Fig. 7. Two mutations usually complement if they are in different genes. Only rarely do they complement when in the same gene. Thus complementation tests provide a preliminary test of whether mutations with similar phenotypes are in the same or different genes. However, complementation tests provide a definitive determination of whether mutations are or are not in the same "complementation group". Mutants that, when combined in a diploid, yield a mutant phenotype are in the same complementation group. Mutants that, when combined in a diploid, produce a normal phenotype are in different complementation groups.