Discrete variation

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.

Bar chart with percentage of population on y axis and blood group - A, B, AB and O on the x axis.  A is approx 40%, B is approx 10%, AB is approx 5% and O is approx 45%.

CharacteristicPossible phenotypes
Hand used for writingLeft or right
Ability to roll tongueYes or no
Snapdragon petal colourRed, white or pink

Continuous variation

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.

Bar chart with number of people in category on y axis and height of category in metres on the x axis ranging from 1.4 - 2.1.  The graph rises to a peak at 40 for height range 1.7-1.8 meters and then falls off.

The table gives some examples of characteristics that show continuous variation.

CharacteristicPossible phenotypes
HeightWide range of values, eg 1.69m
Body massWide range of values, eg 58kg
Circumference of left wristWide range of values, eg 15cm
Milk yield of cowWide range of values, eg 7500 litres/year
Root length of plantWide 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.