Board Thread:Naruto Discussions/@comment-4271790-20140705012633/@comment-1512913-20140705170910

Since Indra's chakra (Sharingan) + Ashura's (whatever power) makes up Hagoromo's (the Rinnegan) it's only logical that he also had to have what his sons were given. Every Rinnegan is a Sharingan as well.