Not sure if it's a bug, random mutation or deliberate (to prevent people breeding 100% terrible beasts?) but it's happened twice, and I'm curious why. Never seen it before.

Amazing, Fabulous

This is the effect of Degeneration. It adds a chance for any gene to mutate into a random negative gene from the respective gene group. In doesn't check if the target gene is negative already or if it is worse than the new gene.

It's your own fault for promoting incest.

I'm sure there's a Monty Python quote that's applicable here somewhere.

Oh, interesting. Maybe it has happened before (it seemed to take so long to "get rid of" some genes), but I didn't realise it. Thanks for the answer!

Excuse me, arete? Wait til Biggus Gryphus hears of this!