Before You Begin
Question #45835
The Question
During a pedigree analysis of a rare genetic disorder, a geneticist observes that an affected child is born to two phenotypically normal parents. While this typically indicates an autosomal recessive trait, molecular analysis reveals the child is heterozygous for a dominant disease-causing allele inherited from the father. Which of the following genetic principles best explains why the father did not express the disorder?
Options
- A.
Variable expressivity, where the father's phenotype was expressed too mildly to be clinically detected.
- B.
Incomplete penetrance, where the dominant allele completely failed to manifest phenotypically in the father.
- C.
Co-dominance, where the wild-type allele completely masked the expression of the dominant disease allele.
- D.
Pleiotropy, where the dominant allele affected a completely different, unobserved physiological trait in the father.
Check Your Answer
Correct Answer
Step-by-Step Solution
Explanation
How the Examiner Thinks
Comparative_Analysis
Sharpen your ability to evaluate multiple scenarios and distinguish between competing potential answers.
Concepts Behind This Question
Pedigree Analysis Traps|Co-dominance Allelic Interaction|Chromosomal Theory of Inheritance
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students often rigidly memorize the NCERT heuristic that 'Autosomal Dominant traits never skip generations.' The trap lies in the absolute nature of this rule.
When confronted with a skipped generation, students reflexively assume Autosomal Recessive inheritance.
The pivot is recognizing that 'incomplete penetrance' allows a dominant allele to remain phenotypically silent, creating a 'pseudo-recessive' pedigree pattern.
Furthermore, the question tests the critical distinction between 'penetrance' (an all-or-nothing manifestation) and 'expressivity' (the severity of manifestation), which is a classic high-yield discriminator in NEET-Plus genetics.
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Principles of Inheritance and Variation - Class 12 |
| Concept | Pedigree Analysis Traps |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |