Board Thread:Theories and Speculation/@comment-26422608-20150522042352/@comment-26090730-20150523135136

As far as we know, this is how it works:

1. Ashura´s chakra+Indra´s chakra+Sharingan=Rinnengan

2. Ashura´s chakra+Indra´s chakra+No sharingan=????

By mixing those chakras alone or Sasuke´s DNA, Naruto can´t awaken Rinnegan