In Labrador retrievers, coat colour is controlled by two genes. Gene B (B = black, b = brown) is epistatic to gene E (E = pigment production, e = no pigment). Dogs with genotype __ee are white regardless of the B locus. Which of the following phenotypic ratios is expected in the F2 generation from a cross between two heterozygous black Labradors (BbEe × BbEe)?

Step-by-step solution

1. Recognise that gene E is required for pigment; ee produces white regardless of B.
2. List possible gametes from a BbEe parent: BE, Be, bE, be.
3. Create a 4×4 Punnett square using these gametes.
4. Classify each genotype: any genotype with at least one E produces colour, and B determines black or brown; ee genotypes are white.
5. Count phenotypes: 9 genotypes have at least one B and at least one E → black; 3 have bb and at least one E → brown; 4 have ee → white.
6. Resulting ratio = 9 black : 3 brown : 4 white.

Concept tested: Recessive epistasis in coat colour

Published 27 September 2026 · How this solution was prepared and checked