Down 's syndorme is due to nondisjunction of

Options

A. X-chromosome
B. Y-chromosome
C. Autosome
D. Second chromosome of Drosophila

Answer

**Step-by-Step Solution:**

1. **Understanding Nondisjunction**: Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division, specifically during meiosis. This can lead to gametes with an abnormal number of chromosomes.

2. **Meiosis Process**: In meiosis, chromosomes align at the metaphase plate during metaphase I and metaphase II. Normally, spindle fibers pull the chromosomes apart at the centromere, leading to the correct distribution of chromosomes into daughter cells.

3. **Consequences of Nondisjunction**: If nondisjunction occurs, it can result in gametes that have an extra chromosome or are missing a chromosome. When such gametes fuse with normal gametes during fertilization, it leads to an individual with an abnormal number of chromosomes.

4. **Down Syndrome Specifics**: Down syndrome, also known as trisomy 21, occurs when there is an extra copy of chromosome 21 due to nondisjunction. This means that instead of the typical two copies of chromosome 21, individuals with Down syndrome have three copies.

5. **Identifying the Chromosome**: Since Down syndrome is specifically associated with chromosome 21, we can conclude that the nondisjunction responsible for this condition occurs with this particular chromosome.

6. **Classifying Chromosome 21**: Chromosome 21 is classified as an autosome (non-sex chromosome). This is important because it distinguishes Down syndrome from other chromosomal disorders that may involve sex chromosomes.

7. **Conclusion**: Therefore, Down syndrome is due to the nondisjunction of chromosome 21, which is an autosome.

**Final Answer**: Down syndrome is due to nondisjunction of chromosome 21 (an autosome).

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