Board Thread:Versus Debates/@comment-33229263-20171019104931/@comment-681696-20171019175332

Hagoromo was likely missing some of the genes that were responsible for his mothers Rinne-Sharingan it is unlikely the Rinne-Sharingan is the result of a single gene but is more likely to be the result of multiple genes working together to produce the Rinne-Sharingan.

As for requiring Hagoromo chakra to gain the Rinnegan it is possible that both of his children lack some of the genes responsible or at least the genes are split between both brothers.