It seems everyone has already agreed on making Dragonmarks a Gift feature! I guess I'll be first voice to suggest another direction, then.
Yes, Dragonmarks are associated with certain houses and bloodlines—some of them anyway. Forgive me if I'm wrong, but someone doesn't have to be a blood member of a certain family (not in all instances, anyway) to have a Dragonmark, yes? People with specific Dragonmarks are expected to know certain people, be trained in certain things, etc. There's as much culture as genetics (if not more) involved in Dragonmarks, so I definitely imagined it fitting at least as well as a character's Culture.