The Diablogger, Mark Aveyard, has a good post up that warns of the slipperly slope that gapes before us if we buy into that argument that:
a) Some people P are more likely to perform behavior X because of genetic predisposition.
b) Therefore, the way society views P and X should be based on that genetic predisposition, and not wholly on the effects and consequences of X on their own.
That way lies legal confusion. If the goal of law is to protect people from the dangerous and harmful effects of other people's actions, then it shouldn't matter why a crime was committed, only that it was, in fact, committed and that it did cause harm to another party.
Some very dangerous behaviors are unquestionably based in genetics, such as psychopathy. Does that mean that society should not act to protect itself from psychotics? Clearly not, and even when a murderer is acquitted due to insanity he is locked up for "treatment" (despite the fact that his malady is entirely untreatable).
The law makes a distinction between motive and intent. If I want your money and decide to rob you, my motive is to get money. However, when I then pull out a gun and shoot at you, my intent is to kill you. If I didn't use deadly force and only threatened you, my intent would have been different even though my motive would have been the same.
Motive should play no part in sentencing, only intent should count. It doesn't matter why you threatened me or why you shot me -- my hair color, race, clothes, money, attitude, revenge, or genetic predisposition? Irrelevent. Judges, juries, and lawyers can't read people's minds, but they can determine intent based on observed actions.
Note: that's not to say that motive is irrelevent when considering the morality of an action or behavior. More on that later, perhaps.