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Question #45600

Difficulty: 8/10 | 🟢 Free

The Question

In a human heterozygous for the ABO blood group ( $I^A I^B$ ), both alleles normally exhibit co-dominance by synthesizing functional glycosyltransferases. Suppose a systemic mutation impairs the proofreading activity of a specific aminoacyl-tRNA synthetase. The mRNA transcript of the $I^A$ allele has a high frequency of codons relying on this specific tRNA, while the $I^B$ transcript does not. Regarding the molecular and phenotypic consequences of this scenario, all of the following statements are correct EXCEPT:

Options

A.
The individual will phenotypically express blood group B, as the $I^A$ glycosyltransferase will lose its functional conformation due to amino acid misincorporation.
B.
The fundamental genetic principle of co-dominance is violated at the DNA level because the $I^A$ allele has effectively mutated into a recessive allele.
C.
The $I^B$ allele transcript will continue to be translated into a functional enzyme because it lacks the codons affected by the synthetase error.
D.
The expected 1:1 functional protein stoichiometry of the co-dominant alleles is disrupted at the translational level, not the transcriptional level.

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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|Translation|Amino_Acid_Incorporation

Why This Question Is Tricky

Common Mistakes

Conceptual Pivot: Students often confuse systemic translational errors with localized DNA mutations.

A common misconception is that a change in Mendelian phenotypic expression (loss of co-dominance) must imply a mutation at the specific allele's DNA level.

Here, the $I^A$ allele is perfectly normal at the DNA and mRNA levels; the failure occurs during translation due to a trans-acting factor (tRNA synthetase).

This tests the deep understanding that dominance/co-dominance is ultimately a function of the final protein product's activity, bridging Mendelian genetics with the Central Dogma.

Quick Facts About This Question

Question Information

Subject	Biology
Chapter	Principles of Inheritance and Variation - Class 12
Concept	Co-dominance Allelic Interaction
Difficulty	8
Cognitive Load	5
Target Solve Time	120 sec
Access	Free