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Coat color in labrador retrievers is determined by two genes, as shown in the image below.

Arellano, Elizabeth. File:LabRetColors.png. Digital Image. Wikipedia. Wikimedia Foundation, Inc., 24 Oct. 2007. 23 Feb. 2017.

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?











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