97% of Thoroughbreds trace to Northern Dancer — inbreeding cuts racing odds by 7%

The global Thoroughbred population traces back to just three foundation stallions from the 1700s, and the gene pool has narrowed dramatically in the modern era: 97% of horses studied carry Northern Dancer bloodlines, 35% of European horses carry Sadler's Wells, and 55% of Australian horses carry Danehill. A peer-reviewed Royal Society study found that a 10% increase in genomic inbreeding coefficient is associated with a 7% lower probability of a horse ever racing at all. A specific haplotype on chromosome ECA14, when homozygous, is linked to a 32.1% reduction in the probability of racing and overlaps a gene (EFNA5) expressed in cartilage — the tissue most commonly implicated in catastrophic breakdowns. This means the breed is literally selecting for fragility. The problem persists because the Jockey Club's stud book has been closed since 1791 (no outside breeds allowed), the most commercially successful stallions get 100-200+ mares per year via the 'popular sire effect,' and breeders optimize for auction price (which rewards fashionable pedigrees) rather than genetic diversity or soundness. There has been no arrest in the rate of inbreeding increase despite decades of scientific warnings.