Down's syndrome is associated with trisomy of chromosome number:

Options

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

Answer

**Step-by-Step Solution:**

1. **Understanding Down Syndrome**: Down syndrome is a genetic disorder that is characterized by the presence of an extra chromosome. It is important to recognize that this condition is a type of chromosomal abnormality.

2. **Definition of Trisomy**: Trisomy refers to the condition where there are three copies of a chromosome instead of the normal two. This means that instead of the usual diploid number of chromosomes (2n), there is an additional chromosome, resulting in a total of 47 chromosomes.

3. **Normal Chromosome Count**: In humans, the normal diploid chromosome count is 46 (23 pairs). In the case of Down syndrome, the total becomes 47 due to the presence of an extra chromosome.

4. **Identifying the Affected Chromosome**: In Down syndrome, the specific chromosome that is affected is the 21st chromosome. This means that individuals with Down syndrome have three copies of chromosome 21.

5. **Conclusion**: Therefore, Down syndrome is associated with trisomy of chromosome number 21.

**Final Answer**: Down's syndrome is associated with trisomy of chromosome number 21.

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