Well, if you take two purebred chickens,and bread them with with each other, the offspring is no longer going to be purebred, so the feathers they have will be a mix of the purebreds.
As the offspring inherits one allele from each parent, it will obtain only the B allele from the female mouse and either the B or b allele from the male mouse. Hence possible offspring genotypes are BB or Bb with 0.50 probability each (each probability of either genotypes being present in the offspring).
