Discontinuous Variation:
This is where individuals fall into a number of distinct classes or categories, and is based on features that cannot be measured across a complete range. You either have the characteristic or you don't. Blood groups are a good example: you are either one blood group or another - you can't be in between, same goes for gender. Such data is called discrete (or categorical) data. Chi-squared statistical calculations work well in this case.
Continuous Variation:
There is a complete range of measurements from one extreme to the other. Height is an example of continuous variation - individuals can have a complete range of heights, for example, 1.6, 1.61, 1.62, 1.625 etc metres high.
This is where individuals fall into a number of distinct classes or categories, and is based on features that cannot be measured across a complete range. You either have the characteristic or you don't. Blood groups are a good example: you are either one blood group or another - you can't be in between, same goes for gender. Such data is called discrete (or categorical) data. Chi-squared statistical calculations work well in this case.
- Monogenic traits (characteristics controlled by a single gene loci) tend to exhibit discrete variation, with individuals expressing one of a number of distinct phenotypes
Continuous Variation:
There is a complete range of measurements from one extreme to the other. Height is an example of continuous variation - individuals can have a complete range of heights, for example, 1.6, 1.61, 1.62, 1.625 etc metres high.
- 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
Understanding:
10.2.U4: The phenotypes of polygenic characteristics tend to show continuous variation.
Objectives:
10.2.U4: The phenotypes of polygenic characteristics tend to show continuous variation.
Objectives:
- Explain polygenetic inheritance using an example of a two gene cross with codominant alleles.
- State that a normal distribution of variation is often the result of polygenic inheritance.
- State example human characteristics that are associated with polygenic inheritance.
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
Each gene has two alleles, which either code for red pigment or white pigment
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
Application:
10.2.A3: Polygenic traits such as human height may be influenced by environmental factors.
Objectives:
10.2.A3: Polygenic traits such as human height may be influenced by environmental factors.
Objectives:
- Outline two example environmental factors that can influence phenotypes.
- Compare continuous to discrete variation.
Phenotypic characteristics are not solely determined by genotype, but are also influenced by environmental factors
One example of a polygenic trait that is influenced by environmental factors is human height
Another example of a polygenic trait that is influenced by environmental factors is human skin colour
- 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