Characteristics controlled by a single gene (one copy inherited from each parent) tend to have phenotypes that fall into separate categories. They show discrete variation.
Discrete variation in a group of individuals can be shown using a bar chart. The table gives some other examples of characteristics that show discrete variation.
|Hand used for writing||Left or right|
|Ability to roll tongue||Yes or no|
|Snapdragon petal colour||Red, white or pink|
Most characteristics are controlled by more than one gene and are described as being polygenic.
Polygenic characteristics have phenotypes that can show a wide range of values, with each value following on from the value before. This is described as continuous variation.
Continuous variation in a group of individuals can be shown using a histogram.
The table gives some examples of characteristics that show continuous variation.
|Height||Wide range of values, eg 1.69m|
|Body mass||Wide range of values, eg 58kg|
|Circumference of left wrist||Wide range of values, eg 15cm|
|Milk yield of cow||Wide range of values, eg 7500 litres/year|
|Root length of plant||Wide range of values, eg 5cm|
Many phenotypes are also affected by environmental factors, such as the conditions to which the individual has been exposed. For example, a person's height is determined by the genes they have inherited and by the availability of food when they are growing.