Cri-du-chat syndrome in humans is caused by the :

Options

A. Fertilization of an XX egg by a normal Y-bearing sperm
B. Loss of half of the short arm of chromosome 5
C. Loss of half of the long arm of chromosome 5
D. Trisomy of 21st chromosome

Answer

**Step-by-Step Solution:**

1. **Understanding Cri-du-chat Syndrome**:
- Cri-du-chat syndrome is a genetic disorder characterized by a distinctive cry that resembles that of a cat. This syndrome is caused by a specific chromosomal abnormality.

2. **Identifying the Chromosomal Cause**:
- The syndrome is specifically linked to a deletion on chromosome 5. More precisely, it is caused by the loss of a portion of the short arm of chromosome 5.

3. **Evaluating the Options**:
- The options provided are:
- A) Fertilization of an XX egg by a normal Y-bearing sperm
- B) Loss of half of the short arm of chromosome 5
- C) Loss of half of the long arm of chromosome 5
- D) Trisomy of the 21st chromosome
- Option A describes a condition leading to Klinefelter syndrome, which is not related to Cri-du-chat syndrome.
- Option C refers to the loss of the long arm of chromosome 5, which is incorrect for Cri-du-chat syndrome.
- Option D describes Down's syndrome, which is caused by trisomy of the 21st chromosome, also unrelated to Cri-du-chat syndrome.

4. **Conclusion**:
- The correct answer is **B) Loss of half of the short arm of chromosome 5**, which is the recognized cause of Cri-du-chat syndrome.