Assertion: Haemophilia shows criss-cross inheritance.
Reason: The gene that causes haemophilia is recessive and lies in the sex (X) chromosome.

Options

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

Answer

### Step-by-Step Solution

1. **Understanding Criss-Cross Inheritance**:
- Criss-cross inheritance refers to the pattern of inheritance where a trait is passed from a parent to a child of the opposite sex. For example, if a father carries a trait on his X chromosome, he can pass it to his daughter, and then the daughter can pass it to her son.

2. **Identifying the Nature of Hemophilia**:
- Hemophilia is a genetic disorder that affects the blood's ability to clot. It is caused by a mutation in a gene located on the X chromosome.

3. **Determining the Type of Inheritance**:
- Since hemophilia is linked to the X chromosome and is recessive, males (who have only one X chromosome) will express the disorder if they inherit the affected X chromosome. Females (who have two X chromosomes) can be carriers if they have one affected X chromosome and one normal X chromosome.

4. **Connecting the Assertion and Reason**:
- The assertion states that hemophilia shows criss-cross inheritance, which is true because the trait can be passed from affected fathers to daughters and from carrier mothers to sons.
- The reason states that the gene causing hemophilia is recessive and located on the X chromosome, which is also true. This explains why the inheritance pattern is criss-cross.

5. **Conclusion**:
- Both the assertion and the reason are true, and the reason correctly explains the assertion. Therefore, the correct answer is that both the assertion and reason are true, and the reason is the correct explanation of the assertion.

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

---