Complementation test

Complementation test is a genetic technique used to determine whether two mutations are in different genes (they complement each other) or in the same gene (they do not complement each other). The complementation test, also known as the cis-trans test, is fundamental in genetics for understanding the allelic relationships between mutations and for defining functional units of genes. It is widely used in various organisms, including bacteria, viruses, and eukaryotes.

Overview

The complementation test is based on the principle that if two mutations are in different genes, each mutant strain can provide the functional product missing in the other, leading to a wild-type or near-wild-type phenotype when the two are combined. This is known as complementation. Conversely, if two mutations affect the same gene, complementation will not occur, and the mutant phenotype will persist. This outcome indicates that the mutations are allelic or within the same gene.

Procedure

The complementation test involves crossing or mating two organisms, each carrying a different mutant allele. In diploid organisms, this typically involves creating a heterozygote by crossing two homozygous mutants. In haploid organisms, such as bacteria and some viruses, complementation can be tested by creating a heterokaryon or by using a mixed infection assay.

1. Selection of Mutants: Mutants with similar phenotypes are selected for the test. 2. Crossing/Mating: The selected mutants are crossed or mated. 3. Phenotypic Analysis: The phenotype of the offspring or the result of the mating is analyzed. If the phenotype is wild-type or near-wild-type, the mutations complement each other, indicating they are in different genes. If the mutant phenotype persists, the mutations are in the same gene.

Applications

The complementation test has numerous applications in genetics, including:

• Identifying whether mutations causing similar phenotypes are in the same or different genes.
• Defining functional units within genes, known as cistrons, through fine-structure genetic mapping.
• Characterizing the genetic basis of diseases in medical genetics.
• Investigating the interactions between different genes and their products.

Limitations

While the complementation test is a powerful tool in genetics, it has limitations. It cannot detect mutations that are in different genes but within the same biochemical pathway, as such mutations may not complement each other. Additionally, the test is not informative for dominant mutations or for mutations that have an effect in trans.

See Also

• Genetics
• Allele
• Mutation
• Phenotype
• Haploid
• Diploid
• Genetic mapping