In a genetic cross having recessive epistasis, `"F"_(2)` phenotypic ratio would be:

Options

A. `9:6:1`
B. `15:1`
C. `9:3:4`
D. `12:3:1`

Answer

To solve the question regarding the F2 phenotypic ratio in a genetic cross exhibiting recessive epistasis, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Epistasis**:
- Epistasis is a genetic phenomenon where the expression of one gene is suppressed or masked by another gene. In the case of recessive epistasis, a recessive allele of one gene masks the effects of either allele of a second gene.

2. **Identify the Genes Involved**:
- Let's denote two genes involved in the cross as A and B. The alleles can be represented as:
- A (dominant) and a (recessive)
- B (dominant) and b (recessive)

3. **Determine the F2 Generation from a Dihybrid Cross**:
- In a typical dihybrid cross (A/a and B/b), the F2 generation would normally show a phenotypic ratio of 9:3:3:1, representing the combinations of dominant and recessive traits.

4. **Apply Recessive Epistasis**:
- In the case of recessive epistasis, the presence of the homozygous recessive genotype (aa) will mask the expression of the B gene. This means that regardless of whether the B gene is dominant or recessive, if the A gene is recessive (aa), the phenotype will be determined solely by the A gene.

5. **Calculate the New Phenotypic Ratio**:
- The typical 9:3:3:1 ratio is modified due to the masking effect of the recessive allele. The new phenotypic ratio will be 9:3:4.
- This means:
- 9 individuals will show the dominant traits for both genes.
- 3 individuals will show the dominant trait for A and the recessive trait for B.
- 4 individuals will show the recessive trait for A, regardless of the B gene.

6. **Conclusion**:
- Therefore, the F2 phenotypic ratio for a genetic cross exhibiting recessive epistasis is 9:3:4.

### Final Answer:
The F2 phenotypic ratio in a genetic cross having recessive epistasis is **9:3:4**.