The ratio of 2:1 is observed in case of

Options

A. Suppressor gene
B. Dominant-recessive epistasis
C. Complementry gene
D. Lethal gene

Answer

To solve the question regarding the observation of a 2:1 ratio, we need to analyze the different genetic concepts mentioned in the options. Here’s a step-by-step breakdown:

### Step 1: Understanding the Options
The question presents four genetic concepts:
1. Suppressor gene
2. Dominant recessive epistasis
3. Complementary gene
4. Lethal gene

### Step 2: Analyzing Each Option
- **Suppressor Gene**: These genes can slow down cell division or repair DNA mistakes. They do not typically produce a 2:1 ratio in offspring.

- **Dominant Recessive Epistasis**: In this case, one gene can mask the expression of another. The typical ratios observed are 12:3:1 (dominant epistasis) or 9:3:4 (recessive epistasis), not 2:1.

- **Complementary Genes**: This involves two independent gene pairs that work together to produce a trait. The expected ratio in the F2 generation is 9:7, which is also not 2:1.

- **Lethal Gene**: A lethal gene typically results in the death of individuals when present in a homozygous condition. In a typical cross involving a lethal gene, the expected phenotypic ratio is 2:1. This occurs because the homozygous lethal individuals do not survive, leading to a reduction in the expected offspring ratio.

### Step 3: Conclusion
Based on the analysis, the correct answer is **lethal gene**, as it is the only option that produces a 2:1 ratio due to the death of homozygous individuals.

### Final Answer
The ratio of 2:1 is observed in the case of **lethal gene**.

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