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

Options

A. Fertilization of an XX egg by normal Y-bearing sperm
B. Loss of half of the short arm of chromosome 5
C. Loss of half of the long arm of choromosome 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 that is characterized by a distinct cry that resembles that of a cat. It is primarily caused by a chromosomal abnormality.

2. **Identifying the Chromosomal Cause**: The syndrome is specifically caused by the deletion of a portion of chromosome 5. More precisely, it involves the loss of the short arm (the P arm) of this chromosome.

3. **Analyzing the Options**:
- **Option A**: "Fertilization of an XX egg by normal Y bearing sperm" - This would result in an XXY condition, which is associated with Klinefelter syndrome, not Cri-du-chat.
- **Option B**: "Loss of half of the short arm of chromosome 5" - This is correct as it directly describes the chromosomal deletion responsible for Cri-du-chat syndrome.
- **Option C**: "Loss of half of the long arm of chromosome 5" - This is incorrect as it refers to the long arm (Q arm) rather than the short arm (P arm).
- **Option D**: "Trisomy of the 21st chromosome" - This condition leads to Down syndrome, not Cri-du-chat syndrome.

4. **Conclusion**: The correct answer is **Option B**: "Loss of half of the short arm of chromosome 5."

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