The evidence that crossing over occurs at four stranded stage of the chromosomes, comes from

Options

A. Studies of meiosis in maize
B. Studies on linkage maps of chromosomes in Drosophilla
C. 4:4 arrangement of ascospores in Neurospora
D. 2:2:2:2 arrangement of ascospores in Neurospora

Explanation

To solve the question regarding the evidence that crossing over occurs at the four-stranded stage of chromosomes, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Crossing Over**:
- Crossing over is a process that occurs during meiosis where homologous chromosomes exchange genetic material. This results in genetic variation in the offspring.

2. **Identifying the Organism**:
- The question refers to *Neurospora crassa*, a type of red bread mold that is commonly used in genetic studies. It belongs to the phylum Ascomycetes.

3. **Meiotic Products in Neurospora**:
- In *Neurospora crassa*, during meiosis, the formation of asci (sac-like structures) occurs. Each ascus typically contains eight ascospores resulting from meiotic division.

4. **Analyzing the Ascus Formation**:
- When examining the meiotic products, it has been observed that the arrangement of ascospores in the asci can show different ratios. For example, the expected ratio of 2:2:2:2 indicates that crossing over has occurred.

5. **Evidence of Four-Stranded Stage**:
- The presence of a 2:2:2:2 arrangement of ascospores suggests that crossing over occurred at a four-stranded stage. This means that during meiosis, the homologous chromosomes were paired and exchanged segments, resulting in this specific ratio of genetic combinations.

6. **Conclusion**:
- Therefore, the evidence that crossing over occurs at the four-stranded stage of chromosomes comes from the observation of the 2:2:2:2 arrangement of ascospores in *Neurospora crassa*.

### Final Answer:
The correct answer is the option that states the 2:2:2:2 arrangement of the ascospores in *Neurospora crassa*.

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