This has been debated to death, see something like: http://en.wikipedia.org/wiki/Nature_versus_nurture, Pinkers "Blank slate" book, or what other people already mentioned. Look at extreme cases of people with genetic diseases on one side of the spectrum, and people like John Von Neumann on the other end, and you really shouldn't have any doubts about this, however bad this makes you feel (I had this reaction first considering this too, and judging by the amount of people who responded to this comment very promptly, I judge this happens to a lot of people). By the way I am just summarizing what seems to be the majority vote of scientists, I don't have any strong personal opinion on it.