Well, except for the hard morphology genes ("make this bone bone-shaped"), genes tend to code for potentialities/ranges. Height is probably the most common physical example: Your genes say you have the potential to grow to be between 5'5" and 5'10", say. No matter how bad your diet is, you'll be at least 5'5", and no matter how great your diet and exercise is, you'll never be over six feet tall.

Same for IQ; some people simply won't ever be good at learning in and of itself, or retaining what's learned, or understanding what it is they were supposed to have learned. Others could be good at it, but they've been inhibited somehow (diet, distractions, not being taught how to think/learn/apply what's learned, are domesticated/institutionalized and don't need to think or shift for themselves.)