The mouse parent probably had a genetic makeup of Bb meaning that there is a gene of the white hair within their own genes of a dominate black hair. Since the white hair mouse had a complete recessive gene makeup of white hair and black hair is dominate in this situation the only genetic makeup that would work would have to be dominate recessive for the dominate gene is the one that shows but that mouse parent cannot be completely dominate or there would be no white haired offspring
