Alien baby from human parents?

Not sure if this is a bug or a cool genetic thing. Basically, my female sim is half alien - her dad is alien and her mum is human. 

For all intents and purposes she is human (no powers, no disguise etc). She had a baby with another human sim and its full alien! 

Is this because her dad is an alien or is it a bug?? I didn't think that children inherited anything from grandparents?