Originally Posted by Dagoro
Its logical to assume that RS did not have the RG before he became the Juubi jink, and that the Juubi was indeed the catalyst that sparked this doujutsu ridiculousness that has consumed the manga.
RS didn't need magic eyes to beat the Juubi, he had an extremely powerful body ( the youngest son inherited it ) to begin with.
That might indeed be the case, RS being an ordinary ninja with an extremely powerful body and that he gained only RG after sealing the Juubi inside him, while the SG part remained dormant until it was awakened in the Uchiha clan.
Maybe, in humans, that RG is dominant over SG, so a some point the Uchiha might have inherited only SG genes. This is then likely the cause of awakening the SG.
I still wonder how BG is related to RG & SG then.