What phenotypic ratio is obtained by selfing of a trihybrid, in which two gene pairs are completely dominant and one gene pair is incompletely dominant ?

Options

A. `3:6:3:1:2:1`
B. `27:9:9:9:3:1:3:3`
C. `18 : 6 : 6 : 2 : 9 : 3 : 3 : 1 : 9 : 3 : 3 : 1`
D. `18 : 6 : 6 : 2 : 9 : 3 : 3 : 1 : 1 : 2 : 1`

Answer

To solve the question regarding the phenotypic ratio obtained by selfing a trihybrid where two gene pairs are completely dominant and one gene pair is incompletely dominant, we can follow these steps:

### Step 1: Understand the Trihybrid Cross
A trihybrid cross involves three different traits. In this case, we have:
- Trait 1: Plant height (complete dominance)
- Trait 2: Seed shape (complete dominance)
- Trait 3: Seed color (incomplete dominance)

### Step 2: Define the Genotypes
Let’s define the genotypes for the traits:
- For plant height: T (tall) is dominant over t (dwarf).
- For seed shape: R (round) is dominant over r (wrinkled).
- For seed color: Y (yellow) is dominant over y (green), but we will also consider the intermediate phenotype due to incomplete dominance.

### Step 3: Parental Generation
Assuming we start with a homozygous dominant parent (TT RR YY) and a homozygous recessive parent (tt rr yy), the F1 generation will be:
- Genotype: Tt Rr Yy (all offspring will be tall, round, and yellow)

### Step 4: Selfing the F1 Generation
When we self the F1 generation (Tt Rr Yy), we need to consider how the gametes will combine. The gametes produced by Tt Rr Yy can be:
- TRY, TRy, TrY, Try, tRY, tRy, trY, try

### Step 5: Determine the Phenotypic Ratio
To find the phenotypic ratio, we need to consider the contributions from each trait:
1. For the two traits with complete dominance (plant height and seed shape), the ratio will be 3:1 (3 dominant: 1 recessive).
2. For the trait with incomplete dominance (seed color), the ratio will be 1:2:1 (1 homozygous dominant: 2 heterozygous: 1 homozygous recessive).

### Step 6: Combine the Ratios
To find the overall phenotypic ratio, we multiply the ratios from the complete dominance traits with the ratio from the incomplete dominance trait:
- For plant height and seed shape: 3 (dominant) : 1 (recessive) = 3:1
- For seed color: 1 (YY) : 2 (Yy) : 1 (yy) = 1:2:1

The combined phenotypic ratio is:
- (3:1) x (1:2:1) = 3 (tall, round, yellow) : 6 (tall, round, intermediate) : 3 (tall, round, green) : 3 (dwarf, round, yellow) : 6 (dwarf, round, intermediate) : 3 (dwarf, round, green)

### Step 7: Final Calculation
After calculating the combinations, we can summarize the phenotypic ratio as:
- 27 (dominant traits) : 9 (intermediate) : 3 (recessive traits)

### Conclusion
The final phenotypic ratio obtained by selfing a trihybrid with two completely dominant and one incompletely dominant gene pair is **27:9:3**.

---