What is the ratio of one pair of contrasting characters in `F_(2)` of a dihybrid cross

Options

A. `5 : 3`
B. `3 : 1`
C. `9 : 3 : 3 : 1`
D. `1 : 2 : 2 : 4 : 1: 2: 1: 2: 1`

Answer

To determine the ratio of one pair of contrasting characters in the F2 generation of a dihybrid cross, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Dihybrid Cross**:
A dihybrid cross involves two traits, each represented by two alleles. For example, consider the traits of seed shape (round vs. wrinkled) and seed color (yellow vs. green).

2. **Identify Parental Genotypes**:
Let’s take two parents:
- Parent 1: Round shape (RR) and Yellow color (YY) - homozygous dominant.
- Parent 2: Wrinkled shape (rr) and Green color (yy) - homozygous recessive.

3. **F1 Generation**:
When these two parents are crossed, all offspring (F1 generation) will be heterozygous for both traits (RrYy). The phenotype will show the dominant traits: Round shape and Yellow color.

4. **F2 Generation**:
To obtain the F2 generation, we cross two F1 individuals (RrYy x RrYy). This will lead to a variety of combinations of alleles.

5. **Use a Punnett Square**:
Construct a 4x4 Punnett square to determine the genotypes of the F2 generation. The resulting combinations will be:
- Round Yellow (RRYY, RRYy, RrYY, RrYy)
- Round Green (RRyy, Rryy)
- Wrinkled Yellow (rrYY, rrYy)
- Wrinkled Green (rryy)

6. **Count the Phenotypes**:
The phenotypic ratio for the F2 generation will be:
- Round Yellow: 9 (from RRYY, RRYy, RrYY, RrYy)
- Round Green: 3 (from Rryy, RRyy)
- Wrinkled Yellow: 3 (from rrYY, rrYy)
- Wrinkled Green: 1 (from rryy)

7. **Determine the Ratio for One Pair of Contrasting Characters**:
When focusing on one pair of contrasting characters (e.g., seed shape), we can see:
- Round (12) vs. Wrinkled (4) = 12:4 = 3:1 ratio.
Similarly, for seed color:
- Yellow (12) vs. Green (4) = 12:4 = 3:1 ratio.

### Final Answer:
The ratio of one pair of contrasting characters in the F2 generation of a dihybrid cross is **3:1**.

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