Final answer:
A mutation that inactivated the lac repressor and prevented it from binding to the lac operator site would result in constitutive expression of the lac operon under all conditions. The disadvantage of having a constitutive lac operon is the wasteful use of energy and resources for the bacterium.
Step-by-step explanation:
If a mutation inactivated the lac repressor and prevented it from binding to the lac operator site, it would result in the constitutive expression of the lac operon under all conditions. This means that the genes of the lac operon would be continuously transcribed and translated, leading to the production of the enzymes involved in lactose metabolism even in the absence of lactose.
The disadvantage of having a constitutive lac operon in a bacterium is that it would waste energy and resources. Since the lac operon is continuously expressed, the bacterium would produce lactose-metabolizing enzymes even when lactose is not available, which is inefficient and costly for the cell.