Identify which one of the following is a test cross.
(i) `Tt xx Tt` (ii) `TT xx tt` (iii) `Tt xx tt`

Explanation

To identify which of the given options is a test cross, we need to understand what a test cross is.

1. **Definition of Test Cross**: A test cross is performed to determine the genotype of an individual with a dominant phenotype. This is done by crossing it with a homozygous recessive individual.

2. **Analyzing the Options**:
- **Option (i) Tt xx Tt**: This is a cross between two heterozygous individuals. This is not a test cross because both parents are not homozygous recessive.
- **Option (ii) TT xx tt**: This is a cross between a homozygous dominant (TT) and a homozygous recessive (tt) individual. This is not a test cross because we are not testing the genotype of a dominant phenotype; we already know it is homozygous dominant.
- **Option (iii) Tt xx tt**: This is a cross between a heterozygous individual (Tt) and a homozygous recessive individual (tt). This is a test cross because we are testing the genotype of the heterozygous individual by crossing it with a homozygous recessive.
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