When there was an outbreak within the population of mice, it seemed to cause worse symptoms in mice that had black hair. Because of the shrinking population, mice with white hair had a higher chance of there traits becoming more frequent.
Prior to the outbreak there was already a small population of white mice. When it comes to mating, white mice mate with white mice and black mice mae with black mice.
Some white mice (very few) have experienced mutations which have changed their hair color to black.
"I am going to take these two mice home!"(Change of Location)
Bobby took home some of the last white mice that were left in the field. This increased the frequency of black mice.