Board Thread:General Discussion/@comment-74.196.223.125-20130804060138/@comment-81.159.105.178-20140426164120

The Lore doesn't state that the next Dragonborn has to be a child of the last. Whilst the Dragon Blood is passed on through the Septim bloodline, the abilities die off (apart from the ability to see in to a mans soul - one our Dragonborn is strangely lacking) because there are no Dragons to beat down on. It's quite possible that whilst the rest of the Septim Bloodline had that ability, they wouldn't technically have been "Dragonborn". Even Martin is gifted his Dragon form by the nine purely to give Dhagon a kicking, and the nine would have known that would cost him his life.