The TB breed was never as fragile as it is today. Horses would run on surfaces as hard as concrete and covered with rocks over 6-7 miles races sometimes 2 times a day and not go lame. The horses that did breakdown were unfortunately put down. Only the soundest horses made it to the breeding shed. Now, horses breakdown all the time even with softer and safer surfaces. Horses that survive these break downs are sent to breeding. These horse pass on their weak leg bones to their foals, making the breed weaker. Some stallions also carry a weak bone gene. Native Dancer is most known for passing this gene onto his foals. Native Dancer was one of the best stallions to enter the breeding shed. He sired many champions and is still influential in pedigrees today. He is in almost every TB pedigree. Most every race horse that breaks down can be traced to Native Dancer. Barbaro and Eight Belles trace back to Native Dancer. Native Dancers son Raise A Native was known for his weak ankles. He can be found in Eight Belles pedigree. Inbreeding is another reason why horse have weaker bones. The inbreeding of horses like Native Dancer, Raise and Native and Bold Ruler, who all carried the weak bone gene, has made TB's weaker. Horses with less inbreeding are sounder and do better at racing.
We should only breed sound horses. If a horse retires because of injury he or she should not breed. They could be used as lead ponies or sent to retirement homes.