FIGURE 6-9 Mutations that result in genetic suppression or synthetic lethality reveal interacting or redundant proteins. (a) The observation that double mutants with two defective proteins (A and B) have a wild-type phenotype but that single mutants have a mutant phenotype indicates that the function of each protein depends on interaction with the other. (b) The observation that double mutants have a more severe phenotypic defect than single mutants is also evidence that two proteins (e.g., subunits of a heterodimer) must interact to function normally. (c) The observation that a double mutant is nonviable but that the corresponding single mutants have the wild-type phenotype indicates that two proteins function in redundant pathways to produce an essential product.