Continuous variation

In the syllabus and the exam

Continuous variation describes a characteristic that shows a full, unbroken range of values between two extremes, with no natural gaps separating individuals into distinct categories, and it is usually controlled by many genes acting together (polygenic inheritance) along with a significant environmental influence. Candidates are expected to name typical human examples, such as height, body mass, hand span or skin colour, and to describe the graph used to display this data correctly: a continuous variation graph is drawn as a line graph or histogram, with a smooth bell-shaped (normal) distribution curve, not a bar chart with separated bars, because the x-axis represents a continuous numerical scale rather than distinct categories. A common error is drawing gaps between the bars of a continuous variation graph, which visually and conceptually misrepresents the data as discontinuous. Candidates should also be able to explain why continuous variation typically involves many genes rather than one, since a single gene with only two alleles could not, on its own, produce the many intermediate values seen in a trait like height. Exam questions frequently present raw data or an unlabelled graph and ask candidates to identify whether the variation shown is continuous based on the shape of the distribution and the nature of the values recorded, before naming the graph type used to represent it correctly. Candidates should also recognise that measuring a continuous characteristic always requires a suitable measuring instrument and a defined unit, such as a metre rule for height in centimetres, since the data collected must be numerical rather than a simple category label.

Example

Plotting the heights of 200 Form 5 students produces a smooth, bell-shaped curve with most students clustered around the average height and progressively fewer students at the very short and very tall extremes.

Related terms

Discontinuous variationVariation

Source:SRC-DSKP-EN