Thoroughbred Pedigrees Show Little Genetic Diversity

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 studbook data, Cunningham's team analyzed the DNA of 211 horses from Coolmore Stud, one of the world's leading racing empires. While pedigree analysis reflects how closely horses are related, DNA analysis reveals how many genes horses have in common, a slightly different measure of kinship. Combining both sources of information, the researchers could then speculate about the amount of genetic diversity present in original foundation stock and compare this to today's horses. Genetic diversity was plentiful during the Thoroughbred's formative years, but it's no surprise that it has declined significantly over time. Some breeders worry that the small initial population size might have contributed to dangerously low levels of genetic diversity, or conversely a high level of inbreeding, leading to decreased reproductive fitness or an increase in genetic defects. Inbreeding in today' s Thoroughbred is estimated to be about 14%, slightly higher than one would expect from a mating between half-siblings. However, much higher levels are seen on other breeds of livestock and some wild populations, and Cunningham and coworkers suggest that despite the small number of founders, the Thoroughbred has sufficient genetic diversity for modern breeding goals.

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