Actually, this has happened to me before in a 100 baby challenge I played when GTW first came out, my founder kept woohooing with aliens in disguise but her babies all came out looking exactly like human sims. They could do the secret handshake with each other though, and talk about weird atmospheric pressure, stuff like that. I remember being frustrated that I couldn't get beautiful blue and green sims... It hasn't happened to me since I stopped playing that file, though.