Final answer:
From a cross between two dihybrid Labrador Retrievers (Bbee), the proportion of offspring with yellow coats is 1/4. This calculation takes into account recessive epistasis, where the presence of the ee genotype results in a yellow coat, irrespective of the B allele's expression.
Step-by-step explanation:
In Labrador Retrievers, fur color is influenced by epistasis involving two genes with alleles E and B. Specifically, the e allele displays recessive epistasis, meaning that when an individual has two copies of this allele (ee), it will have a yellow coat, regardless of the B allele's configuration. Black labs are produced by the B_E_ genotype and chocolate labs by the bbE_ genotype. From a cross between two dihybrid Labradors (Bbee), the proportion of offspring with yellow coats can be calculated using the 9:3:3:1 dihybrid cross ratio.
However, because the e gene is epistatic to the B gene, any offspring with the homozygous recessive genotype (ee) will be yellow, irrespective of the B gene's expression. Consequently, to determine the number of offspring with yellow coats, we would only need to consider the e gene. The cross Bbee x Bbee results in offspring with the following proportions: 9/16 with the B gene expressed (either black or chocolate depending on B or b), 3/16 with the bb genotype (chocolate, because they are not ee), and 4/16 with the ee genotype (which will all be yellow truly because of recessive epistasis by e). Thus, 4/16 or 1/4 of the offspring are expected to have yellow coats.