"I don't know that anyone has looked at speed and endurance individually, but when you look at racing performance as a general term, it has a heritability of 30-35%, which is pretty typical of complex traits," Cothran says. He attributes the remaining 65-70% of a horse's abilities to non-inherited factors such as nutrition, training, and health care. Not everyone agrees. Dick Galley, an equine veterinarian from Texas, believes the correct ratio is "70% heritable and 30% environmental."
How much of a horse's athletic ability is due to genetics? A researcher at the University of Kentucky answers that question in an article written by Sushil Dulai Wenholz that appears in the March edition of The Horse. "I believe there is a heritable component to speed and endurance," Dr. Gus Cothran says. "But they are not just simple traits; the qualities are controlled by a lot of different genes." Each of those genes has a heritability factor of its own. For example, studies have shown that heart girth is 35% heritable in Arabians.