So, as soon as it's not 100% of people two-boxing having the two-boxing gene, but only 99.9%, you assume that you are in the 0.1%?

You didn't specify any numbers. If the actual number was 99.9%, I'd consider that strong evidence against some of my beliefs about the relationship between decisions and genes. I was implicitly assuming a slightly lower number (like 70ish area), which would be somewhat more compatible, and in which case I would expect to be part of that 30% (with greater than 30% probability).

If the number was, in fact, 99.9%, I'd have to assume that genes in general are far more related to specifics of how we think than I currently think, and it might be enough to make this an actual Newcomb's problem. The mechanism for the equivalency Newcomb would be that it creates a causal link from my reaching an opinion to my having a certain gene, in TDT terms. Gene would be another word for "brain state", as I've said elsewhere on this post.