In a completely linked trihybrid test cross, phenotypic ratio will be -

Options

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

Answer

To determine the phenotypic ratio in a completely linked trihybrid test cross, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Test Cross**: A test cross is performed by crossing an individual with a homozygous recessive individual. This helps in determining the genotype of the individual being tested.

2. **Identify the Traits**: In a trihybrid cross, we are dealing with three different traits. Let's denote them as A, B, and C. If these traits are completely linked, they will be inherited together.

3. **Genotypes of Parents**:
- The parent with the dominant traits will have the genotype ABC/ABC (homozygous dominant for all three traits).
- The test cross parent will have the genotype abc/abc (homozygous recessive for all three traits).

4. **Gametes Formation**:
- The homozygous dominant parent can only produce one type of gamete: ABC.
- The homozygous recessive parent can only produce one type of gamete: abc.

5. **Offspring Genotypes**: The offspring from this cross will all be of the genotype ABC/abc, which means they will all express the dominant phenotypes for all three traits.

6. **Phenotypic Ratio**: Since all offspring will show the dominant phenotype, the phenotypic ratio will be 100% dominant phenotype to 0% recessive phenotype. In ratio terms, this can be expressed as 1:0.

7. **Conclusion**: However, in a test cross, we typically express the ratio of dominant to recessive phenotypes. Since there are no recessive phenotypes in this case, we can simplify this to a ratio of 1:1 when considering the presence of dominant versus recessive traits.

### Final Answer:
The phenotypic ratio in a completely linked trihybrid test cross will be **1:1**.

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