Two genes ‘A’ and ‘B’ are linked. In a dihybrid cross the number of phenotypes and genotypes are

Options

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

Explanation

To solve the question regarding the number of phenotypes and genotypes in a dihybrid cross involving two linked genes 'A' and 'B', we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Linkage**:
- Linkage occurs when two genes are located close to each other on the same chromosome and tend to be inherited together. This means that during gamete formation, these genes do not assort independently.

2. **Defining Dihybrid Cross**:
- A dihybrid cross involves two traits, each controlled by two alleles. For example, let’s consider the traits for hair color (A/a) and skin color (B/b).

3. **Setting Up the Cross**:
- Assume we have two parents: one homozygous dominant for both traits (AABB) and one homozygous recessive for both traits (aabb).
- The gametes produced by the first parent will be AB, and the second parent will produce ab.

4. **Gamete Formation**:
- Since the genes are linked, the gametes formed will be AB and ab. There will be no recombination, leading to only two types of gametes.

5. **F1 Generation**:
- The offspring from this cross (F1 generation) will all be heterozygous (AaBb), showing the dominant traits for both characteristics.

6. **F2 Generation**:
- When we self-fertilize the F1 generation (AaBb x AaBb), we would typically expect a 9:3:3:1 phenotypic ratio if the genes were unlinked. However, because the genes are linked, the expected ratio changes.

7. **Phenotypic Ratio**:
- In this case, due to linkage, we will observe only two phenotypes in the offspring:
- Black hair and black skin (A_B_)
- Brown hair and brown skin (aabb)
- Thus, the phenotypic ratio will be 1:1.

8. **Genotypic Ratio**:
- The genotypic ratio will also reflect this linkage, resulting in only two genotypes:
- AABB (black hair and black skin)
- aabb (brown hair and brown skin)
- Therefore, the genotypic ratio will also be 1:1.

9. **Conclusion**:
- The final answer for the number of phenotypes and genotypes in a dihybrid cross with linked genes A and B is 1:1.

### Final Answer:
The ratio of phenotypes and genotypes in a dihybrid cross with linked genes A and B is **1:1**.