Constitutive I mutations

• Wild-type I⁺ (strain 1) is inducible while the mutated I^- (strain 2) is constitutive

• Strain 3 illustrates that the inducible phenotype (I⁺) is dominant to the Constitutive phenotype (I^-). This means that there is enough of the wild-type repressor protein made by one copy of the gene to regulate both copies of the gene.

• I⁺ gene product; the repressor; is trans-acting, meaning that the gene product regulates all lactose structural genes

• The I gene is a standard protein-coding gene in which the protein product can diffuse and act on all operators