Crossing over occurs between ........... Chromatid of ......... chromosome.

Options

A. Sister , homologous
B. Non - sister homologous
C. Non - sister , non homologous
D. Sister , homologous

Answer

To solve the question "Crossing over occurs between ........... Chromatid of ......... chromosome," we need to understand the concepts of chromatids and chromosomes, particularly in the context of meiosis.

### Step-by-Step Solution:

1. **Understand Chromatids**:
- Chromatids are the two identical halves of a replicated chromosome. When a chromosome is duplicated during the S phase of the cell cycle, it consists of two sister chromatids joined at a region called the centromere.

2. **Identify Types of Chromatids**:
- There are two types of chromatids:
- **Sister chromatids**: These are identical copies of a chromosome that are connected at the centromere.
- **Non-sister chromatids**: These are chromatids from different homologous chromosomes. They are not identical and come from different parents.

3. **Understand Homologous Chromosomes**:
- Homologous chromosomes are pairs of chromosomes that have the same genes at the same loci but may have different alleles. One chromosome of each pair is inherited from each parent.

4. **Define Crossing Over**:
- Crossing over is a process that occurs during prophase I of meiosis where non-sister chromatids exchange segments of genetic material. This process increases genetic diversity in the resulting gametes.

5. **Conclusion**:
- Based on the definitions and understanding above, crossing over occurs specifically between non-sister chromatids of homologous chromosomes.

Thus, the complete answer to the question is:
**Crossing over occurs between non-sister chromatids of homologous chromosomes.**