Which of the folloiwng statement/s about test cross is/are correct?
I. Every test cross is a back 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 statements about test crosses, let's analyze each statement step by step.

### Step 1: Understanding Test Cross
A test cross is defined as the cross between an F1 hybrid (which is heterozygous) and a homozygous recessive parent. This helps determine the genotype of the F1 hybrid.

### Step 2: Analyzing Statement I
**Statement I: Every test cross is a back cross, but not every back cross is a test cross.**
- A back cross is a cross between an F1 hybrid and one of its parents (either dominant or recessive).
- Since a test cross specifically involves crossing the F1 hybrid with a recessive parent, it is indeed a type of back cross.
- Therefore, this statement is **correct**.

### Step 3: Analyzing Statement II
**Statement II: Every back cross is a test cross but not every test cross is a back cross.**
- This statement claims that all back crosses are test crosses, which is incorrect. A back cross can involve crossing with a dominant parent, which does not qualify as a test cross.
- Therefore, this statement is **incorrect**.

### Step 4: Analyzing Statement III
**Statement III: The ratio of a test cross is always 1:1.**
- When a test cross is performed, the expected phenotypic ratio is indeed 1:1 if the F1 hybrid is heterozygous. This occurs because the offspring will be either the dominant phenotype or the recessive phenotype.
- Therefore, this statement is **correct**.

### Step 5: Analyzing Statement IV
**Statement IV: A test cross can be considered as a type of reciprocal cross.**
- A reciprocal cross involves switching the roles of the parents to see if the offspring's traits are influenced by the parent of origin. This is typically used to study sex-linked inheritance.
- A test cross, however, is specifically for determining the genotype of an organism and is not a reciprocal cross.
- Therefore, this statement is **incorrect**.

### Conclusion
Based on the analysis:
- **Correct Statements**: I and III
- **Incorrect Statements**: II and IV

Thus, the correct answer is that statements I and III are correct.