Suppose a mutation causes a white fur phenotype to emerge in the population. what might happen to the mouse population after 40 generations

In the event that people with the new phenotype are more fit than the dim or dark mice, the white allele may increment in recurrence in the populace. Dark mice will probably keep on being more typical than alternate phenotypes. 
The phenotype is the shade of their hide and for this situation, the greater part of the posterity will have dark hide, since they are altogether half breed and dark hide is prevailing. The greater part of their posterity will convey the attribute for white hide, yet it won't appear on the other side.

Could you elaborate?