[In TDT] If you desire to smoke cigarettes, this would be observed and screened off by conditioning on the fixed initial conditions of the computation - the fact that the utility function had a positive term for smoking cigarettes, would already tell you that you had the gene. (Eells's "tickle".) If you can't observe your own utility function then you are actually taking a step outside the timeless decision theory as formulated.

Consider a different scenario where people with and without the gene both desire to smoke, but the gene makes that desire stronger, and the stronger it is, the more likely one is to smoke. Even when you observe your own utility function, you don't necessarily have a clue whether the utility assigned to smoking is the level caused by the gene or else by the gene's absence. So your observation of your utility function doesn't necessarily help you to move away from the base-level probability of having cancer here.