Mutations in the same gene cannot complement

KEY TERMS:

• A complementation test determines whether two mutations are alleles of the same gene. It is accomplished by crossing two different recessive mutations that have the same phenotype and determining whether the wild-type phenotype can be produced. If so, the mutations are said to complement each other and are probably not mutations in the same gene.
• Two mutants are said to complement each other when a diploid that is heterozygous for each mutation produces the wild type phenotype.
• A complementation group is a series of mutations unable to complement when tested in pairwise combinations in trans; defines a genetic unit (the cistron).
• A cistron is the genetic unit defined by the complementation test; it is equivalent to the gene.
• A gene (cistron) is the segment of DNA specifying production of a polypeptide chain; it includes regions preceding and following the coding region (leader and trailer) as well as intervening sequences (introns) between individual coding segments (exons).

KEY CONCEPTS:

• A mutation in a gene affects only the protein coded by the mutant copy of the gene, and does not affect the protein coded by any other allele.
• Failure of two mutations to complement (produce wild-phenotype) when they are present in trans configuration in a heterozygote means that they are part of the same gene. 

How do we determine whether two mutations that cause a similar phenotype lie in the same gene? If they map close together, they may be alleles. However, they could also represent mutations in two different genes whose proteins are involved in the same function. The complementation test is used to determine whether two mutations lie in the same gene or in different genes. The test consists of making a heterozygote for the two mutations (by mating parents homozygous for each mutation).

If the mutations lie in the same gene, the parental genotypes can be represented as:

m1  and  m2

m1         m2

The first parent provides an m₁ mutant allele and the second parent provides an m₂ allele, so that the heterozygote has the constitution:

m1

m2

No wild-type gene is present, so the heterozygote has mutant phenotype.

If the mutations lie in different genes, the parental genotypes can be represented as:

m1+  and +m2

m1+        +m2

Each chromosome has a wild-type copy of one gene (represented by the plus sign) and a mutant copy of the other. Then the heterozygote has the constitution:

m1+

+m2

in which the two parents between them have provided a wild-type copy of each gene. The heterozygote has wild phenotype; the two genes are said to complement.

The complementation test is shown in more detail in Figure 1.27. The basic test consists of the comparison shown in the top part of the figure. If two mutations lie in the same gene, we see a difference in the phenotypes of the trans configuration and the cis configuration. The trans configuration is mutant, because each allele has a (different) mutation. But the cis configuration is wild-type, because one allele has two mutations but the other allele has no mutations. The lower part of the figure shows that if the two mutations lie in different genes, we always see a wild phenotype. There is always one wild-type and one mutant allele of each gene, and the configuration is irrelevant. The basic test and some exceptions to it are discussed in 32.9 Complementation.

Failure to complement means that two mutations are part of the same genetic unit. Mutations that do not complement one another are said to comprise part of the same complementation group. Another term that is used to describe the unit defined by the complementation test is the cistron. This is the same as the gene. Basically these three terms all describe a stretch of DNA that functions as a unit to give rise to an RNA or protein product. The properties of the gene with regards to complementation are explained by the fact that this product is a single molecule that behaves as a functional unit.