Polygenic inheritance

Polygenic inheritance is a pattern responsible for many features that seem simple on the surface. Many traits such as height, shape, weight, color, and metabolic rate are governed by the cumulative effects of many genes. Polygenic traits are not expressed as absolute or discrete characters, as was the case with Mendel's pea plant traits. Instead, polygenic traits are recognizable by their expression as a gradation of small differences (a continuous variation).



The results form a bell shaped curve, with a mean value and extremes in either direction.

Height in humans is a polygenic trait, as is color in wheat kernels.


Height in humans is NOT discontinuous. If you line up the entire class a continuum of variation is evident, with an average height and extremes in variation (very short [vertically challenged?] and very tall [vertically enhanced]).

Traits showing continuous variation are usually controlled by the additive effects of two or more separate gene pairs. This is an example of polygenic inheritance. The inheritance of EACH gene follows Mendelian rules.

Usually polygenic traits are distinguished by:

1. Traits are usually quantified by measurement rather than counting.


2. Two or more gene pairs contribute to the phenotype.
   
3. Phenotypic expression of polygenic traits varies over a wide range.
   

Human polygenic traits include:

• Height
  
• SLE (Lupus) an autoimmune disease


• Weight
  
• Eye Color
  
• Intelligence


• Skin Color


• Many forms of behaviour
  

The following diagrams illustrate the process of polygenic inheritance of skin colour in humans.

In this example, the trait (skin colour) is controlled by 3 genes.

The first cross is between an homozygous recessive indivivdual and an homzygous dominant indivivdual.

Click here to view graphics about human polygenic inheritance from McGill University's Genetics pages.

Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (http://www.sinauer.com/) and WH Freeman (http://www.whfreeman.com/), used with permission.