It is inherited though, which is what Garp implied at marineford
¨so you were really born with it¨ or whatever
¨so you were really born with it¨ or whatever
Did Big Mom parents had it? Why only Katakuri had it among her children?
Does it skip generations? We know Ryuma is Zoro's ancestor. Did his parents have it? Pinzoro?
etc.
It's not like some "eye color" genetic.
Don't think about it too hard or scientifically. It's One Piece.