[A]: Copy choice theory for crossing over was proposed by Belling.
[R]: According to this theory, paired chromosomes in Meiosis I, duplicate their genes before the development of fibres that join them in tandem.

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

To solve the question, we need to analyze both statements (A) and (R) regarding the copy choice theory proposed by Belling.

### Step-by-Step Solution:

1. **Understanding Statement A**:
- The assertion states that "Copy choice theory for crossing over was proposed by Belling."
- We need to verify if Belling indeed proposed this theory.
- **Conclusion**: This statement is true as J. Belling, a cytologist, proposed the copy choice theory in 1931.

2. **Understanding Statement R**:
- The reason states that "According to this theory, paired chromosomes in Meiosis I duplicate their genes before the development of fibres that join them in tandem."
- We need to check if this accurately reflects what the copy choice theory states.
- According to the theory, paired chromosomes do duplicate their genes during Meiosis I before the formation of the fibers that facilitate crossing over.
- **Conclusion**: This statement is also true.

3. **Analyzing the Relationship Between A and R**:
- Although both statements A and R are true, we need to determine if R adequately explains A.
- The copy choice theory discusses the process of crossing over and gene duplication but does not specifically state that the duplication of genes is the reason for the assertion made in A.
- **Conclusion**: R does not correctly explain A, even though both are true.

4. **Final Answer**:
- Both A and R are true, but R is not the correct explanation of A.