The incorrect statement with regard to haemophilia is

Options

A. It is a dominant disease
B. A single protein involved in the clotting of blood is affected
C. it is a sex-linked disease
D. it is a recessive disease

Answer

**Step-by-Step Solution:**

1. **Understanding Hemophilia**: Hemophilia is an inherited bleeding disorder where blood does not clot properly. This condition is primarily due to a deficiency in certain clotting factors.

2. **Identifying the Nature of Hemophilia**: Hemophilia is classified as a recessive disorder, specifically an X-linked recessive disorder. This means that the gene responsible for hemophilia is located on the X chromosome and that the disorder will manifest in individuals who have two copies of the recessive allele (in females) or one copy (in males).

3. **Evaluating the Statements**:
- **Statement 1**: "It is a dominant disease." - This is incorrect because hemophilia is a recessive disorder.
- **Statement 2**: "A single protein involved in the clotting of blood is affected." - This is correct as hemophilia is caused by a deficiency in specific clotting factors (proteins).
- **Statement 3**: "It is a sex-linked disease." - This is correct since the gene for hemophilia is located on the X chromosome.
- **Statement 4**: "It is a recessive disease." - This is correct as hemophilia is indeed a recessive disorder.

4. **Conclusion**: The incorrect statement regarding hemophilia is that "it is a dominant disease." Therefore, the answer is Statement 1.