From Steven Pinker's article, "My Genome, My Self," in the New York Times Magazine:
Even when the effect of some gene is indubitable, the sheer complexity of the self will mean that it will not serve as an oracle on what the person will do. The gene that lets me taste propyl thiouracil, 23andMe suggests, might make me dislike tonic water, coffee and dark beer. Unlike the tenuous genes linked to personality or intelligence, this one codes for a single taste-bud receptor, and I don't doubt that it lets me taste the bitterness. So why hasn't it stopped me from enjoying those drinks? Presumably it's because adults get a sophisticated pleasure from administering controlled doses of aversive stimuli to themselves. I've acquired a taste for Beck's Dark; others enjoy saunas, rock-climbing, thrillers or dissonant music. Similarly, why don't I conform to type and exploit those fast-twitch muscle fibers (thanks, ACTN3 genes!) in squash or basketball, rather than wasting them on hiking? A lack of coordination, a love of the outdoors, an inclination to daydream, all of the above? The self is a byzantine bureaucracy, and no gene can push the buttons of behavior by itself. You can attribute the ability to defy our genotypes to free will, whatever that means, but you can also attribute it to the fact that in a hundred-trillion-synapse human brain, any single influence can be outweighed by the product of all of the others.
See also The Useless Trees' Daoist response here.