Total number of genotypes and phenotypes in a dihybrid test cross is -

Options

A. 9
B. 8
C. 16
D. 13

Answer

To solve the question regarding the total number of genotypes and phenotypes in a dihybrid test cross, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Dihybrid Cross**:
A dihybrid cross involves two traits, each represented by two alleles. For example, consider two traits: seed shape (round 'R' vs. wrinkled 'r') and seed color (yellow 'Y' vs. green 'y'). The F1 generation from a cross of two homozygous parents (RRYY x rryy) will be heterozygous for both traits (RrYy).

2. **Perform a Test Cross**:
A test cross is performed by crossing the F1 individual (RrYy) with a homozygous recessive individual (rryy). This helps to determine the genotype of the F1 individual based on the phenotypes of the offspring.

3. **Determine the Gametes**:
The F1 individual (RrYy) can produce four types of gametes due to independent assortment:
- RY
- Ry
- rY
- ry

4. **Cross with Homozygous Recessive**:
When we cross RrYy (producing the four gametes) with rryy, we can create a Punnett square to visualize the combinations:
- RY x ry → RrYy (Round Yellow)
- Ry x ry → Rryy (Round Green)
- rY x ry → rrYy (Wrinkled Yellow)
- ry x ry → rryy (Wrinkled Green)

5. **Count the Offspring Phenotypes**:
From the combinations above, we can see that there are 4 distinct phenotypes:
- Round Yellow (RrYy)
- Round Green (Rryy)
- Wrinkled Yellow (rrYy)
- Wrinkled Green (rryy)

6. **Count the Offspring Genotypes**:
Each phenotype corresponds to a unique genotype, so we also have 4 distinct genotypes:
- RrYy
- Rryy
- rrYy
- rryy

7. **Conclusion**:
Therefore, in a dihybrid test cross, the total number of phenotypes is 4 and the total number of genotypes is also 4. Thus, the total number of distinct combinations (genotypes + phenotypes) is 8.

### Final Answer:
The total number of genotypes and phenotypes in a dihybrid test cross is **8**.