"In theory, an Eternal Mangekyō Sharingan can only be created by fusing the Mangekyō Sharingan of two brothers.
"But throughout Uchiha history, only Madara Uchiha ever awakened the Eternal Mangekyō Sharingan.
"Indra awakened the Mangekyō Sharingan, but because of his bloodline, his eyes would never go blind. As for me, certain circumstances let me use Susanoo even though I never awakened the Mangekyō Sharingan." Uchiha Yi let out a long sigh. "Ultimately, it comes down to the decline of the Uchiha Clan's bloodline.
"In theory, there shouldn't be a distinction between the Mangekyō Sharingan and the Eternal Mangekyō Sharingan. Brothers have similar genes, which allows their eyes to fuse. I think that fusion resolves a genetic defect."
"So it makes up for a deficiency in their bloodline?" Tobirama Senju nodded thoughtfully.
If the Uchiha were deficient, didn't that make the Senju Clan superior? The Senju didn't have to worry about going blind.
Their bloodline had declined too, but that was because the Senju Clan had continually married into other families. Those marriages had diluted their bloodline and weakened it.
Even so, an exceptional person would emerge among the Senju from time to time, with a far stronger bloodline. That person's descendants would then have a much purer bloodline than other Senju.
Tobirama now knew this was due to Asura Chakra's reincarnation. In theory, Indra's Chakra reincarnating should have a similar effect.
But although Indra's Chakra's reincarnations were indeed powerful and had purer bloodlines than ordinary Uchiha, they rarely seemed to leave descendants.
Madara Uchiha, for instance, should have had exceptional descendants if he had fathered children. Unfortunately, he had shown no interest in women and left none.
Seen that way, the decline of the Uchiha Clan's bloodline was understandable.
There had been a time when the Uchiha Clan was so