My offspring are always mermaids

I’m playing generations and I’ve run into a problem. My generation 3 heir got with a mermaid and since then every single child born has been a mermaid. My heirs have always been partnered up with either a human or some other occult but never a mermaid since gen 3, it’s been 3 generations since then and I have had at least 2 kids every time.

I don’t want to just let my sims wait it out and turn human again, I prefer to roll with what I’m given, but honestly the mermaid gameplay is getting old and at this point I might just let them adapt to land.