Definition for Complementation
From Biology Forums Dictionary
In genetics, complementation refers to a relationship between two different strains of an organism which both have homozygous recessive mutations that produce the same phenotype (for example, a change in wing structure in flies). These strains are true breeding for their mutation. If, when these strains are crossed with each other, some offspring show recovery of the wild-type phenotype, these strains show "genetic complementation". When this occurs, each strain's haploid supplies a wild-type allele to "complement" the mutated allele of the other strain's haploid, causing the offspring to have heterozygous mutations in all related genes. Since the mutations are recessive, the offspring display the wild-type phenotype.