Biggest Mystery In all of NARUTO!

[Draphsin]

Its implied that the rikudou sennin created his sons

 

[Ventalus]

We don't know that.

1) We know that the Sage had Rinnegan
2) We know that the elder son had a weird eye, that became Sharingan later.

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Juubi doesn't have a Rinnegan. But he doesn't have a Sharingan either.

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This thing isn't a Sharingan at all.
It seems more like a mixture of both. Maybe Rinnegan isn't the ultimate doujutsu. Maybe there is this thing...

Sharingan is a portion of the 'Sharinnegan' with was a fan named from the Sharingan. But Juubi has 9 tomes meanwhile a human can only have 3 tomes. You can see that Uchichas only have the middle ring of the Sharingan, meanwhile Juubi has those 3 last lines and two of them contains 3 other tomes.

And yes it have been talking about Rinnegan + Sharingan is the ultimate doujutsu, but as well Rinnegan can as well only be a mutation from mixing Senju (Rikudou) With Juubi (Uchicha) and with mutations it can disabled some abilities but as well enabled new abilities, etc Rinnegan's power to have all chakra natures as his own and perhaps Juubi doesn't.

On the elder son it could as well been a mix between the Sharingan and the Rinnegan, other words if we go by logic Juubi's DNA is more dominant than Rikudou's with would pretty much push away Rikudou's DNA and give the child same abilities as Juubi, etc the doujutsu Sharingan. You can for example see that on the Uchicha clan that most people actually had the Sharingan with would probably mean that Juubi's DNA was so damn dominant so it actually still follows strongly with the Uchichas.

So Elder son is/was probably more likely walking to Juubi's bloodline.

But never mind, nothing of these will be stated in Manga so it's no need to actually talk about this lol.