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Question #45848
The Question
Consider a biosynthetic pathway for a phenotypic trait controlled by two unlinked loci in a diploid organism. Locus 1 codes for a regulatory repressor protein, while Locus 2 contains the structural gene for a pigment-synthesizing enzyme. The repressor normally binds to the operator of Locus 2 but dissociates in the presence of a specific inducer. A dominant mutation at Locus 1 () alters the repressor's allosteric site, completely preventing inducer binding. Which of the following best describes the genetic phenomenon and the molecular consequence of the allele?
Options
- A.
It demonstrates recessive epistasis, because the mutant repressor requires the absence of the inducer to bind to the promoter region.
- B.
It exhibits dominant epistasis over Locus 2, as the allele produces a repressor that permanently blocks transcription, masking any allelic variation at Locus 2.
- C.
It acts as a hypostatic gene, causing the structural gene at Locus 2 to undergo independent assortment and bypass regulatory control.
- D.
It results in pleiotropy, where the mutated repressor simultaneously activates multiple independent operons across different chromosomes.
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Correct Answer
Step-by-Step Solution
Explanation
How the Examiner Thinks
Regulatory_Failure
Understand how NOT, EXCEPT, and logical constraints are used to manipulate exam results.
Concepts Behind This Question
Transcriptional Unit Boundaries|Mendelian Dihybrid Exceptions|Lac Operon Inducibility
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students often memorize Epistasis purely as a deviation in Mendelian dihybrid ratios (e.g., 12:3:1 or 9:3:4) without understanding its molecular basis.
This question forces the student to realize that hierarchical gene regulation (like a permanently active repressor from one locus shutting down a structural gene at another locus) is the actual molecular mechanism that causes epistatic masking and overrides the Law of Independent Assortment.
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Principles of Inheritance and Variation - Class 12 |
| Concept | Mendelian Dihybrid Exceptions |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |