Complementation (genetics)

A complementation test is used in genetics to decide if two recessive mutant phenotypes are determined by mutations in the same gene or two different genes. True breeding mutants are crossed bringing together the mutant genotypes in an F1 individual. If that individual is mutant, then the complementation has failed, and thus the two alleles are the same gene. If no mutant phenotype is observed in the F1 individual, then the mutant alleles are complemented and must be different genes.

That is:

• If the combination of two haploid genomes containing different recessive mutations yields a mutant phenotype, then the mutations must be in the same gene (alleles).
• If the combination of two haploid genomes containing different recessive mutations yields the wild type phenotype, then the mutations must be in different genes.

There is an exception to this rule. Two non-allelic mutants may fail to complement (called "non-allelic non-complementation" or "unlinked non-complementation"). This situation is rare and is dependent on the particular nature of the mutants being tested. For example, two mutations may be synthetically dominant negative.