Which is not true for test cross ?

Options

A. `F_2` dihybrid test cross ratio `1:1:1:1`.
B. It is performed know tha alleles of a gene in an individual.
C. A test cross is a type of back cross
D. Dominant offspring's in test cross is always homozygous.

Answer

To determine which statement is not true for a test cross, we need to analyze the provided options based on the principles of genetics, specifically the concept of a test cross.

### Step-by-Step Solution:

1. **Understanding Test Cross**: A test cross is a genetic cross between an individual exhibiting a dominant phenotype (whose genotype is unknown) and a homozygous recessive individual. This helps to determine whether the dominant individual is homozygous or heterozygous.

2. **Analyzing the Statements**:
- **Statement 1**: "F2 dihybrid test cross ratio is 1:1:1:1."
- This statement is **true**. In a dihybrid test cross, the expected phenotypic ratio is indeed 1:1:1:1.

- **Statement 2**: "It is performed to know the alleles of a gene in an individual."
- This statement is also **true**. A test cross is used to determine the alleles present in the dominant phenotype individual.

- **Statement 3**: "A test cross is a type of back cross."
- This statement is **true**. A test cross can be considered a specific type of back cross, as it involves crossing an F1 hybrid with one of its parents (the recessive one).

- **Statement 4**: "Dominant offspring in test cross is always homozygous."
- This statement is **not true**. The dominant offspring can be either homozygous or heterozygous. For example, if the dominant phenotype is tall (T), the offspring could be either TT (homozygous) or Tt (heterozygous).

3. **Conclusion**: The statement that is not true for a test cross is: "Dominant offspring in test cross is always homozygous."

### Final Answer:
The statement that is not true for a test cross is: **"Dominant offspring in test cross is always homozygous."**