A : Chiasmata counting stage is diplotene.
R : Dissolution of the synaptonemal complex occurs except at the sites of cross overs.

Options

A. IF both Assertion & Reason are true and the reason followed is the correct explanation of the assertion then mark (1).
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion then mark (2).
C. If Assertion is true statement but Reason is false, then mark (3).
D. If both Assertion and Reason are false statement then mark (4).

Answer

To solve the assertion-reason question, we need to analyze both the assertion (A) and the reason (R) statements provided.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that "Chiasmata counting stage is diplotene."
- Chiasmata are the points where homologous chromosomes exchange genetic material during crossing over. They become visible during the diplotene stage of prophase I in meiosis.
- Therefore, this assertion is **true**.

2. **Understanding the Reason (R)**:
- The reason states that "Dissolution of the synaptonemal complex occurs except at the sites of crossovers."
- The synaptonemal complex is a protein structure that holds homologous chromosomes together during prophase I. It begins to dissolve during the diplotene stage, but it remains intact at the sites where crossing over has occurred, allowing the chromosomes to stay connected at those points (the chiasmata).
- Thus, this reason is also **true**.

3. **Determining the Relationship**:
- We need to check if the reason correctly explains the assertion.
- The assertion is about the visibility of chiasmata during the diplotene stage, while the reason explains the process of synaptonemal complex dissolution and its relation to chiasmata.
- Since the dissolution of the synaptonemal complex leads to the formation of chiasmata, the reason provides a correct explanation for the assertion.

4. **Conclusion**:
- Both the assertion and the reason are true, and the reason is the correct explanation for the assertion.

### Final Answer:
Both the assertion (A) and the reason (R) are true, and the reason is the correct explanation of the assertion.

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