[Theory] [w/Proof] [w/Math] The Uchiha and Senju Clan have low chakra reserves

[Mr Hiru]

Uchiha can use great-scale Katon techniques not using too much chakra. It's just their chakra is too powerful.

Lol, this is just bullshit, since you have no evidence for any of it. Why doesn't Tobirama has mokuton then? What about Hashiramas father? You need to prove that someone else has mokuton too. For what we know, mokuton is unique to Hashiramas DNA only. It might as well be a mutation from someone else marrying into the clan, and thus the DNA created in his cells had mutated from that point.
Also you have no evidence, that they wouldn't marry with people outside of the clan. For a clan to be created, people need to join it. It doesn't have to be by blood.

Naruto has the Yang half of Kurama. He and Hashirama have a common feature: Yang dominant (phyisical energy).

That's the reason Naruto can awake the Mokuton element given the conditions.

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You explained yourself the reason of why Tobirama didn't awake Mokuton: The Ancestors of Uchiha/Senju had wives, so the bloodline may be thinned, but they are still different than other people (for example, in the case of Senju, their lifeforce makes them look very young). Hashirama had strong physical force, and the younger son of the sage had strong physical force. Well, even if the bloodline was thinned, the tendence was mantained... you don't have proof that the father of Hashirama didn't have the Mokuton element either.

 

[ZealotTyreal]

This is a lot of speculation on your part without evidence. Mastering the Sharingan ( 3 tomoe) by adulthood, for an Uchiha who awakened it, is pretty normal. We have seen it happen with the police force and pretty much any adult Uchiha we have seen. There was never any evidence for inbreeding to maintain the bloodline pure. As for moukuton being the Senju gekke kenkai, that is another assumption without evidence; no one else other than Hashirama has been shown to have it. If it was like the Sharingan of the Senju, some one like Tobirama would be sure to have it also. These two clans have large chakra reserves compared to the average ninja, and of course compared to Rikoduo, the amount is going to be small. Though you don't know how the chakra was exactly distributed and you don't account for bloodlines mixing through marriage, which some one else already mentioned. Sure some people like Kushina have unique chakra, but it does not go for everybody.

 

[Mr Hiru]

This is a lot of speculation on your part without evidence. Mastering the Sharingan ( 3 tomoe) by adulthood, for an Uchiha who awakened it, is pretty normal. We have seen it happen with the police force and pretty much any adult Uchiha we have seen. There was never any evidence for inbreeding to maintain the bloodline pure. As for moukuton being the Senju gekke kenkai, that is another assumption without evidence; no one else other than Hashirama has been shown to have it. If it was like the Sharingan of the Senju, some one like Tobirama would be sure to have it also. These two clans have large chakra reserves compared to the average ninja, and of course compared to Rikoduo, the amount is going to be small. Though you don't know how the chakra was exactly distributed and you don't account for bloodlines mixing through marriage, which some one else already mentioned. Sure some people like Kushina have unique chakra, but it does not go for everybody.

a) I posted the page of the manga that explains how this was distributed. The names of the Clans were mentioned with a Panel of the sage using the "Creation of All Things" jutsu in the background. It doesn't matter how much chakra did the Sage have originally, the quantity of Spiritual power passed to the younger son was minimum, as well as the Physical power passed to the elder son... "minimum".

b) I did take it in account, that explains why not everybody in Hashirama's generation inherited the Mokuton element. But his elders had more DNA of the Younger son (and physical energy), that's for sure. Hashirama is the special case since the dominant genes manifested strongly in him.