Board Thread:Theories and Speculation/@comment-3965545-20160511191323/@comment-1512913-20160512110354

Fact: Uzumaki and Senju are counsin Clans, sharing common ancestor. Fact: Black Zetsu said that Nagato is of Senju lineage.

Its statement has been commonly interpreted as Uzumaki being descended/branched from Senju. But as stated, had one of Nagato's parents been an Uzumaki and the other one a Senju, it could have meant exactly that. We simply don't know what BZ meant, we just assume the former.