complementation

In genetics, complementation refers to the interaction between nonallelic genes. When two different genes, each with a mutation that causes a specific phenotype (like deafness) in its homozygous state, are present together in a heterozygote, they can restore normal function. This means the mutations are in different genes and "complement" each other.1

The complementation test is used to determine if two mutations associated with a phenotype are in the same gene (alleles) or different genes. If the mutations are in different genes, the offspring will be heterozygous and may not express the recessive trait because the functional gene from one parent compensates for the mutated gene from the other. This phenomenon is distinct from the effect of a single gene, where two or more genes together produce a unique outcome.213