Or more specifically, descendants of Alessia, who made the original pact with Akatosh, the dragon god. I suspect her blood is well spread out in the noble human families of Tamriel. Recall that only one of the Septims after Tiber, Pelagius I, was descended from Tiber Septim. After than it was a line descended from his brother, tehn a consort of no known Septim blood, one from a distant cousin line (Lariat), and then a closer cousin line.