It just depends on the individual horse. He may be cut out for it, or he might not be. Generally, if they are stockier, then they'll do better, but, again, it just depends on the individual horse. I've heard both that they can and that they can't. I've met TONS of 1D Thoroughbreds here in my area. (I race against Brittany Pozzi!) So, knowing that I'm up against some of the toughest competition, I'd say if there is a 1D Thoroughbred, then they CAN race and be good at it! ...but, again, not every single horse is the same. It just depends on his personality, his work ethic, his conformation, etc.