Down's syndrome is related to

Options

A. Increase in chromosome number of 21 st pair
B. Decrease in chromosome number of 21 st pair
C. Increase in chromosome number of 18th pair
D. Decrease in chromosome number of 18th pair

Answer

**Step-by-Step Solution:**

1. **Understanding Down's Syndrome**:
- Down's syndrome is a genetic disorder that is characterized by certain physical and mental developmental challenges. It is important to recognize that it is classified as an autosomal disorder.

2. **Identifying Chromosomal Abnormality**:
- The disorder is caused by a chromosomal abnormality, specifically involving the autosomal chromosomes. Autosomal chromosomes are those that do not determine sex.

3. **Chromosome 21 Involvement**:
- In Down's syndrome, there is an abnormality specifically related to chromosome 21. This chromosome is involved in the genetic makeup of the individual.

4. **Extra Genetic Material**:
- The key feature of Down's syndrome is the presence of an extra copy of chromosome 21, which is referred to as trisomy 21. This means that instead of the usual two copies of chromosome 21, individuals with Down's syndrome have three copies.

5. **Effects of Extra Chromosome**:
- The additional genetic material from the extra chromosome 21 can lead to various developmental changes, resulting in both physical and mental abnormalities in the affected individuals.

6. **Conclusion**:
- Therefore, Down's syndrome is related to an increase in the number of chromosomes in the 21st pair, making the correct answer option one: "increase in chromosome number of 21st pair."

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