Crossing over is the exchange of genetic material between:

Options

A. Non-sister chromatids of the homologous chromosomes
B. Sister chromatids of the homologous chromosomes
C. Chromatids of non-homologous chromosome
D. The gene those are completely linked

Answer

**Step-by-Step Solution:**

1. **Understanding Crossing Over**:
- Crossing over is a process that occurs during meiosis, specifically in prophase I, where homologous chromosomes exchange genetic material.

2. **Identifying Chromosomes**:
- Homologous chromosomes are pairs of chromosomes that have the same genes at the same loci but may have different alleles (variations of a gene). For example, one chromosome may carry a gene for brown hair, while the other carries a gene for black hair.

3. **Sister vs. Non-Sister Chromatids**:
- Each chromosome consists of two identical halves called sister chromatids, which are joined at a region called the centromere. Non-sister chromatids are the chromatids of homologous chromosomes (i.e., chromatids from different homologous pairs).

4. **The Process of Crossing Over**:
- During crossing over, segments of non-sister chromatids from homologous chromosomes are exchanged. This exchange results in new combinations of alleles, contributing to genetic diversity.

5. **Conclusion**:
- Therefore, crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes.

6. **Final Answer**:
- The correct answer is: Crossing over is the exchange of genetic material between **non-sister chromatids of homologous chromosomes**.

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