I do believe that there is a genetic component to underbites. Not sure exactly how, though.
I do believe there is a comorbidity rate between underbite and other conditions that might have genetic links. I do believe there is, for example, a link between autism and underbites, though. I saw a thread on another forum once where someone brought up this idea, and a dental hygienist chimed in that in her autistic patients (at least among girls), she saw common traits, one of them being a higher rate of underbites.