Polygenic inheritance describes how multiple genes collectively influence a single observable characteristic. This mechanism is responsible for many complex human traits. This article explores polygenic inheritance through examples, discussing how these traits are influenced and passed down.
What Makes a Trait Polygenic?
A polygenic trait is determined by the combined action of multiple genes. Each gene contributes a small, additive effect. This contrasts with monogenic, or Mendelian, traits, which are controlled by a single gene and exhibit distinct categories.
The additive nature of polygenic inheritance leads to continuous variation within a population. For instance, human height exists along a continuum. This continuous distribution is a hallmark of polygenic traits, reflecting the cumulative impact of genetic contributions.
Common Human Polygenic Traits
Height serves as an example of a polygenic trait. Over 12,000 genetic variants influence adult height. The combined effects of these genes and environmental factors result in the broad range of heights.
Skin color is another illustration of polygenic inheritance. Multiple genes regulate the production and distribution of melanin. Variations in these genes lead to the wide spectrum of skin tones.
Cognitive abilities are also polygenic. Intelligence is influenced by thousands of genetic variants. While environmental factors play a role, the genetic component is polygenic, with common genetic variants accounting for substantial variation.
Environmental Factors and Polygenic Traits
Polygenic traits are multifactorial, influenced by environmental factors. The observable characteristic, or phenotype, arises from the interplay between an individual’s genetic predisposition and external conditions. This interaction means genetics alone do not solely determine the final expression of such traits.
For example, while genes contribute to potential height, nutritional intake during childhood impacts whether that genetic potential is realized. Similarly, sun exposure directly affects melanin production, altering skin pigmentation. Cognitive development is also shaped by factors like access to education and and early childhood experiences.
Inheriting Polygenic Traits
The inheritance of polygenic traits does not follow the dominant or recessive patterns of Mendelian traits. Instead, offspring inherit a combination of genes contributing to the trait from both parents. This complex genetic contribution often results in offspring exhibiting a phenotype within their parents’ range, though it can extend beyond due to the combination of small genetic effects.
The distribution of polygenic traits in a population follows a bell-shaped curve, or normal distribution. Most individuals exhibit an average expression of the trait, while fewer are found at the extreme ends of the spectrum. This pattern reflects the cumulative and additive nature of the genes.