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Question #46353
The Question
The sickle cell allele (HbS) acts as a structural inhibitor to the normal erythrocyte lifecycle but provides significant ecological fitness in malaria-endemic regions. Which of the following statements regarding the genetic, physiological, and evolutionary dynamics of this phenomenon is INCORRECT?
Options
- A.
The defect is caused by the substitution of Glutamic acid by Valine at the sixth position of the beta-globin chain due to a single base substitution from GAG to GUG in the mRNA.
- B.
In heterozygotes (HbA HbS), Plasmodium infection induces localized low oxygen tension, triggering premature sickling and subsequent splenic clearance of the infected erythrocytes.
- C.
The high frequency of the HbS allele in malaria-endemic regions is maintained by directional selection, which continuously favors the mutant allele to eliminate the parasite.
- D.
The mutant haemoglobin molecule undergoes polymerization under low oxygen tension, causing the change in the shape of the RBC from a biconcave disc to an elongated sickle-like structure.
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Correct Answer
Step-by-Step Solution
Explanation
How the Examiner Thinks
Evolutionary_Exception
Tackle problems where standard rules evolve or change based on specific environmental constraints.
Concepts Behind This Question
Sickle_Cell_Trait|Malaria_Resistance|Heterozygote_Advantage
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students frequently confuse 'Natural Selection favoring a trait' with 'Directional Selection'.
While the HbS allele provides an advantage against malaria, it is lethal in the homozygous state.
Therefore, natural selection favors the heterozygote (HbA HbS), making this a case of Balancing Selection.
Failing to account for the lethality of the homozygous recessive state leads students to incorrectly choose Directional Selection.
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Body Fluids and Circulation - Class 11 |
| Concept | Erythrocyte Lifecycle and Location |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |