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 why certain traits are not distinctly occurring but are spread over a gradient. This indicates that we are dealing with traits that show a range of phenotypes rather than clear-cut categories.

2. **Introduction to Mendelian Genetics**: Gregor Mendel's work laid the foundation for understanding inheritance. He studied traits in garden peas and identified laws of inheritance based on contrasting traits, such as tall and dwarf plants. These traits were distinct and easy to categorize.

3. **Contrasting Traits vs. Continuous Traits**: Mendel's experiments focused on contrasting traits, which are easily observable and fit into two distinct categories. However, many traits in nature do not fit this model. Instead, they show a range of phenotypes, such as human height, which varies continuously rather than being limited to just "tall" or "short".

4. **Definition of Quantitative Inheritance**: Traits that exhibit a gradient are referred to as continuous traits and are governed by quantitative inheritance. This means that multiple genes (polygenic inheritance) contribute to the expression of these traits, leading to a variety of phenotypes.

5. **Polygenic Inheritance**: Continuous traits are influenced by multiple genes, which work together to produce a range of phenotypes. For example, human height is determined by several genes, each contributing a small effect, resulting in a spectrum of heights.

6. **Conclusion**: The reason many traits are spread over a gradient is due to polygenic inheritance, where multiple genes influence a single trait, leading to continuous variation in phenotypes.

### Final Answer:
Many traits that are not distinctly occurring and are spread over a gradient are due to **polygenic inheritance**, where multiple genes control the expression of these traits, resulting in continuous variation.

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