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Question #45854
The Question
Aneuploid conditions such as Down's syndrome (Trisomy 21) and Turner's syndrome (45, XO) impose severe reproductive fitness costs, often resulting in sterility. Despite this strong negative selection pressure, these chromosomal disorders have persisted at a relatively constant frequency throughout human evolutionary history. Which of the following best explains this genomic paradox?
Options
- A.
The conditions are maintained by balanced polymorphism, where phenotypically normal carrier parents possess a heterozygote advantage.
- B.
The disorders are continuously replenished in the population by de novo meiotic non-disjunction events during gametogenesis.
- C.
High mutation rates in the non-coding regions of the affected chromosomes confer resistance against the 'Evil Quartet' of extinction.
- D.
The persistence is due to euploidy, where the addition or deletion of a single chromosome stabilizes the genomic mutation load.
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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
Down/Klinefelter/Turner Syndrome Karyotypes|Evolutionary Chronology|Hominid Cranial Capacity
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students often conflate the evolutionary maintenance of Mendelian genetic disorders (via carrier survival/heterozygote advantage) with chromosomal disorders.
The trap relies on the assumption that all persistent genetic conditions must offer some hidden evolutionary advantage.
Students must synthesize NCERT knowledge of non-disjunction to realize that aneuploidies are almost entirely de novo errors in gametogenesis, meaning natural selection cannot 'weed them out' of the gene pool because the error arises anew in each generation.
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Principles of Inheritance and Variation - Class 12 |
| Concept | Down/Klinefelter/Turner Syndrome Karyotypes |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |