Down's syndrome is due to

Options

A. Crossing over
B. Linkage
C. Sex linked inheritance
D. Nondisjunction of chromosomes

Answer

### Step-by-Step Solution:

1. **Understanding Down's Syndrome**:
- Down's syndrome is a genetic disorder characterized by the presence of an extra chromosome 21. This condition is also referred to as trisomy 21.

2. **Identifying the Cause**:
- The primary cause of Down's syndrome is non-disjunction of chromosomes. Non-disjunction occurs when homologous chromosomes fail to separate properly during cell division (meiosis).

3. **Mechanism of Non-Disjunction**:
- Normally, during meiosis, chromosomes are supposed to separate into different gametes. If non-disjunction occurs, one gamete may receive an extra copy of chromosome 21, leading to an embryo with three copies of this chromosome instead of the usual two.

4. **Consequences of Non-Disjunction**:
- When fertilization occurs, the resulting embryo will have three copies of chromosome 21. This genetic anomaly leads to the various physical and developmental characteristics associated with Down's syndrome.

5. **Exclusion of Other Options**:
- The other options provided (crossing over, linkage, and sex-linked inheritance) do not cause Down's syndrome.
- **Crossing Over**: This is a process that occurs during meiosis where homologous chromosomes exchange genetic material, leading to genetic diversity, but it does not result in an extra chromosome.
- **Linkage**: This refers to genes located close together on the same chromosome that tend to be inherited together, which is unrelated to the occurrence of Down's syndrome.
- **Sex-Linked Inheritance**: This involves traits that are associated with genes located on sex chromosomes (X or Y), which is not the case for Down's syndrome.

6. **Conclusion**:
- Therefore, the correct answer to the question "Down's syndrome is due to" is **non-disjunction of chromosomes**.