Assertion (a) : Supernumerary chromosomes do not usually have any effect on the phenotype and Hence ,are genetically unnecessary.
Reason( R ) : In some plants supernumerary chromosomes result in decrased vigour.

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

To analyze the given assertion and reason, let's break it down step by step:

### Step 1: Understand the Assertion
The assertion states that "Supernumerary chromosomes do not usually have any effect on the phenotype and hence are genetically unnecessary."

- **Explanation**: Supernumerary chromosomes, also known as B-chromosomes, are extra chromosomes that do not typically contribute to the organism's phenotype because the genes they carry are not expressed. Therefore, they are considered genetically unnecessary.

### Step 2: Understand the Reason
The reason states that "In some plants, supernumerary chromosomes result in decreased vigour."

- **Explanation**: While it is true that in some instances, supernumerary chromosomes can lead to decreased vigour in plants, this does not serve as an explanation for the assertion. The assertion is about the general effect of supernumerary chromosomes, while the reason discusses a specific case where they may have a negative impact.

### Step 3: Evaluate the Relationship Between Assertion and Reason
Now, we need to determine if both statements are correct and if the reason correctly explains the assertion.

- **Both Statements**: Both the assertion and reason are indeed correct. Supernumerary chromosomes generally do not affect phenotype, but in certain cases, they can lead to decreased vigour.
- **Explanation Relationship**: However, the reason does not explain the assertion. The assertion is about the general lack of phenotypic effect, while the reason points out a specific situation where there is an effect.

### Conclusion
Both the assertion (A) and reason (R) are true, but R is not the correct explanation of A.

### Final Answer
- **Assertion (A)**: True
- **Reason (R)**: True
- **Explanation Relationship**: R is not the correct explanation of A.

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