Board Thread:Naruto Discussions/@comment-3568954-20140523074558/@comment-24068519-20140713053506

DNA has nothing to do with awakening the Rinnegan. Madara awakened the Rinnegan by biting off a bit of Hashirama's flesh, putting it in to his wound and when his life span was nearly ending he awakened the Rinnegan. He was basically mixing Indra's chakra with a part of Ashura's chakra and that brought out Hagoromo's chakra. This worked because Madara was the reincarnation of Indra and Hashirama was the reincarnation of Ashura and both Indra and Ashura are the sons of Hagoromo who welds the Rinnegan. They're not literal reincarnations, they're just basically inheritors of the chakra. I got this from page 6 of chapter 671 "Naruto and the Hermit of the six paths...!!".