Re: lack of genes or lack of trying?
I'd say you need a degree of aptitude simply to get started, but success is more dependent on interest.
After all, if you're not interested you won't apply yourself to a subject even if you've got the aptitude.
Where as if you're interested, you can't succeed, no matter how hard you try, if you've no aptitude (don't believe me: Listen to me try and play the Saxaphone. Or don't: It'll save your ears).
So the limit is a combination: Interest, in order to want to pursue a subject, and aptitude, to see just how far you can get with it. Study can compensate for aptitude to a degree, but you'd still need the greater aptitude to progress far.
As for where aptitude comes from: Well, part is nature, part is nurture. How our brains wire up gives us the base line tools we then need to learn how to use. It's quite a study, particularly if you're interested in cognitive psychology and Artificial Intelligence.