Crossing over takes place between :
(a) Two sister chromatids
(b) Two non-sister chromatids
(c) Three homologous chromosomes
(d) Four non-homologous chromosomes

Options

A. Two sister chromatids
B. Two non-sister chromatids
C. Three homologous chromosomes
D. Four non-homologous chromosomes

Answer

To solve the question regarding where crossing over takes place, let's analyze each option step by step.

### 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, which contributes to genetic diversity in offspring.

2. **Analyzing the Options**:
- **Option (a): Two sister chromatids**:
- Sister chromatids are identical copies of a chromosome formed by DNA replication. They do not undergo crossing over since they are identical and do not contribute to genetic variation.
- **Option (b): Two non-sister chromatids**:
- Non-sister chromatids are chromatids from homologous chromosomes (one from each parent). Crossing over occurs between these non-sister chromatids, allowing for the exchange of genetic material and increasing genetic variability.
- **Option (c): Three homologous chromosomes**:
- Crossing over involves pairs of homologous chromosomes, not three. This option is incorrect.
- **Option (d): Four non-homologous chromosomes**:
- Non-homologous chromosomes do not pair up during meiosis, and crossing over does not occur between them. This option is also incorrect.

3. **Conclusion**:
- The correct answer is **(b) Two non-sister chromatids**, as crossing over specifically occurs between non-sister chromatids of homologous chromosomes.

### Final Answer:
The correct option is **(b) Two non-sister chromatids**.

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