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Question #45827
The Question
In a diploid organism, a quantitative trait is controlled by three unlinked polygenes (A, B, and C) with additive effects, normally producing a classic bell-shaped distribution curve. If an individual inherits an autosomal trisomy (2n+1) for the chromosome carrying gene 'A' and possesses only dominant alleles for all three loci, which of the following best describes the disruption to the phenotypic curve?
Options
- A.
The curve shifts, exhibiting a transgressive phenotype beyond the normal maximum range due to a chromosome dosage effect.
- B.
The curve becomes strictly bimodal because the extra chromosome induces pleiotropy across the other two loci.
- C.
The phenotypic expression remains within the normal maximum limit because dosage compensation will silence the extra autosome.
- D.
The trait loses its continuous variation and exhibits a discrete Mendelian ratio due to the disruption of genetic continuity.
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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
Polygenic Inheritance Curve|Aneuploidy vs Polyploidy|Gene Expression
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students often memorize that polygenic traits form a fixed bell curve (e.g., max 6 alleles for human skin color).
The trap tests the synthesis of Mendelian genetics with chromosomal disorders.
The student must realize that non-disjunction (aneuploidy) physically adds an extra locus, mathematically breaking the normal maximum limit via an additive dosage effect.
Option C is a high-level distractor testing the misapplication of X-inactivation (dosage compensation) to autosomes.
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Principles of Inheritance and Variation - Class 12 |
| Concept | Polygenic Inheritance Curve |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |