Given below are two statements, one is labelled as Assertion (A) and other labelled as Reason (R).
Assertion (A): In a breeding program, it is relatively easy to handle the 'major genes' for resistance, but it leads to the boom and bust cycle due to the quick breakdown of resistance in the varieties.
Reason (R): Polygenic or partial resistance is durable but breeding for this type of resistance is problematic.
In light of the above statements, choose the correct answer from the options given below.
Assertion (A): In a breeding program, it is relatively easy to handle the 'major genes' for resistance, but it leads to the boom and bust cycle due to the quick breakdown of resistance in the varieties.
Reason (R): Polygenic or partial resistance is durable but breeding for this type of resistance is problematic.
In light of the above statements, choose the correct answer from the options given below.
Show Hint
- Both (A) and (R) are true and (R) is the correct explanation of (A).
- Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
- (A) is true but (R) is false.
- (A) is false but (R) is true.
The Correct Option is B
Solution and Explanation
Plant breeding programs rely on major-gene (vertical) resistance and polygenic (horizontal) resistance to control pathogens.
Major-gene resistance is monogenic, which exerts severe directional selection pressure on the pathogen, leading to rapid evolution of virulent strains.
Polygenic resistance is governed by multiple minor genes, which reduces disease progress without completely blocking pathogen reproduction.
Step 2: Detailed Explanation:
Handling major genes in breeding is straightforward because they display simple Mendelian inheritance, making them easy to select using phenotypic screens or molecular markers.
However, relying solely on single major genes leads to the "boom and bust cycle."
When a highly resistant variety is widely cultivated, the pathogen population rapidly selects for mutants that overcome the specific resistance gene.
This results in a sudden, catastrophic breakdown of resistance in the field, making Assertion (A) true.
On the other hand, polygenic (partial) resistance is highly durable because it does not impose absolute selection pressure on the pathogen.
The multi-genic nature makes it difficult for the pathogen to overcome all host defense barriers simultaneously.
However, breeding for polygenic resistance is highly problematic because the inheritance is complex, quantitative, and heavily influenced by environmental conditions.
Hence, Reason (R) is also a true statement.
The durability and breeding challenges of polygenic resistance, however, do not explain why major-gene resistance fails and leads to the boom and bust cycle.
The boom and bust cycle of major genes is explained by the directional evolutionary pressure exerted on the pathogen population.
Thus, both (A) and (R) are true, but (R) is not the correct explanation of (A).
Step 3: Final Answer:
Both (A) and (R) are true, but (R) is not the correct explanation of (A).
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