I dont know what manga you are reading but it's not the same I am, there are actually very few cross-overs in the series, the two main being Tsunade and Naruto, and both clearly don't look exactly like their ancestors.
They are called "Senju" and "Uchiha", the burden of proof is on you. Sasuke and Itachi's parents were clearly Uchiha by what you could tell of their hair color. Prove to me that those other Uchiha were not pure blood, because they do seem to be.
Sir Armathyx, I advise you cease your spouting of fallacious "counter-arguments."
If you weren't aware of what fallacies are, then I will explain it to you.
Simply put, they are examples of faulty logic.
Allow me to point out the fallacies present within this pitifully constructed response:
Regarding the portion in blue
There are only a few "cross overs" that we
know exist. There may be several that exist which haven't been exposed. You are making a false assumption that these are the
only cases of inter-clan breeding in the NarutoVerse (not intra-clan breeding, there's a distinct difference). It has never been stated that these are the only cases, so it is gaudily erroneous to make such a claim.
Hence, you are committing the fallacy known as the 'argument from silence,' which denotes a conclusion based on the lack of evidence, rather than the existence of evidence.
Regarding the portion in red
It is apparent that you fail to comprehend what the "burden of proof" actually means. The burden of proof most certainly is not on me, because the obligation to exhibit evidence rests on the shoulders of the individual who is making the original assertion.
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This is also related to the 'argument from ignorance,' which denotes assuming a claim is true merely because it has not yet been proven false.
You're advocating the following: "You can't prove they're not pure-blooded, so they must be pure-blooded!"
That's like saying the following: "You can't prove that God doesn't exist, so he definitely must exist!"
Do you see how wrong this pattern of thought is?
I repeat again,
you are the one making a claim so
you are responsible for supporting
your claim with evidence. Otherwise, this assertion lacks any vindication whatsoever.