Describe the difference between continuous phenotypic variation and discontinuous variation. Explain how polygenic inheritance could be the basis of a trait showing continuous phenotypic variation. Explain how polygenic inheritance can be the basis of a threshold trait.

Key Concepts

Continuous vs. Discontinuous Variation

Continuous variation refers to traits that show a range of phenotypes, such as height or skin color, where individuals can have any value within a spectrum. In contrast, discontinuous variation involves distinct categories or classes, such as blood type or flower color, where traits are clearly defined and do not blend. Understanding these differences is crucial for analyzing how traits are inherited and expressed.

Polygenic Inheritance

Polygenic inheritance occurs when multiple genes contribute to a single trait, resulting in a continuous range of phenotypes. Each gene may have a small additive effect, leading to variations that can be measured on a continuum. This concept is essential for explaining traits like height, where many genes interact to produce a wide array of possible outcomes.

Threshold Traits

Threshold traits are characteristics that are influenced by multiple genes but only manifest when a certain genetic or environmental threshold is crossed. For example, a person may have a genetic predisposition for a condition, but it only appears if environmental factors trigger it. This concept illustrates how polygenic inheritance can lead to traits that are not continuously variable but instead appear in a binary or categorical manner.