Q.70 An E.coli mutant constitutive for the lac operon was mated with a wild type strain. The merodiploid
thus obtained was inducible by lactose. This observation indicates that the original mutation is
(A) dominant (B) trans-dominant (C) recessive (D) cis-dominant
The original mutation is recessive (C).
Question Breakdown
Lac operon constitutive mutants express lacZYA continuously (no lactose needed). Mating with wild-type creates merodiploid F’lacI⁺O⁺Z⁺Y⁺A⁺ / lacIˣO⁺Z⁺Y⁺A⁺. Inducible phenotype shows wild-type lacI⁺ complements mutation, restoring regulation.
Option Analysis
(A) Dominant
Wild-type would show constitutive expression. Ruled out (merodiploid inducible).
(B) Trans-dominant
lacIᵈ (dominant negative) mutations disrupt tetramer function. Ruled out.
(C) Recessive
Correct. lacI⁻ (repressor null) recessive to lacI⁺. Wild-type repressor binds both operators:
F'lacI⁺ → Repressor⁺ → binds both O⁺ → inducible
(D) Cis-dominant
Oᶜ (operator constitutive) mutations affect only adjacent genes. Ruled out (both operons regulated).
Correct Choice
(C) recessive
E.coli mutant constitutive lac operon identified as recessive when merodiploid with wild-type shows lactose inducibility.
Genetic Test Results
Original: lacI⁻ O⁺ Z⁺ → Constitutive (always ON)
Merodiploid: F'lacI⁺O⁺Z⁺ / lacI⁻O⁺Z⁺ → Inducible (lacI⁺ complements)
lacI⁺ repressor diffuses, regulates both operons
Dominance Classification
| Mutation | Merodiploid Phenotype | Type |
|---|---|---|
| lacI⁻ | Inducible | Recessive |
| lacIᵈ | Constitutive | Trans-dominant |
| Oᶜ | Constitutive | Cis-dominant |
| lacIˢ | Non-inducible | Dominant (IPTG-insensitive) |
GATE Exam Relevance
Classic Genetics PYQ tests complementation analysis. Key: lacI⁻ (recessive, trans-acting), Oᶜ (cis-dominant, DNA site). Partial diploids distinguish regulator vs operator mutations.
