Board Thread:Naruto Discussions/@comment-3568954-20140523074558/@comment-24022934-20140714124323

You have to have both Indra's and Asura's chakra in order to awaken the Rinnegan. It cannot be awaken by those who aren't either of the sons' reicarnation or don't have both of the sons' chakra. Obito had both Senju and Uchiha DNA, but he didn't awaken the Rinnegan himself: he had Asura's chakra, but he lacked Indra's chakra.