A complementation test is also known as a test for allelism. The basic idea of this test is to determine whether or not two particular mutants differ from wild type as a result of mutations in the same gene OR as a result of mutations in different genes.
The concept: if you cross two mutants, both recessive to wild type, and observe wild type progeny (as we will see through the course, this does not require that ALL progeny are wild type, in some situations that arise later, only SOME of the progeny will be wild type) then the mutations complement. The interpretation of this outcome is that the mutations are in DIFFERENT genes. If you cross the two mutant and observe only mutant progeny then the two do NOT complement and this is interpreted as meaning that the mutations are in the SAME gene.
Why is this the case?
To start, make sure you understand the idea of genetic pathways or networks. Every trait you observe in an organism is the result of the actions of many different genes (their products really). Even though we usually study the phenotypic effect of a change in just one or two genes that does not mean that other genes are not important, only that we are ignoring their influence, usually because the organisms we study are all homozygous for a certain allele for each of those genes. This means that they don't contribute to DIFFERENCES amongst the organisms that we are studying but they are still there and are still important.
As a result, if you observe two different organisms that both differ from wild type in a certain trait an important question arises: do they have mutations in the same gene or in different genes that both affect that trait?
The complementation test is conducted to answer the question just raised. An important point is that the test answers it for mutations that are RECESSIVE to wild type! Mutations are said to complement one another if a wild type phenotype is produced when the two mutations are introduced into the same organism.
The test is based on the idea that, if the two mutations occur in the same gene, the progeny of true-breeding mutant strains will not have a dominant (wild type) allele amongst them and the progeny will all have the recessive phenotype. On the other hand, if the mutations are in different genes then strain 1 carries a dominant allele for the mutation in strain 2 and vice versa. In this case, progeny will have a dominant wild type allele for both genes and have the wild type phenotype. Here we consider the possibilities separately:
1) First possibility - mutations in the same gene
We will designate the mutant as a and the wild type as a+ (remember, lower case means the mutant allele a is recessive to wild type). If the mutation is in the same gene, both strains will be aa genotype.
If this is the case, then when you cross the two mutants the cross you are performing is aa x aa. All progeny will be aa (recessive phenotype).
Therefore, if both mutations are in the same gene, all progeny will have the mutant phenotype.
2) Second possibility - mutations are in 2 different genes
We will designate the two mutant alleles as a and b since we have to diagram the genetics for 2 genes. If the mutations happen to be in different genes then the first strain will have the genotype aa b+b+ (since we assume it has wild type alleles at the gene that is mutated in the second strain) and the second strain will have the genotype a+a+ bb. You should keep in mind that, if either aa or bb causes the same mutant phenotype, a fly only has to be homozygous recessive for a or b to show this phenotype.
If this is the case then the cross you are performing - when you cross the two mutants - is a+a+ bb x aa b+b+. All progeny will be a+a b+b so all would have the wild type phenotype. This is COMPLEMENTATION.
Therefore, by crossing the mutants you can use the phenotypes of the progeny to determine whether or not the mutations are in the same gene or in different genes. Note that this does not cover all possibilities - for example, only autosomal genes are considered! In other sections, such as sex-linkage, the complementation test will be expanded to include more complex genetic situations although the basic concept will remain the same.
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