Complementation is therefore a test of function.
Each allele ought to be present in a single copy number in the cell and it is crucial that the entire relevant region be present in diploid in the cell. Such a partially diploid cell is termed merodiploid. The two alleles can either be present on the chromosome or on extra-chromosomal elements. Mutations affecting genes whose products are not essential for the desired phenotype will not be tested for in complementation analysis. Figure 29 gives an idea of the results one could expect from straightforward complementation tests.