I think there is definitely a genetic component, but certainly no guarantee. If you are going to breed look to the foundation Quarter Horses - King, Doc Bar, Two Eyed Jack, etc. - in both the mare and stallion. One of the recent editions of Western Horseman had a list of the top 10 sires of working ranch horses if you want a better list. The other plus is that you can imprint, train, and handle them as they grow up. You never quite know what you're getting when you buy a horse from someone else.
With that said, now is not a good time to be breeding horses. The market out west has collapsed. It's not so bad in Florida right now, but I expect it to get worse. We are in the process of downsizing now.