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| Modern Genetic Analysis Chapter 6. Gene Interaction Gene Interaction Leads to Modified Dihybrid Ratios Figure 6-12. Coat color inheritance in Labrador retrievers. Two alleles B and b, of a pigment gene, determine (a) black and (b) brown, respectively. At a separate gene, E allows color deposition in the coat and e/e prevents deposition, resulting in (c) the gold phenotype. This is a case of recessive epistasis. Thus the three homozygous genotypes are (a) B/B ; E/E, (b) b/b ; E/E, and (c) B/B ; e/e or b/b ; e/e. The dog in (c) is most likely B/B ; e/ethe animal still has the ability to make black pigment, as witnessed by the black nose and lips, but not to deposit this pigment in the hairs. The progeny of a dihybrid cross would produce a 9:3:4 ratio of black:brown:golden. (Anthony Griffiths.)

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