The rinnegan grants its user the power of the six paths. With it one wouldn't need their dna, the fact that the rinnegan was awakened in one of his descendants shows that it has stayed in the family. Its not clear which side the uzumaki clan comes from or if its a mix of both. In the manga and anime when the list is shown of the descendants of his sons some intermarried and since nagato had enough chakara to destroy a village and create a huge chibaku tensei and bring back the dead of konoha(all while controlling six bodies and don't forget chakara wasted on the walk there) and has the rinnegan I'm leaning towards the intermarriage side. However the rinnegan is the sages bloodline trait, him awakening it is proof of him being his descendant. With the rinnegans powers nagato can use all six nature types which includes yin yang release, which is unheard of, which is the release used to create izanagi. The sages power is the rinnegan and possessing it means to have his power. Because izanagi is genjutsu its possible that nagato can't use it well. None of jiraiya's students, including the teacher himself, were good at genjutsu. That said it could be why its never been used by nagato, but we have no way of knowing that. However he may vary well use it now that he is complete. It is implied that if he has the power of the six paths and to use it you need the power of the six paths he has the potential to use it but its unknown if he has. We'll have to wait to see if he uses it in the coming chapters. I'll find the chapter and scan it in so you can see madara calls nagato the third sage of six paths and not the third of the six paths. I gotta go its late and I need to get to bed.
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