Q.2 A mutation in the operator locus of lac operon that confers constitutive expression of β–
galactosidase is ________.
(A) cis dominant (B) trans dominant (C) co-dominant (D) dominant negative

The correct answer is (A) cis dominant. A mutation in the lac operon operator that causes constitutive β-galactosidase expression acts only on the adjacent DNA copy, making it cis dominant.

The lac operon operator mutation constitutive expression of β-galactosidase represents a classic example of cis-acting genetic regulation in prokaryotes, crucial for CSIR NET Life Sciences preparation.

Question Analysis

Operator mutations (O^c) prevent repressor binding, leading to continuous transcription of lacZ (β-galactosidase), lacY, and lacA regardless of lactose presence.

In merodiploids like I^+ O^c Z^+ / I^+ O^+ Z^+, only the operon with O^c shows constitutive expression, proving cis action on adjacent genes.

Option Explanations

• (A) Cis dominant: Correct. O^c mutations affect only the linked operon copy and override wild-type regulation locally, as repressor cannot bind mutated operator DNA.

• (B) Trans dominant: Incorrect. Applies to diffusible factors like I^-d (dominant negative repressor) or I^s (super-repressor) that affect both operon copies via protein interactions.

• (C) Co-dominant: Incorrect. Not applicable here; co-dominance involves simultaneous expression of both alleles (e.g., blood groups), absent in operon cis-elements.

• (D) Dominant negative: Incorrect. Refers to defective proteins (like multimeric I^-d repressors) poisoning wild-type complexes, causing trans repression; operator is DNA, not protein.

Dominance in Merodiploids

This table illustrates how operator mutations uniquely show cis dominance.