few of the recent findings brought to light by a research team at Trinity College in Dublin, Ireland. The group, led by Professor Patrick Cunningham, MS, PhD, used pedigree records from nearly 1,000,000 registered Thoroughbreds, dating back more than three centuries, to perform the most complete pedigree analysis to date for the breed.

The researchers found that horses in today’s Thoroughbred population can trace their paternal lineage to one of just three foundation stallions, with one, the Darley Arabian, accounting for more than 95% of today’s sire lines. In human terms, this is analogous to saying that 95% of Thoroughbreds have the same surname. The superiority of the Darley Arabian has been increasing for nearly 200 yea  rs, eclipsing the contributions of the other two foundation stallions, the Byerley Turk and the Godolphin Arabian. Switching to the bottom side of the pedigree, the researchers also discovered that a surprisingly small number of foundation mares were key in developing today’s female families. More than 70% of modern-day Thoroughbreds can trace their maternal lineage to just 10 mares, while 94% of horses come from only 30 foundation mares.

When the entire pedigree was considered, it was discovered that just 10 foundation horses are responsible for 45% of the genetic makeup of modern-day Thoroughbreds. Foremost among these is the Godolphin Arabian, who contributed about 14% of the genes found in the average Thoroughbred today. Amazingly, the relative contribution of foundation horses has not changed significantly since the late 1700s, indicating that the population has been relatively homogenous for centuries and reflecting the breed’s effectively closed studbooks.

Not content to work solely with studbo