Board Thread:Naruto Discussions/@comment-29393962-20161111215453/@comment-734582-20170116090605

LegionZero wrote: Rinnegan seems to have a few pre-reqs to activate. You need Indra and Ashura's chakra. That's it. Madara had very little of Hashirama's flesh which is why it took so long for it to awaken.

It does mean that regular people ought to be able to awaken it if they happen to have enough fragments of their chakra, but it's fairly clear that this is a very rare occurrences (after all, Orochimaru tried a whole bunch and it didn't work) to the point that BZ didn't bother.

The Rinnegan is also not a recessive trait, as it's clearly dominant over Sharingan.