A dihybrid test cross ratio for two linked genes in a hybrid is

Options

A. `1:1`
B. `1:1:1:1`
C. `7:1:1:7`
D. `1:7:7:1`

Answer

To solve the question regarding the dihybrid test cross ratio for two linked genes in a hybrid, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concepts**:
- A **dihybrid cross** involves two traits, where the parents differ in two characteristics.
- A **test cross** is performed by crossing an F1 hybrid (heterozygous for both traits) with a homozygous recessive individual for both traits.
- **Linked genes** are genes that are located close to each other on the same chromosome and tend to be inherited together.

2. **Define the Traits**:
- Let's consider two traits:
- Trait 1: Seed shape (Round - R, Wrinkled - r)
- Trait 2: Seed color (Yellow - Y, Green - y)
- The parental genotypes can be represented as:
- Parent 1: Round Yellow (RRYY)
- Parent 2: Wrinkled Green (rryy)

3. **Determine the F1 Generation**:
- The F1 generation from this cross will be heterozygous for both traits: RrYy.

4. **Gamete Formation**:
- In a normal dihybrid cross, the gametes would be RY, Ry, rY, and ry. However, since we are dealing with linked genes, the gametes will be:
- RY (Round Yellow)
- ry (Wrinkled Green)
- This is because linked genes do not assort independently.

5. **Perform the Test Cross**:
- The F1 hybrid (RrYy) is crossed with a homozygous recessive individual (rryy).
- The gametes from the homozygous recessive parent will only be: ry.

6. **Offspring Genotypes**:
- The possible offspring from this test cross will be:
- RrYy (Round Yellow)
- rryy (Wrinkled Green)

7. **Determine the Phenotypic Ratio**:
- The phenotypic ratio of the offspring will be:
- 1 Round Yellow : 1 Wrinkled Green
- Therefore, the ratio is 1:1.

### Final Answer:
The dihybrid test cross ratio for two linked genes in a hybrid is **1:1**.