Yeah it's a total bummer that mermaid genetics weren't fixed in the latest patch. What I did when my mermaid twin babies aged to toddlers is went straight into CAS, deleted them and did random genetics with both parents. I only hit the randomize button once for each twin, and the girls are child age now (in my avatar pic), so luckily this worked for me. But it's a crappy work-around. This bug is really messing up my 100 baby challenge as I wanted my matriarch to have a few mermaid children.