A dihybrid test cross ratio for two completely linked genes will be

Options

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

Answer

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

### Step-by-Step Solution:

1. **Understand Dihybrid Cross**: A dihybrid cross involves two traits, where we observe the inheritance of two different genes simultaneously. For example, consider two traits in pea plants: seed shape (round vs. wrinkled) and seed color (yellow vs. green).

2. **Identify Parental Genotypes**: In a typical dihybrid cross, we start with two pure parental lines. For instance, one parent could be homozygous dominant for both traits (RRYY - round yellow), and the other could be homozygous recessive for both traits (rryy - wrinkled green).

3. **F1 Generation**: When these two parents are crossed, the F1 generation will all be heterozygous (RrYy) for both traits and will exhibit the dominant phenotype (round yellow).

4. **F2 Generation in Independent Assortment**: If the genes assort independently (not linked), the F2 generation would show a phenotypic ratio of 9:3:3:1 (round yellow: round green: wrinkled yellow: wrinkled green).

5. **Understanding Complete Linkage**: If the two genes are completely linked, they will not assort independently. This means that the alleles for these genes are inherited together because they are located very close to each other on the same chromosome.

6. **Test Cross with Completely Linked Genes**: In a test cross, we cross the F1 hybrid (RrYy) with a homozygous recessive parent (rryy). For completely linked genes, the F1 hybrid can only produce two types of gametes: RY and ry.

7. **Gamete Combinations**: The gametes from the F1 hybrid (RrYy) will combine with the recessive gametes (ry) from the homozygous recessive parent. The possible offspring will be:
- Round Yellow (RY/ry) - phenotype: Round Yellow
- Wrinkled Green (ry/ry) - phenotype: Wrinkled Green

8. **Phenotypic Ratio**: Since only the parental types appear (Round Yellow and Wrinkled Green) and no recombinant types (Round Green and Wrinkled Yellow) are produced, the phenotypic ratio in this test cross will be 1:1.

### Final Answer:
The dihybrid test cross ratio for two completely linked genes will be **1:1**.

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