I'm genuinely curious to know how you think about that line. What is the transgression here in your mind? Merely mentioning that genes affect behavior is not allowed (even though it's true, or maybe it doesn't matter if it's true), or you actually believe the claim "genes affect behavior within the human species" is false?
I agree with you that we can't tell from a single gene whether someone is likely to be a criminal or not. I'd like you to consider this example, where hopefully I only use premises you already believe:
Some people are predisposed to becoming addicted to drugs. We can make better-than-chance bets about who these people are based on their genomes. We also know that people who are addicted to drugs are more likely to be on the wrong side of the law (and, sadly, are often by definition on the wrong side of the law). So, we conclude that, given someone's genome, we can make better-than-chance bets about that person's likelihood of being on the wrong side of the law.
It's a simplistic example, but hopefully that helps get the idea across. Doing this sort of thing isn't super practical right now, but it will be soon! [oh boy.]