Complementation

Complementation tests if two different alleles that result in observable phenotypes are part of the same gene or are alleles of different genes. If you have generated a range of mutants phenotypes for a trait it allows these mutations to be quickly grouped by genes (rather than, for example, conducting fine scaled recombinant mapping). Complementation assumes simple dominance where the mutant phenotype is recessive to wild type.

If there are mutations in the same gene then when homozygous lines are crossed together there is not a wild type allele present in the heterozygote. The heterozygote has a mutant phenotype. This is non-complementation and indicates that the two mutants occur at the same gene.

If there are mutations at two different genes then crossing true-breeding (homozygous) lines together generates double heterozygote offspring with a wild type phenotype. This is complementation and indicates the two mutants occur at different genes.