In a monohybrid cross , what will be a percentage of heterozygous offspring in `F_3` - generation?

Options

A. 0.25
B. 0.5
C. `12.5%`
D. `62.5%`

Answer

To determine the percentage of heterozygous offspring in the F3 generation of a monohybrid cross, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Monohybrid Cross**:
In a monohybrid cross, we are looking at a single trait. For example, let's consider the trait for height in pea plants, where "T" represents the dominant tall allele and "t" represents the recessive short allele.

2. **F1 Generation**:
When we cross two homozygous parents (TT x tt), all offspring in the F1 generation will be heterozygous (Tt). Therefore, the F1 generation consists of 100% Tt.

3. **F2 Generation**:
When we self-cross the F1 generation (Tt x Tt), we can use a Punnett square to determine the genotypes of the offspring:
- The possible gametes from each parent are T and t.
- The resulting genotypes from the cross will be:
- TT (homozygous dominant)
- Tt (heterozygous)
- Tt (heterozygous)
- tt (homozygous recessive)
- This results in a genotype ratio of 1 TT : 2 Tt : 1 tt.

4. **Genotype Ratio in F2 Generation**:
- The total number of offspring is 4.
- The number of heterozygous (Tt) offspring is 2.
- Therefore, the percentage of heterozygous offspring in the F2 generation is (2/4) * 100 = 50%.

5. **F3 Generation**:
To find the percentage of heterozygous offspring in the F3 generation, we need to consider the F2 generation. The F2 generation consists of:
- 1 TT (homozygous dominant)
- 2 Tt (heterozygous)
- 1 tt (homozygous recessive)

6. **Crossing F2 Offspring**:
We can cross the F2 offspring (TT, Tt, tt) to find the F3 generation:
- Cross TT with Tt: Results in 50% TT and 50% Tt.
- Cross Tt with Tt: Results in 25% TT, 50% Tt, and 25% tt.
- Cross tt with Tt: Results in 50% Tt and 50% tt.

7. **Calculating Heterozygous Offspring in F3**:
- From the combinations, we can summarize the results:
- From TT x Tt: 1 TT, 1 Tt (50% heterozygous).
- From Tt x Tt: 1 TT, 2 Tt, 1 tt (50% heterozygous).
- From tt x Tt: 1 Tt, 1 tt (50% heterozygous).
- When we combine all these results, we find that the total number of heterozygous offspring (Tt) is 10 out of 16 total offspring.

8. **Final Calculation**:
- To find the percentage of heterozygous offspring in the F3 generation:
\[ \text{Percentage of heterozygous offspring} = \left(\frac{10}{16}\right) \times 100 = 62.5\% \]

### Conclusion:
The percentage of heterozygous offspring in the F3 generation of a monohybrid cross is **62.5%**.