Cistron The term which Benzer (1957) introduced, derived from cis and trans, for the functional unit of the hereditary material defined by the phenotype of the trans (repulsion)heterokaryon or heterozygote for two recessive mutations: if this phenotype is mutant, the mutations are said to be alleles and to belong to the same cistron; if the phenotype is normal (wild-type), the mutations are said to be non-allelic and to belong to different cistrons. Benzer's definition has since been modified to accommodate allelic complementation, and a cistron is now defined (Fincham 1959) as a segment of the hereditary material within which pairs of mutations in the trans configuration are either deficient for a particular enzyme, or give rise to that enzyme in a structurally abnormal form. There is a good case for equating Fincham's definition of a cistron with the term gene.