<p>How can there be any SasuKarin fans? Sasuke and Sakura are married. This is a canon fact. There aren't even any indicators for Sasuke and Karin ever having any sexual or romantic interaction with each other. Just because Karin is the genetic mother, it doesn't mean she was engaged in sexual intercourse with Sasuke or even gave birth to Sarada. The "pairing" basically never existed, because it was never based on mutual feelings.
</p><p>With regard to the heir though, at Sarada's age, Sakura had a score of 0.5 at strength. There is no chance she is currently so physically strong because of her genes, because if she were, she wouldn't have been this weak back then. Genjutsu was her natural skill, and that could have been her genetic advantage, but not strength. She learned how to perform strong attacks, she didn't genetically inherited this skill from Tsunade. It's not an exclusive clan ability that is passed down through blood. She was not a natural at this back then either. In fact she was the exact opposite. Like the Byakugou, it's passed down through training. Sarada's strength wasn't inherited, it was taught to her. She can't genetically inherit something that was merely taught to her mother. It's definitely no indicator for Sakura being the biological mother after all.
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