Reader: Kokoro is a mermaid, so why is her granddaughter, Chimney, a regular human? Don’t the mermaid genes pass down?
Oda: When Kokoro was young, a very long time ago, she came with Tom from Fish-Man Island. She married a man from Water Seven and had a son that was half mermaid. That son married a human girl, and they had Chimney. Chimney is a quarter mermaid, so she is very good at swimming. In volume 42, chapter 403, you can see that even a small child like her could save the drowning Luffy from a whirlpool. That’s because she has mermaid blood in her.