The coupling and repulsion theory of Bateson and Punnett later on modified linkage and crossing over by Morgan. Two completely linked genes show a dihybrid ratio of

Options

A. `1:1`
B. `3:1`
C. `1:1:1:1`
D. `9:3:3:1`

Answer

To solve the question regarding the dihybrid ratio of two completely linked genes, we can follow these steps:

### Step 1: Understand the Concepts of Coupling and Repulsion
- **Coupling** refers to the situation where two dominant alleles are linked together (e.g., A-B).
- **Repulsion** refers to the situation where one dominant allele is linked with one recessive allele (e.g., A-b).

### Step 2: Recognize the Implications of Complete Linkage
- When two genes are completely linked, they do not assort independently. This means that they will be inherited together as a single unit.

### Step 3: Determine the Dihybrid Cross
- In a typical dihybrid cross (AaBb x AaBb), if the genes are completely linked, the gametes produced will be limited to the parental combinations (AB and ab) and will not produce recombinant types (Ab and aB).

### Step 4: Analyze the Gametes
- The gametes produced from a cross of two completely linked genes (e.g., AABB x aabb) will be:
- Parental gametes: AB and ab
- The resulting offspring will be either dominant for both traits (AB) or recessive for both traits (ab).

### Step 5: Determine the Phenotypic Ratio
- Since there are only two types of gametes (AB and ab), the resulting phenotypic ratio will be:
- 1 (AB) : 1 (ab)

### Conclusion
- Therefore, the dihybrid ratio of two completely linked genes is **1:1**.

### Final Answer
- The correct answer is **1:1**.

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