Complementation test A mating test to determine whether two different recessive mutations (a1;a2) on opposite chromosomes (trans, a1+/+a2) of a diploid or partial diploid will not complement (ie have a mutant phenotype) each other; but the same two recessive mutations on the same chromosome (cis, a1a2/++) in a diploid or partial diploid show a wild-type phenotype; a test for allelism. A test to determine whether two mutant sites are in the same functional unit or gene.