Down 's syndrome has trisomy of chromosome

Options

A. 20
B. 21
C. 22
D. 23

Answer

**Step-by-Step Solution:**

1. **Understanding Down's Syndrome**: Down's syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21. This results in three copies of this chromosome instead of the usual two.

2. **Identifying the Chromosome**: The question specifically asks about which chromosome is involved in Down's syndrome. We need to determine which of the provided options corresponds to chromosome 21.

3. **Analyzing the Options**:
- **Option A: 20** - This is incorrect as Down's syndrome does not involve chromosome 20.
- **Option B: 21** - This is correct, as Down's syndrome is caused by trisomy of chromosome 21.
- **Option C: 22** - This is incorrect as it does not relate to Down's syndrome.
- **Option D: 23** - This is also incorrect as it does not pertain to Down's syndrome.

4. **Conclusion**: Based on the analysis, the correct answer is **Option B: 21**.

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