Many traits which are not so distinct in their occurrence and are spread over gradient is due to

Options

A. dominance
B. pleiotropy
C. qualitative inheritance
D. polygenic inheritance

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about traits that are not distinctly separated and instead show a range or gradient in their occurrence. This suggests that the traits in question are influenced by multiple factors rather than a single gene.

2. **Identifying Key Terms**:
- **Traits**: Characteristics or features of an organism.
- **Gradient**: A gradual change or transition in traits rather than distinct categories.
- **Inheritance**: The process by which traits are passed from parents to offspring.

3. **Analyzing the Options**:
- **Dominance**: Refers to the relationship between alleles where one allele masks the effect of another. This does not explain traits that show a gradient.
- **Pleiotropy**: A single gene influencing multiple traits. While it involves multiple effects, it does not account for traits that are spread over a gradient.
- **Qualitative Inheritance**: Involves traits that are distinctly categorized (e.g., tall vs. short). This does not apply to traits that show a continuous range.
- **Polygenic Inheritance**: Involves multiple genes contributing to a single trait, resulting in a continuous range of phenotypes (e.g., human height).

4. **Conclusion**: The correct answer is **polygenic inheritance**. This type of inheritance explains traits that are not distinctly categorized and are instead spread over a gradient due to the influence of multiple genes.

### Final Answer:
Many traits which are not so distinct in their occurrence and are spread over a gradient are due to **polygenic inheritance**.

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