Oh you should not be feeling guilty! You are doing a great job as a mother, and nobody is responsible for the genes. Plus, you wrote that he is "usually A's, and pretty sharp" and if I may venture a guess, he got the genes necessary to be "usually A's, and pretty sharp" from you.

Nobody has yet figured out a way to pass on some genes but not others, selectively. So the kids end up sharp but sometimes depressed.