Thank you both for your experience.
And it's not a cop out to say it's a mutation! It's actually quite likely. There is a genetic disorder called turner's syndrome in humans which is where the baby only receives 1 copy of the X chromosome. Maybe something similar could happen in mice but the baby only recieved one copy of the c gene instead of both cs from it's parents.

John, I would say maybe you had himilayans with really terrible points, but if that were the case, then the babies would have been siamese! THAT is a very interesting case you have!