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Question #45591
The Question
A plant geneticist crosses an autotetraploid crop variety (AAAA) with its wild-type diploid ancestor (AA). To map a specific gene, the geneticist attempts a standard test cross using the resulting F1 offspring. Which of the following cytological mechanisms best explains why the expected Mendelian test-cross ratios will completely fail to materialize?
Options
- A.
The F1 offspring is triploid (3n), which prevents the formation of normal bivalents during Zygotene, leading to unequal Anaphase I segregation and highly sterile aneuploid gametes.
- B.
The F1 offspring produces equal proportions of balanced haploid (n) and diploid (2n) gametes, which triggers pre-zygotic incompatibility during pollen tube growth.
- C.
The presence of three homologous chromosomes causes complete suppression of the recombinase enzyme during Pachytene, resulting in absolute linkage.
- D.
The F1 offspring undergoes spontaneous somatic endoreduplication to restore fertility, shifting the expected 1:1 test-cross ratio to a 15:1 duplicate gene interaction ratio.
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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
Aneuploidy vs Polyploidy|Test Cross vs Back Cross Ratios
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students often memorize the fact that 'triploids are sterile' (e.g., seedless bananas) but fail to connect it to the underlying cytological mechanism (failure of bivalent formation in Zygotene) and its mathematical consequence on Mendelian genetics.
The trap in Option B exploits the misconception that a 3n cell simply divides its genome into '2n' and '1n' halves during meiosis, ignoring the independent assortment of each individual chromosome pair which guarantees aneuploidy.
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Principles of Inheritance and Variation - Class 12 |
| Concept | Test Cross vs Back Cross Ratios |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |