A polygene is a group of genes that collectively influence a single observable characteristic or trait. This inheritance involves multiple genes. Each individual gene contributes a small amount to the overall trait. The combined effect of these genes results in the wide range of variations seen in many human characteristics.
The Mechanism of Polygenic Inheritance
Unlike traits determined by a single gene, which often present in distinct “either/or” categories, polygenic inheritance produces a continuous spectrum of phenotypes. This continuous variation arises from the additive effects of multiple genes, where each gene contributes a small, cumulative impact to the overall trait.
This additive mechanism means that no single gene completely dominates or masks the others involved in the trait’s expression. The more “contributing” alleles an individual possesses, the stronger the expression of the trait. When observed across a large population, the distribution of these traits forms a bell-shaped curve, or normal distribution. This curve illustrates that most individuals fall within an average range for the trait, with fewer individuals at the extremes.
Examples in Human Traits
Human height is an example of polygenic inheritance, influenced by over 400 genes that contribute to bone length and growth regulation. These genes collectively determine a person’s stature, leading to a wide continuum of heights across the population rather than just “tall” or “short” categories.
Skin color also demonstrates polygenic inheritance, involving at least three primary genes. The amount of melanin, a dark pigment in the skin, is determined by these multiple genes. Different combinations of alleles from these genes lead to the spectrum of human skin tones, from very light to very dark. Eye color, similarly, is influenced by multiple genes, including OCA2 and HERC2, which determine melanin production in the iris, creating the diverse array of blue, green, and brown hues.
Influence of Environmental Factors
Polygenic traits are not solely determined by an individual’s genetic makeup; environmental factors also play a role in their expression. This interaction means that both inherited predispositions and external influences shape the final observable characteristic. For instance, a person’s genetic potential for height can be influenced by their nutrition during childhood. Adequate diet and overall health support the full expression of genetic height potential, while malnutrition or illness can limit growth.
Skin pigmentation, while genetically influenced, also responds to environmental factors like sun exposure. Increased exposure to ultraviolet (UV) light triggers the skin to produce more melanin as a protective response, leading to a darkening of the skin. This demonstrates how the environment can modify the expression of a genetically determined trait.
Polygenic Risk and Disease
Polygenic inheritance extends to many common diseases, where multiple genes collectively contribute to an individual’s susceptibility. Conditions such as type 2 diabetes, coronary artery disease, and certain cancers are examples of diseases with a polygenic component. These genes create a genetic predisposition or increased risk for developing the disease, rather than guaranteeing its onset.
A person with a higher polygenic risk score for a particular disease has inherited a combination of genetic variants that increase their likelihood of developing that condition. However, environmental and lifestyle factors, like diet, exercise, and smoking, can interact with this genetic risk. These external factors can either trigger the disease in predisposed individuals or, conversely, help prevent it, even in those with a higher genetic risk.