complementation test (cis-trans test) A test to determine whether two mutant sites of a gene are on the same cistron. It is performed by introducing two mutant chromosomes into the same cell and observing whether the wild-type phenotype will be expressed, which it will if each chromosome complements the defect of the other. If complementation occurs, the two sites must be in different cistrons. The genotype of the complementing hybrid would be symbolized as a +/+b (where + represents the wild-type allele).

complementation test A test to determine whether two mutant sites of a gene are on the same cistron. It is performed by introducing two mutant chromosomes into the same cell and observing whether the wild-type phenotype will be expressed, which it will if each chromosome complements the defect of the other. If complementation occurs, the two sites must be in different cistrons. The genotype of the complementing hybrid would be symbolized as a+/+b. This test is also known as the cis-trans test.