I think that HD is part genetic but also part environmental. Things like diet, too much hard exercise too young, allowing them to grow too fast, all contribute to conditions that can cause HD. If your pup is growing too fast, you are running him hard, and not feeding him the right food, you are just as much at fault if he gets it especially if he comes from lines that have not had many instances of HD. Too many people want their GSD to get real big and grow real fast. Slow and steady wins the race. A dog's size is genetic (aside from being overweight). A GSD that is going to get to 25" and 85lbs is going to get there no matter what, so by feeding him the correct diet and allowing him to grow slowly, you are going to have a healthier dog in the long run. If he's 80lbs at 6 months, that's too big and you might end up with problems. People who have these 120-130lb GSDs are so far out of the breed standard, and most likely their dogs are way overweight, and conditions like that contribute greatly to poor health. This is all theoretical, not saying you are doing any of these things, but I wanted to put this out there.