User blog comment:Jack Pistol/Immortal Dovahkiin?/@comment-5558140-20121004222153

im pretty sure not every septim was a dragonborn. theres quite a few lore-holes when it comes to the septim bloodline. even in oblivion, martin septim was not a direct descendant of tiber septim. tiber septims last son was palagius the first, who died childless. the septim line was continued by tiber's brother angorinth. there may have been more, but the only ones we know about are tiber septim, and our own, so its quite possible that being dragonborn is in no way a genetic trait, just luck of the draw.