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 crossing over, chromatids, and chromosomes.

### Step-by-Step Solution:

1. **Understanding Crossing Over**:
- 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 Chromatids**:
- Chromatids are the two identical halves of a duplicated chromosome. They are referred to as sister chromatids when they are identical and connected by a centromere.

3. **Homologous Chromosomes**:
- Homologous chromosomes are pairs of chromosomes that have the same structure and gene sequence but may have different alleles. Each pair consists of one chromosome from each parent.

4. **Correct Pairing for Crossing Over**:
- During crossing over, the exchange occurs between non-sister chromatids of homologous chromosomes. This is crucial because sister chromatids are identical and do not contribute to genetic diversity.

5. **Filling in the Blanks**:
- Based on the understanding that crossing over occurs between non-sister chromatids of homologous chromosomes, we can fill in the blanks:
- Crossing over occurs between **non-sister** chromatids of **homologous** chromosomes.

### Final Answer:
Crossing over occurs between **non-sister** chromatids of **homologous** chromosomes.