[A]: A gene affecting the expression of an- other nonallelic gene is called epistatic gene.
[R]: Epistatic gene hides the expression of an allele at a different locus

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

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "a gene affecting the expression of another nonallelic gene is called an epistatic gene."
- This means that there exists a gene which can influence the expression of another gene that is located at a different locus (nonallelic).

2. **Understanding the Reason**:
- The reason states that "an epistatic gene hides the expression of an allele at a different locus."
- This indicates that the epistatic gene can mask or inhibit the expression of another gene's allele, which is referred to as the hypostatic gene.

3. **Defining Key Terms**:
- **Epistatic Gene**: A gene that can suppress or mask the expression of another gene (the hypostatic gene).
- **Hypostatic Gene**: The gene whose expression is masked or inhibited by the epistatic gene.

4. **Examples of Epistasis**:
- **Dominant Epistasis**: An example is seen in *Cucurbita pepo* (squash), where the phenotypic ratio of offspring is 12:3:1 (12 white, 3 yellow, and 1 green).
- **Recessive Epistasis**: An example is the skin color of mice, which shows a phenotypic ratio of 9:3:4 (9 agouti, 3 black, and 4 albino).

5. **Conclusion**:
- Both the assertion and reason are true. The assertion correctly defines an epistatic gene, and the reason accurately explains the function of an epistatic gene in relation to a hypostatic gene. Therefore, the correct answer is that both the assertion and reason are true, and the reason is a correct explanation of the assertion.