In rats, gene b produces black coat color if the genotype is b-, but black pigment is not produced if the genotype is bb. at an independent locus, gene d produces yellow pigment if the genotype is d-, but no pigment is produced when the genotype is dd. production of both pigments results in brown coat color. if neither pigment is produced, coat color is cream. the genotype of parents of litters with the following phenotype distribution 3 brown, 3 yellow, 1 black, 1 cream is: