A plant that is heterozygous for two genes was selfed. A total of 1000 seeds were collected. Find out the total number of seeds heterozygous for the first gene and homozygous for the second gene.

Options

A. 125
B. 250
C. 300
D. 500

Answer

To solve the problem of finding the total number of seeds that are heterozygous for the first gene and homozygous for the second gene from a selfed plant that is heterozygous for two genes, we can follow these steps:

### Step-by-Step Solution

1. **Understand the Genetic Cross**:
- The plant is heterozygous for two genes, which we can denote as AaBb, where 'A' and 'a' are alleles for the first gene, and 'B' and 'b' are alleles for the second gene.

2. **Determine the Expected Ratios**:
- When a heterozygous plant (AaBb) is selfed (crossed with itself), the expected phenotypic ratio in the F2 generation, according to Mendel's dihybrid cross, is 9:3:3:1. This means:
- 9 will be dominant for both traits (A_B_)
- 3 will be dominant for the first trait and recessive for the second (A_bb)
- 3 will be recessive for the first trait and dominant for the second (aaB_)
- 1 will be recessive for both traits (aabb)

3. **Identify the Required Genotype**:
- We are interested in the seeds that are heterozygous for the first gene (Aa) and homozygous for the second gene (bb). This corresponds to the genotype Aa bb.

4. **Calculate the Ratio for Aa bb**:
- From the 16 total combinations (from the 9:3:3:1 ratio), the Aa bb genotype appears in the 3rd category. The fraction of Aa bb in the total is 3 out of 16.

5. **Calculate the Total Number of Seeds**:
- Given that there are 1000 seeds in total, we can calculate the number of seeds that are heterozygous for the first gene and homozygous for the second gene using the ratio:
\[ \text{Number of Aa bb seeds} = \left(\frac{3}{16}\right) \times 1000 \]

6. **Perform the Calculation**:
\[ \text{Number of Aa bb seeds} = 3 \times 62.5 = 187.5 \]
Since we cannot have half a seed, we round to the nearest whole number, which gives us 188 seeds.

### Final Answer
The total number of seeds that are heterozygous for the first gene and homozygous for the second gene is **188**.