Validity of Mendel's law of segregation is established when

Options

A. Two hybrids are crossed
B. A parent is crossed with `F_(1)` hybrid
C. Two pure breeding constrasting traits are crossed.
D. None of the above

Answer

### Step-by-Step Solution

1. **Understanding Mendel's Law of Segregation**: Mendel's law of segregation states that during the formation of gametes, the two alleles responsible for a trait separate from each other. This means that each gamete carries only one allele for each trait.

2. **Identifying the Conditions for Validity**: For Mendel's law of segregation to be validated, it is essential to observe the behavior of alleles during the reproduction process. This can be effectively studied through specific types of genetic crosses.

3. **Analyzing the Options**:
- **Two hybrids are crossed**: This option involves crossing two F1 hybrids, which allows us to observe how the alleles segregate during gamete formation.
- **A parent is crossed with F1 hybrid**: This option does not directly test the segregation of alleles since it involves one pure parent and one hybrid.
- **Two pure breeding traits are crossed**: This option would not demonstrate segregation because both parents would contribute only one type of allele.
- **None of the above**: This option is incorrect since one of the previous options is valid.

4. **Conclusion**: The validity of Mendel's law of segregation is established when two hybrids (F1 hybrids) are crossed. This allows for the observation of how alleles segregate and combine in the offspring.

### Final Answer
The validity of Mendel's law of segregation is established when **two hybrids are crossed**.

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