Board Thread:Naruto Discussions/@comment-1512913-20170417105433/@comment-734582-20170417160933

Ravenlot 27 wrote: In general, I can summarise my opinion like that: the original owner of a certain doujutsu isn't necessary the one whom the eyes themselves naturally belong to, but the one who awakened these eyes into the said doujutsu I want to add to this.

The origin of the rinnegan lies with Ashura and Indra chakra. Specifically, combining these two chakras causes a reaction that turns it back to Hagoromo's. Not unlike how the Biju reform into the Juubi. That means that when Madara took in Hashi's cells, his own chakra changed to become Hagoromo's.

The second important keystone is that the EMS are physically transplanted eyes (Sasuke does say to Edotachi "these are your eyes"). We know from Minato and Naruto that each shinobi has a specific "chakra" (represented as a shape) and that transferring chakra requires you to change it to the receiver's "shape". One can thus argue that an EMS is formed when the donor's chakra type is compatible with that of the acceptor, which is the sibling/close relative requirement.

Thus we can connect these two pieces: When Madara took in Izuna's eyes, they essentially became his own eyes. And when he took in Hashirama's cells, they transformed his chakra into Hagoromo's chakra. Since his chakra is also present in Izuna's eyes, they turned into Rinnegan. Note that when Madara had regular sharingan in his eyes, they didn't turn to rinnegan since they aren't compatible in the same way.

All that rests is to reverse the reasoning: If Madara returns Izuna's eyes, Izuna's own chakra would be present and he would be able to use EMS. After all, if EMS works one way it should work the other way too. The Rinnegan however exists due to Madara's contribution, and the rest of Izuna's body would not possess the same Six Paths chakra. Thus, while he would likely be able to control the eye on a level that other transplanters could not, he won't be the True Owner of the eye.