Crossing over is the exchange of genetic material between ______chromatids of homologous chromosomes.

Answer

**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 chromosomes.

2. **Identifying Chromatids**:
- In a homologous chromosome pair, each chromosome consists of two sister chromatids. However, during crossing over, the exchange occurs between non-sister chromatids.

3. **Defining Non-Sister Chromatids**:
- Non-sister chromatids are the chromatids from different homologous chromosomes. For example, if we have two homologous chromosomes (one from the mother and one from the father), the chromatids from these two chromosomes are considered non-sister chromatids.

4. **Conclusion**:
- Therefore, the correct answer to the question is that crossing over is the exchange of genetic material between **non-sister chromatids** of homologous chromosomes.

**Final Answer**: Non-sister chromatids

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