Brent Cornell

    
Variation in phenotypes for a particular characteristic can be either discrete (discontinuous) or continuous

• Monogenic traits (characteristics controlled by a single gene loci) tend to exhibit discrete variation, with individuals expressing one of a number of distinct phenotypes
• Polygenic traits (characteristics controlled by more than two gene loci) tend to exhibit continuous variation, with an individual’s phenotype existing somewhere along a continuous spectrum of potential phenotypes

In the case of polygenic inheritance:

• Increasing the number of loci responsible for a particular trait increases the number of possible phenotypes 
• This results in a phenotypic distribution that follows a Gaussian (bell-shaped) normal distribution curve

Monogenic → Polygenic Inheritance

Maize Grain Colour

An example of a polygenic trait is grain colour in maize (wheat), which is controlled by three gene loci

• Grain colour can range from white to dark red, depending on the amount of pigment that is expressed

Each gene has two alleles, which either code for red pigment or white pigment

• The most frequent combinations have an equal number of the two allele types
• Conversely, combinations of one extreme or the other are relatively rare
• The overall pattern of inheritance shows continuous variation

Continuous Variation in Maize Grain Colour

    
Phenotypic characteristics are not solely determined by genotype, but are also influenced by environmental factors

• The added effect of environmental pressures functions to increase the variation seen for a particular trait

One example of a polygenic trait that is influenced by environmental factors is human height

• Human height is controlled by multiple genes (polygenic), resulting in a bell-shaped spectrum of potential phenotypes
• Environmental factors such as diet and health (disease) can further influence an individual human’s height

Another example of a polygenic trait that is influenced by environmental factors is human skin colour

• Skin colour is controlled by multiple melanin producing genes, but is also affected by factors such as sun exposure 

TED Talks: Inheritance of Human Skin Colour