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Question #45846
The Question
In a genetic experiment, a dihybrid plant with the genotype AaBb (where dominant alleles 'A' and 'B' are in the cis arrangement) is test-crossed with a homozygous recessive plant (aabb). The loci for genes A and B are located on the same chromosome at a distance of 8 map units (centimorgans). Which of the following accurately represents the expected genotypic ratio of the progeny, and the cytological basis for this deviation from Mendel's Law of Independent Assortment?
Options
- A.
25% AaBb, 25% aabb, 25% Aabb, 25% aaBb; due to random alignment of bivalents at the metaphase plate during Meiosis I.
- B.
46% AaBb, 46% aabb, 4% Aabb, 4% aaBb; due to physical linkage restricting independent assortment, with crossing over occurring during the pachytene stage.
- C.
42% AaBb, 42% aabb, 8% Aabb, 8% aaBb; due to recombination frequency being equal to map distance, overriding the law of segregation.
- D.
50% AaBb, 50% aabb; due to complete linkage at distances under 10 centimorgans preventing chiasmata formation.
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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
Mendelian Dihybrid Exceptions|Recombination Frequency (RF)
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students frequently confuse the total recombination frequency (8%) with the frequency of individual recombinant classes (which must be divided by 2, yielding 4%).
Additionally, it tests the integration of Mendelian genetics with the cytological basis of crossing over (pachytene of Prophase I), reinforcing that linkage breaks the 1:1:1:1 test cross ratio expected from independent assortment.
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Principles of Inheritance and Variation - Class 12 |
| Concept | Mendelian Dihybrid Exceptions |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |