Down's syndrome is due to

Options

A. crossing over
B. linkage
C. sex-linked inheritance
D. non-disjunction of chromosomes

Explanation

**Step-by-Step Text Solution:**

1. **Understanding Down's Syndrome**: Down's syndrome is a genetic disorder that results from the presence of an extra chromosome 21. This condition is also known as trisomy 21.

2. **Identifying the Cause**: The primary cause of Down's syndrome is non-disjunction of chromosomes during cell division. Non-disjunction occurs when chromosomes fail to separate properly during meiosis, the process that produces gametes (sperm and egg cells).

3. **Meiosis and Chromosome Separation**: During meiosis, homologous chromosomes are supposed to separate into different gametes. If non-disjunction occurs, one gamete may end up with an extra chromosome, while the other gamete may have one less chromosome.

4. **Formation of Trisomy 21**: If a gamete with an extra chromosome 21 fuses with a normal gamete (with the standard number of chromosomes), the resulting zygote will have three copies of chromosome 21 instead of the usual two. This condition is known as trisomy 21.

5. **Conclusion**: Therefore, Down's syndrome is specifically due to the non-disjunction of chromosomes during meiosis, leading to the presence of an extra chromosome 21.

**Final Answer**: Down's syndrome is due to non-disjunction of chromosomes.

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