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. Thus, 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.

Say we have three mutations, a[-], b[-], and c[-], that result in mutant wing phenotypes in Drosophila. These mutant phenotypes are recessive to wild-type. First we cross a[-] and b[-] together.