This is a point I have trouble making whenever that genetics term gets tossed around on the forum. As soon as it's uttered people look at is an excuse... it's being realistic.
Someone will know within the first 2 years of training if they are something special, if you have King Kamali's arm insertion points you won't have 22s anytime soon but you can surely still build some big ass arms if you are determined enough and do things right.
People that think they are going to turn pro but don't even stand out in a crowd after 3-5 years of training... forget about it and be more freaking realistic. Train hard, but set goals that are within reason.
It's like being dead set on being a fighter pilot if you've got terrible vision... it's not for you but you can still do something important, you just need to understand your strengths/weaknesses. Make sense?