[A]: Mendelian recombinations are due to crossing over.
[R]: Crossing over brings about exchange of genes through chiasma formation.

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 the assertion (A) and the reason (R) statements provided.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that "Mendelian recombinations are due to crossing over."
- This is true because Mendelian recombination refers to the generation of new combinations of alleles, which occurs during meiosis when homologous chromosomes exchange genetic material.

2. **Understanding the Reason (R)**:
- The reason states that "Crossing over brings about exchange of genes through chiasma formation."
- This is also true. During prophase I of meiosis, homologous chromosomes align and form structures called chiasmata where they exchange segments of genetic material, leading to genetic variation.

3. **Linking A and R**:
- The assertion (A) is supported by the reason (R). The process of crossing over, which is described in R, is the mechanism that leads to the Mendelian recombinations mentioned in A.
- Therefore, R provides a correct explanation for A.

4. **Conclusion**:
- Both statements A and R are true, and R is the correct explanation of A. Hence, the correct answer is that both A and R are true, and R explains A.

### Final Answer:
Both A and R are true, and R is the correct explanation of A.

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