Inter-allelic or intra-cistronic (Schlesinger and Levinthal, 1963) complementation is now recognized as a very general phenomenon. Wherever there is an extensive series of mutants structurally defective with respect to some particular enzyme, it is likely that some pairs of them will be found to be capable of complementing one another (in heterocaryon, heterozygote or heterogenote, depending on the organism) to form more enzyme activity than either could produce by itself. Not to find examples of such complementation seems to be more the exception than the rule.
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Fincham et al. (1963) studied this question.