Jacob and Monod’s merodiploid analysis of the lac operon. (a) A simplified view of the wild-type lac operon. (b) Jacob and Monod isolated two mutant strains of otherwise normal (haploid) bacterial cells with mutations resulting in constitutive expression from the lac operon. These strains had mutations in either lacI (the repressor gene) or lacO (the operator region) that rendered the gene or region nonfunctional. (c) Results from Jacob and Monod’s analysis of merodiploid (partial diploid) strains carrying both a mutant and a wild-type lac operon suggested that the product of the lacI gene acts in trans (i.e., is diffusible; top) and the lacO region functions in cis (i.e., does not produce a diffusible product; middle). A double mutant analysis confirmed this hypothesis (bottom).