How can you tell if two genes are epistatic?

• A. The F₂ offspring from a cross show a 9:3:3:1 phenotypic ratio
• B. The F₂ offspring all show a mutant phenotype
• C. The F₂ offspring show a phenotypic ratio that is NOT 9:3:3:1
• D. The F₂ offspring all show a wild-type phenotype

Answer: (C)

Verified step by step guidance

1. Understand the concept of epistasis: Epistasis occurs when the effect of one gene is dependent on the presence of one or more 'modifier genes'. It can alter the expected phenotypic ratios in genetic crosses.
2. Recognize the typical Mendelian ratio: In a dihybrid cross, the expected phenotypic ratio for independent assortment is 9:3:3:1, which indicates no epistasis.
3. Identify deviations from the 9:3:3:1 ratio: If the F2 offspring show a phenotypic ratio that deviates from 9:3:3:1, it suggests that epistasis may be occurring.
4. Consider the phenotypes of the F2 offspring: If all F2 offspring show a mutant phenotype or all show a wild-type phenotype, this indicates a deviation from the expected ratio, suggesting epistasis.
5. Analyze the genetic interaction: Determine which gene is masking the effect of the other, leading to the observed phenotypic ratio. This will help confirm the presence of epistasis.