Brief overview: There is a mutation in the lacI gene, called the Id mutation, which results in the lac operon being uninducible; that is, no β-galactosidase is made in the presence or in the absence of lactose (these results are also unaffected by the presence or absence of glucose in the medium). In partial diploids (chromosome + plasmid), lacId acts as a dominant gene. In other words, even a good copy of the lacI gene on a plasmid cannot overcome the defect of the Id mutation on the genomic lacI gene.