complementation (genetics)

The process by which two recessive mutant genes at different loci in a chromosome can supply each other's deficiency. An individual carrying both genes (a double heterozygote) appears phenotypically normal. For example, if two dwarfs marry they may have normal children as their genetic deficiencies are at different loci.

For complementation in algebra, see complementation (algebra).

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.

Two different true breeding mutants are crossed with each other, bringing together the mutant genotypes of each parent in a single F1 individual (or transheterozygote). If that individual shows the mutant phenotype, then the complementation has failed, and the two alleles must be in the same gene. If no mutant phenotype is observed in the F1 individual, then the mutant alleles have complemented each other, and must be in different genes.

In other words:

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.