In
genetics,
complementation occurs when two
strains of an organism with different homozygous recessive
mutations that produce the same mutant
phenotype (for example, a change in wing structure in flies) produce offspring with the
wild-type phenotype when mated or crossed. Complementation will occur only if the mutations are in different genes. In this case, each strain's genome supplies the wild-type
allele to "complement" the mutated allele of the other strain's genome. Since the mutations are recessive, the offspring will display the wild-type phenotype. A
complementation test (sometimes called a "
cis-trans" test) can be used to test whether the mutations in two strains are in different genes. Complementation will not occur if the mutations are in the same gene. The convenience and essence of this test is that the mutations that produce a phenotype can be assigned to different genes without the exact knowledge of what the gene product is doing on a molecular level. The complementation test was developed by
American geneticist Edward B. Lewis.