Which one of the following crosses shows phenotypic ratio of 3:1?

Options

A. AaBb`xx`AaBb
B. aabb`xx`AABb
C. Aabb`xx`Aabb
D. AaBb`xx`aabb

Explanation

To determine which cross shows a phenotypic ratio of 3:1, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Phenotypic Ratio**: The 3:1 phenotypic ratio typically arises from a monohybrid cross between two heterozygous parents (Aa x Aa). This means both parents have one dominant allele (A) and one recessive allele (a).

2. **Set Up the Cross**:
- Parent 1: Aa (heterozygous)
- Parent 2: Aa (heterozygous)

3. **Determine the Gametes**: Each parent can produce two types of gametes:
- Parent 1 can produce gametes A and a.
- Parent 2 can produce gametes A and a.

4. **Create a Punnett Square**:
- Draw a 2x2 Punnett square to visualize the possible combinations of gametes.
- Fill in the Punnett square:
- Top row: A, a (from Parent 1)
- Side column: A, a (from Parent 2)

| | A | a |
|-----|-----|-----|
| **A** | AA | Aa |
| **a** | Aa | aa |

5. **Count the Genotypes**: From the Punnett square, we can see the resulting genotypes:
- 1 AA (homozygous dominant)
- 2 Aa (heterozygous)
- 1 aa (homozygous recessive)

6. **Determine the Phenotypes**:
- The phenotypes corresponding to these genotypes are:
- AA and Aa will express the dominant phenotype (let's say "A").
- aa will express the recessive phenotype (let's say "a").
- Thus, the phenotypic ratio is:
- 3 A (dominant phenotype) : 1 a (recessive phenotype)

7. **Conclusion**: Therefore, the cross between two heterozygous parents (Aa x Aa) results in a phenotypic ratio of 3:1.

### Final Answer:
The cross that shows a phenotypic ratio of 3:1 is the monohybrid cross between two heterozygous individuals (Aa x Aa).

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