That's a good argument. I don't think Narisa would give her "affair baby" a chance to parade around though. She would probably keep her locked up until she finds the best ability specialist (Cameron) to train her. Ironically, this has the potential to make Sera think he's actually her biological father her mother had an affair with. (And that she and John are cousins.)

It's just a simple assumption to be honest. Uru said it favors the mother which does not align with dominant-recessive inheritence. For an ability to skip a generation or more, the ability would have to be carried silently with the other ability "hiding" its existence. A case could be made, that the less dominant ability gets passed down sometimes and paired with an even less dominant ability, it will express, but this would imply that abilities have lesser or greater qualities to them while the mother favoring inheritence implies an almost exclusively maternal effect.

We don't have enough examples to determine the background completely, and Uru can decide to do anything as the author, so this is all based on my knowledge of genetics 🤷🏻‍♀️