Fundamentals of Molecular Biology. Part 2: Molecular Genetic Mechanisms - A. N. Ohurtsov 2011

Alleles and Mutations
Analysis of Mutation Complementation

Often, several different recessive Mutations lead to the same phenotypic outcome. Determining whether these mutations result from alterations in the same Gene or affect different genes reveals The phenomenon of genetic complementation.

Genetic complementation refers to the restoration of the original wild-type phenotype by crossing two different mutants.

If two recessive mutations a and b affect the same gene, then a diploid Organism heterozygous for both mutations (i.e., containing one allele a and one allele b) will exhibit a mutant phenotype, since neither allele can provide a functional copy of the gene.

Conversely, if recessive mutations a and b affect different genes, heterozygotes containing a single copy of each mutant allele will not display a mutant phenotype, because a normal wild-type allele for each gene will also be present. In this case, the mutations are said to Complement each other.

By performing complementation analysis on a set of mutations that manifest in the same phenotype, it is possible to distinguish individual genes within a set of functionally related genes, all of which must function together to ensure the expression of that phenotypic trait.

For example, the screening of the cdc mutations in Saccharomyces cerevisiae described above reveals numerous Temperature-sensitive mutants arrested at the same stage of the Cell Cycle. To determine how many genes lead to such a mutation, Hartwell and colleagues performed complementation tests for all pairwise combinations of cdc mutants following the protocol illustrated in Figures 78 and 79.

Complementation analysis was carried out by mating haploid a and a Cells carrying recessive mutations in order to form diploid cells.

In the (hypothetical) example shown in Figure 78, the cdcX and cdcY mutants complement each other and therefore carry mutations in different genes, whereas the cdcX and cdcZ mutants have mutations in the same gene. Such a test has identified more than 20 different CDC genes. Subsequent characterization of these genes and the Proteins they encode provided the foundation for understanding the Regulation of Cell division across organisms, ranging from Yeast to humans.

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Figure 78 - Scheme for determining whether recessive mutations reside in the same or different genes using complementation analysis

To analyze the cdc mutations, pairs of different haploid temperature-sensitive cdc strains were systematically crossed, and the resulting diploids were tested for growth at permissive and restrictive temperatures.

Figure 79 - Interpretation of complementation analysis results