Which of the following statements does not provide an explanation for hybrid vigour ?

Options

A. Under certain circumstances, heterozygotes are superior to either possible homozygotes.
B. Disease-causing, homozygous recessive phenotypes from either parent are masked in the hybrids.
C. Offspring from a hybrid cross usually possess the best of two desirable parents.
D. Inherently, hybrids have no deleterious mutations.

Explanation

To answer the question "Which of the following statements does not provide an explanation for hybrid vigour?" we need to analyze each of the provided options regarding hybrid vigour.

### Step-by-Step Solution:

1. **Understanding Hybrid Vigour**:
- Hybrid vigour, or heterosis, refers to the phenomenon where hybrid offspring exhibit superior qualities compared to their parents. This can include increased growth rates, higher yields, and better resistance to diseases.

2. **Analyzing the Options**:
- **Option 1**: "Under certain circumstances, heterozygotes are superior to either possible homozygotes."
- This statement is true as it describes a key aspect of hybrid vigour. Heterozygotes, which are hybrids, often outperform homozygotes due to genetic diversity.

- **Option 2**: "Disease-causing homozygous recessive phenotypes from either parent are masked in the hybrid."
- This statement is also true. In hybrids, harmful recessive traits from the parents can be masked, leading to healthier offspring.

- **Option 3**: "Offspring from a hybrid cross usually possess the best of two desirable parents."
- This statement is correct as hybrids are often bred to combine the best traits from both parent species.

- **Option 4**: "Inherently, hybrids have no deleterious mutations."
- This statement is incorrect. While hybrids may mask harmful recessive traits, it does not mean they are free from deleterious mutations. They can still carry such mutations, but they may not express them due to the dominance of other traits.

3. **Conclusion**:
- The statement that does not provide a correct explanation for hybrid vigour is **Option 4**: "Inherently hybrids have no deleterious mutations."

### Final Answer:
- The correct answer is **Option 4**.