In a dihybrid cross, if one trait is showing complete dominance and other trait follows lethality, what will be phenotypic ration in F2 generation?

Options

A. `3:6:3:1:2:1`
B. `1:2:1:2:4:2:1:2:1`
C. `9:3:3:1`
D. `6:3:2:1`

Answer

To determine the phenotypic ratio in the F2 generation of a dihybrid cross where one trait shows complete dominance and the other trait follows lethality, we can follow these steps:

### Step 1: Understand the Traits
In a dihybrid cross, we are dealing with two traits. Let's denote them as:
- Trait A (showing complete dominance)
- Trait B (following lethality)

### Step 2: Set Up the Parental Generation
Assume we have two pure breeding parents:
- Parent 1: AA BB (homozygous dominant for both traits)
- Parent 2: aa bb (homozygous recessive for both traits)

### Step 3: Determine the F1 Generation
When we cross these two parents, all offspring (F1 generation) will be heterozygous for both traits:
- F1: Aa Bb

### Step 4: Perform the Dihybrid Cross for F2 Generation
Now, we will perform a dihybrid cross using the F1 generation:
- F1 x F1: Aa Bb x Aa Bb

### Step 5: Create a Punnett Square
The gametes produced by the F1 generation (Aa Bb) will be:
- AB, Ab, aB, ab

Setting up a 4x4 Punnett square will give us the following combinations in the F2 generation:
- AABB, AABb, AaBB, AaBb, AAbb, Aabb, aaBB, aaBb, Aabb, aabb

### Step 6: Apply the Lethality Condition
Since one trait follows lethality, we need to consider that certain combinations will not be viable. For example, if we assume that the homozygous recessive condition (aa) for Trait B is lethal, then:
- AABB, AABb, AaBB, AaBb, AAbb, and Aabb are viable.
- aaBB and aaBb are not viable (lethal).
- aabb is also viable.

### Step 7: Count the Viable Phenotypes
Now, let's count the viable phenotypes:
- Dominant for Trait A and dominant for Trait B (A_B): 6 (AABB, AABb, AaBB, AaBb)
- Dominant for Trait A and recessive for Trait B (A_bb): 2 (AAbb, Aabb)
- Recessive for Trait A and recessive for Trait B (aa bb): 1 (aabb)

### Step 8: Calculate the Phenotypic Ratio
The viable phenotypes can be summarized as follows:
- A_B: 6
- A_bb: 2
- aa bb: 1

Thus, the phenotypic ratio in the F2 generation is:
- 6:2:1 or simplified to 6:2:1.

### Conclusion
The final phenotypic ratio in the F2 generation, considering one trait shows complete dominance and the other trait follows lethality, is **6:2:1**.

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