You always have to remember that British Thoroughbreds are a closed book and have been for well over 200 years. The only influx of new genes occurred when the American Running Horse was accepted as a Thoroughbred and allowed to be used for breeding. Because of that, all of the modern day Thoroughbred horses are mixes of the same genes over and over. The sire lines have very often gone through bottlenecks, and we may be at the last possible one now, but the mare lines are far more diverse. So, far example, Hermit, one of the best known sources of jumping talent, is dead in the sire line–the last one I’ve found from him was sent to Japan in the 1950s or thereabouts–but he is still quite present in the breed through mares who carried him forward.
So even when a horse carries lines that appear to be almost invisible when added up, the ancestors of that horses may be present in quite high numbers. A few years ago, it was calculated that Herod had provided more genes to the modern TB than any other horse–something in the neighborhood of 14%–and he’s been dead for over 200 years.
