Monkey Family: Are They Truly Related by Blood?

What if Father and Son aren't Blood Related?[]

Okay, hear me out. What if Dragon is not Monkey D. Garp's biological son? I mean, in the entire series, I have never heard that Garp and Dragon are father and son, except when Helmeppo questioningly said so (Chapter 432 and Episode 314, Helmeppo just gave his hypothesis based on what he heard).

There are two possibilities to support my theory:

Garp is Luffy's maternal grandfather (meaning he came from Luffy's mother's side).

For some reason, Dragon adopted the Monkey family name when he married with Luffy's mother.
Similarly, Portgas D. Ace adopted his mother's surname instead of keeping his father's.

Garp adopted Dragon.

Similar as he did with his adoptive grandson Ace from Roger and Rouge.
Of course, this would make Garp and Luffy not biological grandfather and grandson.

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If anyone has any other potential theories, please do share.