Which of the following crossess would produce a genotypic ratio of 1:2:1

Options

A. AB `xx` AB
B. Ab `xx` ab
C. Ab `xx` Ab
D. ab`xx`ab

Answer

To determine which cross produces a genotypic ratio of 1:2:1, we can analyze the options provided step by step.

### Step-by-Step Solution:

1. **Understanding Genotypic Ratios**:
- A genotypic ratio of 1:2:1 typically arises from a monohybrid cross between two heterozygous individuals (e.g., Aa x Aa).

2. **Analyzing the First Option**:
- The first option is a cross between A B and A B (both homozygous).
- Since both parents are the same, all offspring will also be A B.
- Result: 100% A B (genotypic ratio = 1:0:0).
- **Conclusion**: This option does not produce a 1:2:1 ratio.

3. **Analyzing the Second Option**:
- The second option is a cross between a homozygous dominant (e.g., A A) and a homozygous recessive (e.g., a a).
- All offspring will be heterozygous (A a).
- Result: 100% A a (genotypic ratio = 1:0:0).
- **Conclusion**: This option does not produce a 1:2:1 ratio.

4. **Analyzing the Third Option**:
- The third option is a cross between A B (heterozygous) and a b (homozygous recessive).
- Possible gametes: A B and a b.
- Using a Punnett square:
- Parent 1: A B
- Parent 2: a b
- Gametes: A B, a b
- Offspring combinations: A B, A b, a B, a b.
- Result: 1 A B : 1 A b : 1 a b (genotypic ratio = 1:1:1).
- **Conclusion**: This option does not produce a 1:2:1 ratio.

5. **Analyzing the Fourth Option**:
- The fourth option is a cross between A B (heterozygous) and selfing (A A).
- Possible gametes: A A and A B.
- Using a Punnett square:
- Parent 1: A A
- Parent 2: A B
- Gametes: A, A and A, B
- Offspring combinations: A A, A B, A B, A b.
- Result: 1 A A : 2 A B : 1 a b (genotypic ratio = 1:2:1).
- **Conclusion**: This option produces the desired 1:2:1 ratio.

### Final Answer:
The correct cross that produces a genotypic ratio of 1:2:1 is the fourth option: **A B (heterozygous) crossed with A A (homozygous)**.