Which of the following crosses would result in 1:2:1 `F_(2)` ratio?

Options

A. Yy x yy
B. Yy x Yy
C. YY x yy
D. Yy x yy

Answer

To determine which of the given crosses results in a 1:2:1 phenotypic ratio in the F2 generation, we need to understand the principles of inheritance, particularly focusing on monohybrid crosses.

### Step-by-Step Solution:

1. **Understanding Monohybrid Crosses**:
A monohybrid cross involves two individuals that differ in a single trait. For example, let’s consider a trait controlled by a single gene with two alleles: a dominant allele (Y) for yellow color and a recessive allele (y) for green color.

2. **Setting Up the Parental Generation (P Generation)**:
We start with two homozygous parents:
- Parent 1: Homozygous dominant (YY) - Yellow
- Parent 2: Homozygous recessive (yy) - Green

3. **F1 Generation**:
When we cross these two parents (YY x yy), all offspring in the F1 generation will be heterozygous (Yy) and exhibit the dominant phenotype (yellow). Thus, the F1 generation is:
- F1: All Yy (Yellow)

4. **Selfing the F1 Generation**:
To obtain the F2 generation, we perform a self-cross of the F1 individuals:
- F1 Cross: Yy x Yy

5. **Gamete Formation**:
Each F1 individual can produce two types of gametes: Y and y.

6. **Punnett Square Setup**:
We can set up a Punnett square to visualize the cross:
```
Y | y
-----------------
Y | YY | Yy
-----------------
y | Yy | yy
```

7. **Results from the Punnett Square**:
From the Punnett square, we can see the following genotypes:
- 1 YY (homozygous dominant)
- 2 Yy (heterozygous)
- 1 yy (homozygous recessive)

8. **Genotypic Ratio**:
The genotypic ratio from this cross is:
- 1 YY : 2 Yy : 1 yy

9. **Phenotypic Ratio**:
In terms of phenotype:
- YY and Yy both express the dominant yellow color, while yy expresses the recessive green color. Therefore, the phenotypic ratio is:
- 3 Yellow : 1 Green

10. **Identifying the Correct Cross**:
To achieve a 1:2:1 ratio in the F2 generation, we need to consider a different cross. The 1:2:1 ratio typically arises from a cross between two heterozygous individuals (Yy x Yy).

11. **Conclusion**:
The correct cross that results in a 1:2:1 ratio in the F2 generation would be:
- **Yy x Yy** (heterozygous parents).

### Final Answer:
The cross that results in a 1:2:1 F2 ratio is **Yy x Yy**.