Board Thread:Naruto Discussions/@comment-26488946-20150630094329/@comment-5407586-20150630101048

One could say that the Sharingan is the manifestation of Indra's chakra. Every single one of Indra's descendants inherited their ancestor's power, only in a weakened version compared to the trasmigrants, who inherited the full strength of Indra's power.

Hagoromo says that the Rinnegan can be awakened by gaining his chakra. So as long as one has the chakra of both Indra's and Asura's trasmigrants, or just Hagoromo's chakra, in one's body, it should be enough to awaken the Rinnegan. The only problem is that it takes many years, if not decades, unless you're a reincarnation and have been given chakra directly from Hagoromo. I dunno if Asura's incarnation would awaken the Sharingan first by gaining Indra's chakra, or if he/she would awaken the Rinnegan directly.