The biggest constraint of plant breeding is

Options

A. availability of desirable gene in the crop and its wild relatives
B. infrastructure
C. trained manpower
D. transfer of genes from unrelated sources.

Explanation

### Step-by-Step Text Solution:

1. **Understanding Plant Breeding**:
- Plant breeding is the science of changing the traits of plants in order to produce desired characteristics. This involves the introduction of desirable genes from wild varieties into cultivated crops.

2. **Identifying Constraints in Plant Breeding**:
- The question asks for the biggest constraint in plant breeding. The options provided include:
- Availability of the desirable gene in the crop and its wild varieties
- Infrastructure
- Trained manpower
- Transfer of genes from unrelated sources

3. **Analyzing Each Option**:
- **Availability of Desirable Gene**: This is crucial because if the gene is not available in the wild relatives or the crop itself, breeding cannot occur.
- **Infrastructure**: While infrastructure is important for many agricultural practices, it is not the primary constraint in the process of plant breeding itself.
- **Trained Manpower**: Skilled personnel are important, but the basics of plant breeding can be learned, and there are many trained professionals available.
- **Transfer of Genes from Unrelated Sources**: While this can be a challenge, the primary issue remains the availability of the gene in related species.

4. **Conclusion**:
- After analyzing the options, the biggest constraint in plant breeding is the **availability of the desirable gene in the crop and its wild relatives**. This is because without the gene, the breeding process cannot even begin.

### Final Answer:
The biggest constraint of plant breeding is **the availability of the desirable gene in the crop and its wild varieties**.

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