The ability of two different mutations to produce a wild type phenotype in a double heterozygote.

In genetics, the restoration of a wild-type function (e.g., ability to grow on galactose) in diploid heterozygotes generated from haploids, each of which carries a mutation in a different gene whose encoded protein is required for the same biochemical pathway. If two mutants with the same mutant phenotype (e.g., inability to grow on galactose) can complement each other, then their mutations are in different genes. ( Figure 8-11)

The production of a wild-type phenotype when two different mutations are combined in a diploid or a heterokaryon.

The ability of a gene to produce a functional gene product which compensates for the mutant phenotype caused by a mutation in another gene. Typically, the complementing gene produces a gene product (e.g. a repressor protein or an enzyme for synthesis of a metabolite) that is diffusible and thus functions in trans.

The ability of two different genetic defects to compensate for one another.

A complementation test (sometimes called a "cis-trans" test) is used in genetics to decide if two independently derived recessive mutant phenotypes are caused by mutations in the same gene or in two different genes. American geneticist Edward B. Lewis is credited with developing the test.