Assertion (A): Sickle cell anaemia is an example of point mutation.
Reason ( R): It is caused by addition or deletion of nitrogenous bases in the DNA or mRNA.

Options

A. Both assertion (A) and reason ( R) are true and reason ( R) is the correct explanation of assertion (A).
B. Both assertion (A) and reason ( R) are true but reason ( R) is not the correct explanation of assertion (A).
C. Assertion (A) is true but reason ( R) is false.
D. Assertion (A) is false but reason ( R) is true.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Assertion (A)**: The assertion states that "Sickle cell anaemia is an example of point mutation."
- A point mutation refers to a change in a single nucleotide base pair in DNA. Sickle cell anaemia is indeed caused by a single nucleotide substitution in the beta-globin gene.

2. **Understanding the Reason (R)**: The reason states that "It is caused by addition or deletion of nitrogenous bases in the DNA or mRNA."
- This statement is incorrect in the context of sickle cell anaemia. Sickle cell anaemia is specifically caused by a substitution mutation (the replacement of one base with another), not by the addition or deletion of bases.

3. **Evaluating the Relationship**: We need to determine if both the assertion and reason are correct, and if the reason correctly explains the assertion.
- The assertion is correct (sickle cell anaemia is a point mutation).
- The reason is incorrect (it does not involve addition or deletion).

4. **Conclusion**: Based on the evaluation, the correct option is that the assertion is correct, but the reason is wrong.

**Final Answer**: The assertion is correct, but the reason is wrong.

---