As to the thing about genetics.... Have you seen Jay train? I watched a back training video, and though he was working hard, he wasn't doing anything most hardcore trainers do. He was using really crappy form on pulldowns with something like 250 lbs. Why would that even be hard for a man his size? I'm tiny compared to him and use about 10 lbs more with better form. I'm sure he's doing more volume than me, but you just can't discount genetics. Yes, he works hard. Yes, he diets like a mad man and has amazing discipline! Yes, he probably uses a boatload of supplements, legal and otherwise. But, the dude looked better at 17 yrs old than most of us will after years of training. Get real.
Consider Mike Mentzer. The dude was 5'8"-5'9" same as Jay and competed at 225 at his biggest. Mentzer was known for high intensity training, working to absolute failure and all that. Why wasn't he as big as Jay is (280 ripped on stage)? I mean, we're talking 45 lbs difference here. The answer is: 1) no insulin or GH 2) He's not Jay, was not born to be Jay or have his potential for muscle growth.
Jacob