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, let's analyze each option step by step.

### Step 1: Understand what a test cross is
A test cross is a genetic cross performed to determine the zygosity of an individual expressing a dominant phenotype. This is done by crossing the individual with a homozygous recessive individual.

### Step 2: Analyze the options
1. **F2 Dihybrid test cross ratio 1:1:1:1**:
- This statement is true. In a dihybrid test cross, the expected phenotypic ratio of offspring is indeed 1:1:1:1 when crossing a dihybrid with a homozygous recessive individual.

2. **It is performed to know the alleles of a gene in an individual**:
- This statement is also true. The purpose of a test cross is to determine whether the individual with the dominant phenotype is homozygous or heterozygous for that trait.

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 where the individual is crossed with a homozygous recessive parent.

4. **Dominant offspring test cross is always homozygous**:
- This statement is false. The dominant offspring from a test cross can either be homozygous (e.g., TT) or heterozygous (e.g., Tt). Therefore, it is incorrect to say that the dominant offspring from a test cross is always homozygous.

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