Which of the following is not observed in a monohybrid cross-

Options

A. Recessive parental trait is expressed without any blending in the `F_(2)` generation
B. Recessive parental trait is expressed without any blending in the `F_(1)` generation
C. Dominance also explains the proportion of `3:1` obtained at the `F_(2)`
D. Genotype ratio is `1:2:1`

Answer

To solve the question "Which of the following is not observed in a monohybrid cross?", we need to analyze each statement provided and determine which one is incorrect.

### Step-by-Step Solution:

1. **Understand Monohybrid Cross**: A monohybrid cross involves a single trait with two alleles (dominant and recessive). It typically follows Mendel's laws of inheritance.

2. **Evaluate Each Statement**:
- **Statement A**: "Recessive parental traits is expressed without any blending in the F2 generation."
- This statement is **correct**. In the F2 generation, recessive traits can be expressed without blending, as seen in the classic 3:1 phenotypic ratio.

- **Statement B**: "Recessive parental trait is expressed without any blending in the F1 generation."
- This statement is **incorrect**. In the F1 generation, the dominant trait is expressed, and the recessive trait does not appear. The recessive trait is masked by the dominant trait, hence there is blending in the F1 generation.

- **Statement C**: "Dominance also explains the proportion of 3:1 obtained in the F2."
- This statement is **correct**. The 3:1 ratio in the F2 generation is due to the dominance of one allele over the other.

- **Statement D**: "Genotype ratio is 1:2:1."
- This statement is **correct**. The genotype ratio for a monohybrid cross is indeed 1:2:1, representing the homozygous dominant, heterozygous, and homozygous recessive genotypes.

3. **Identify the Incorrect Statement**:
- After evaluating all statements, we find that **Statement B** is the one that is not observed in a monohybrid cross, as the recessive trait does not express in the F1 generation.

### Final Answer:
The statement that is not observed in a monohybrid cross is **Statement B**.