Which of the folloiwng statement/s about test cross is/are correct?
I. Every test cross but not every back cross is a test cross.
II. Every back cross is a test cross but not every test cross is a back cross.
III. The ratio of a test cross is always 1:1.
IV. A test cross can be considered as a type of reciprocal cross.

Options

A. I and III
B. I, III and IV
C. II and IV
D. I

Answer

To solve the question regarding the correctness of the statements about test crosses, let's analyze each statement step by step.

### Step-by-Step Solution:

1. **Understanding Test Cross**:
- A test cross is performed by crossing an individual with a homozygous recessive individual. This helps determine the genotype of the individual in question (whether it is homozygous dominant or heterozygous).

2. **Analyzing Statement I**:
- **Statement I**: "Every test cross but not every back cross is a test cross."
- This statement is **correct**. A test cross is a specific type of back cross (crossing offspring with one of its parents), but not all back crosses are test crosses since a back cross can involve a homozygous dominant parent.

3. **Analyzing Statement II**:
- **Statement II**: "Every back cross is a test cross but not every test cross is a back cross."
- This statement is **incorrect**. While every test cross is a back cross, not every back cross qualifies as a test cross because a back cross can involve crossing with a homozygous dominant parent.

4. **Analyzing Statement III**:
- **Statement III**: "The ratio of a test cross is always 1:1."
- This statement is **incorrect**. The ratio in a test cross can vary depending on the genotype of the parent being tested. For example, if the test cross involves a homozygous dominant and a homozygous recessive, the offspring will all show the dominant phenotype. If it involves a heterozygous individual, the ratio can be 1:1.

5. **Analyzing Statement IV**:
- **Statement IV**: "A test cross can be considered as a type of reciprocal cross."
- This statement is **incorrect**. A reciprocal cross involves switching the sexes of the parents (male and female), while a test cross specifically involves crossing with a homozygous recessive individual. They are not the same.

### Conclusion:
- The only correct statement is **Statement I**. Therefore, the answer to the question is that only **Statement I** is correct.

### Final Answer:
- **Correct Statement(s)**: I only.

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