Board Thread:Naruto Discussions/@comment-945885-20140709082448/@comment-24519434-20140718061002

Tunade is technicaly a senju her grand mother was the only one in her family who was Uzumaki, redhair is a recessive trait so minato's blonde hair showed up in naruto. The other Uzumaki are all born from two people in the Uzumaki clan, thats how they got red hair.

It seems rare that people marry outside the clan.