Epitasis in which dihybrid ratio of 9:3:3:1 between Aa Bb `xx` AaBb is modified is

Options

A. Dominance of one allele on another allele at the same locus
B. Dominance of one alleles on another allele at both its loci
C. Interaction of two alleles at different loci
D. Interaction between two alleles at same locus

Answer

To solve the question regarding epistasis and its effect on the dihybrid ratio of 9:3:3:1, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Epistasis**:
- Epistasis is a genetic interaction where the effect of one gene (or allele) masks or modifies the effect of another gene (or allele). This can alter the expected phenotypic ratios in offspring.

2. **Identifying the Dihybrid Ratio**:
- In a typical dihybrid cross (AaBb x AaBb), the expected phenotypic ratio is 9:3:3:1. This ratio represents the combinations of dominant and recessive traits for two genes.

3. **Types of Epistasis**:
- There are different types of epistasis, including:
- **Dominant Epistasis**: One dominant allele masks the expression of alleles at another locus.
- **Recessive Epistasis**: Two recessive alleles at one locus mask the expression of alleles at another locus.

4. **Analyzing the Options**:
- The question provides several options regarding the nature of epistasis:
- A) Dominance of one allele on another allele at the same locus.
- B) Dominance of one allele on another allele at both its loci.
- C) Interaction of two alleles at different loci.
- D) Interaction between two alleles at the same locus.

5. **Determining the Correct Option**:
- **Option A** is incorrect because it refers to dominance at the same locus, which does not describe epistasis.
- **Option B** is correct because it describes dominant epistasis where one allele can dominate over alleles at both loci.
- **Option C** is incorrect as it does not specify dominance and refers to interaction without masking.
- **Option D** is also incorrect because it refers to interaction at the same locus without dominance.

6. **Conclusion**:
- In dominant epistasis, the typical 9:3:3:1 ratio is modified to a new ratio, often 12:3:1, due to the dominance of one allele over the others.

### Final Answer:
The correct answer is **B) Dominance of one allele on another allele at both its loci**.

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