Some bloodlines of thoroughbreds are definitely known to have weaker bones, more easily fractured.
There have been a few fillies who could compete against the colts... there has been at least one winner of the Kentucky Derby who was a filly (Regret, can't remember the year she won).
Some years back, there was another gallant filly, Ruffian, who also fractured a leg and had to be destroyed. I remember reading that her bloodlines were known to be full of horses with weak legs. Her bloodlines had Bold Ruler on both sides of the pedigree, IIRC, and he was known to pass on weak legs.
If you Google Ruffian's name and find a picture, you can see that she had very delicate-looking legs, even for a thoroughbred. She was also a relatively large filly.
Or could the horses be started racing too early in their development? Anyone out there know anything about this?
Some folks think they race them too young, which may be part of it. All thoroughbreds are considered to have the same birthday (not sure, but I think it's January 1), regardless of when they were actually born.
In this country, thoroughbreds commonly start racing as two-year-olds. But they're not actually full-grown until they're about four years old.
Friday’s WSJ had an article explaining the bloodline of the Kentucky Derby racehorses. Every horse in this year’s Kentucky Derby is a descendant of Native Dancer who raced in the early 50’s. The article questioned whether the sport has become too reliant on one set of genes.
Because of heavily muscular legs and a jerky running style Native Dancer and his descendants have trouble with their feet. And over-breeding has made the problem worse.
Barbaro was a great-great-great-grandson of Native Dancer.