and I have to add, that OFA health testing is much more than just hips and elbows (though those are the ones that get stressed alot). For the "English" Mastiff the OFA/CHIC recommend not only hip and elbow dysplasia testing, but also eye exams, heart exams, thyroid testing, and urine testing for cystinuria. And last I checked the Mastiff Club of American takes it even further, recommending several pieces of DNA testing be done (I can't get a list right this minute as it appears their site might be down?).
Hips and elbows CAN be affected by a huge list of things besides genetics, current studies indicate a link between early spay/neuter in dogs to hip dysplasia. HOWEVER, that does not absolve a breeder from having their dog tested or offering a good health guarantee IMO. Because we don't know how much of a genetic link there is we have to assume that there is recessive genes involved and EVERY dog should be checked, and any dog showing dysplasia shouldn't be breed.