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 specific deletion on chromosome 5. It is named after the distinctive cry of affected infants, which sounds like a cat's meow.

2. **Identifying the Cause:**
- The primary cause of Cri-du-chat syndrome is the loss of a portion of the short arm of chromosome 5. This deletion leads to various developmental issues in affected individuals.

3. **Analyzing the Options:**
- The question provides several options regarding the causes of Cri-du-chat syndrome:
- A) Fertilization of a XY egg by a normally bearing Y 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.

4. **Evaluating Each Option:**
- **Option A:** This describes a different genetic condition (Klinefelter syndrome), not Cri-du-chat.
- **Option B:** This correctly identifies the loss of the short arm of chromosome 5, which is the cause of Cri-du-chat syndrome.
- **Option C:** This is incorrect as Cri-du-chat syndrome is not caused by the loss of the long arm of chromosome 5.
- **Option D:** This refers to Down syndrome, which is caused by trisomy of the 21st chromosome and is unrelated to Cri-du-chat syndrome.

5. **Conclusion:**
- The correct answer is **B) Loss of half of the short arm of chromosome 5**, as this deletion is the defining characteristic of Cri-du-chat syndrome.

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