Board Thread:Naruto Discussions/@comment-3232093-20140617193601/@comment-4271790-20140819024533

Maybe the power of the Sharingan was a recessive trait. There might not have been a physical Sharingan eye to start out with. Kaguya had the Byakugan eyes and a Rinnegan eye with the power of the Sharingan (never stated to have been a Sharingan eye, just its power, that's why I think the power could be a recessive trait). It could have skipped Hagoromo and appeared in his son, Indra. Or maybe Hagoromo did have the Sharingan. All I know is that Hagoromo and his descendants inherited his mother's Rinnegan and Sharingan, whereas Hamura and his descendants inherited his mother's Byakugan.