Complementation TestingOccasionally, multiple mutations of a single wild type phenotype are observed. The appropriate genetic question to ask is whether any of the mutations are in a single gene, or whether each mutations represents one of the several genes necessary for a phenotype to be expressed. The simplest test to distinguish between the two possibilities is the complementation test. The test is simple to perform --- two mutants are crossed, and the F1 is analyzed. If th e F1 expresses the wild type phenotype, we conclude each mutation is in one of two possible genes necessary for the wild type phenotype. When it is shown that shown genetically that two (or more) genes control a phenotype, the genes are said to form a complementation group. Alternatively, if the F1 does not express the wild type phenotype, but rather a mutant phenotype, we conclude that both mutations occur in the same gene.
These two results can be explained by considering the importance of genes to phenotypic function. If two separate genes are involved, each mutant will have a lesion in one gene while maintaining a wild type copy of the second gene. When the F1 is produc ed, it will expresses the mutant allele of gene A and the wild type allele of gene B (each contributed by one of the mutant parents). The F1 will also express the wild type allele for gene A and the mutant allele for gene B (contributed by the other muta nt parent). Because the F1 is expressing both of the necessary wild type alleles, the wild type phenotype is observed.
Conversely, if the mutations are in the same gene, each homolog will express a mutant version of the gene in the F1. Without a normal functioning gene product in the individual, a mutant phenotype occurs.
Eye color in Drosphila is a good model to demonstrate the complementation test. A wide array of spontaneous mutations have been studied. These experiments demonstrated that five genes (white, ruby, vermillion, garnet and carnation) controlling ey e color reside within 60 cM of each other on the X chromosome. The dominant wild type allele for each gene produces the deep red eyes. The mutant alleles produce a different color. If mutants from any of these five genes are crossed, the F1 would expre ss deep red eye color (wild type phenotype).
Five different alleles (buff, coral, apricot, white and cherry) are also known to exist for the white gene, each representing a mutation at a different position in the gene. If mutant flies for any of the five white alleles are crossed, the F1 offspring would have a mutant eye color. Therefore, two genes involved in the expression of a phenotype complement each other. But complementation bet ween two alleles of the same gene does not occur.
Copyright © 1999. Phillip McClean