Meiotic crossing over involves :

Options

A. Non-sister chromatids of a pair of homologous chromosomes
B. Sister chromatids
C. chromatids of non-homologous chromosomes
D. None of the above

Answer

### Step-by-Step Solution

1. **Understanding Meiotic Crossing Over**: Meiotic crossing over is a process that occurs during meiosis, specifically in prophase I. It involves the exchange of genetic material between homologous chromosomes.

2. **Identifying Homologous Chromosomes**: Homologous chromosomes are pairs of chromosomes, one inherited from each parent. They have the same genes at the same loci but may carry different alleles.

3. **Non-Sister Chromatids**: In a homologous pair, each chromosome consists of two sister chromatids. However, crossing over occurs between non-sister chromatids of homologous chromosomes. Non-sister chromatids are the chromatids from different homologous chromosomes.

4. **Exchange of Genetic Material**: During crossing over, segments of DNA are exchanged between the non-sister chromatids. This exchange increases genetic diversity in the resulting gametes.

5. **Evaluating the Options**:
- **Option 1**: Non-sister chromatids of a pair of the homologous chromosome. This is correct because crossing over occurs between non-sister chromatids.
- **Option 2**: Sister chromatids. This is incorrect as crossing over does not occur between sister chromatids.
- **Option 3**: Chromatids of non-homologous chromosomes. This is also incorrect because crossing over specifically involves homologous chromosomes.
- **Option 4**: None of the above options are correct. This is incorrect since option 1 is correct.

6. **Conclusion**: The correct answer is that meiotic crossing over involves non-sister chromatids of a pair of homologous chromosomes.