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 is a genetic disorder caused by the presence of an extra chromosome. This condition is known as trisomy.

2. **Chromosomal Basics**: In humans, the normal chromosomal count is 46, organized into 23 pairs (2N). This includes 22 pairs of autosomes and one pair of sex chromosomes (XX or XY).

3. **Defining Trisomy**: Trisomy refers to the presence of three copies of a particular chromosome instead of the usual two. In the case of Down's syndrome, this specifically refers to chromosome 21.

4. **Identifying the Extra Chromosome**: In Down's syndrome, there is an extra copy of chromosome 21, resulting in a total of 47 chromosomes (46 normal + 1 extra).

5. **Mechanism of Trisomy**: The extra chromosome arises due to a process called non-disjunction, where chromosomes fail to separate properly during cell division. This leads to one gamete having an extra chromosome.

6. **Conclusion**: Therefore, the correct answer to the question is that Down's syndrome has a trisomy of chromosome 21.

**Final Answer**: Down's syndrome has trisomy of chromosome **21**.

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