Coat color in labrador retrievers is determined by two genes, as shown in the image below.
The presence of a B allele allows synthesis of a black pigment, and the recessive allele of this gene, b, leads to the synthesis of a brown pigment. Thus, black labradors have the genotype B/- while brown labradors have the genotype b/b.
A second, unlinked locus, the E gene, determines whether or not pigment will be deposited in the fur. Labradors of genotype E/- will have dark-colored fur (either black or brown) while yellow labradors have the recessive genotype e/e. Since having the genotype e/e will result in a yellow coat no matter whether the dog is B/- or b/b, carrying two copies of the e allele is recessively epistatic to the B gene.
If a dihybrid cross is done between two black labradors heterozygous for both genes, what is the probability of obtaining a yellow labrador as offspring?