IGCSE Biology / Cell division
Continuous and discontinuous variation
Edexcel International GCSE Biology (4BI1), chapter 17: Cell division, specification point 3.33. Free lesson.
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Variation can be continuous, with a range of values, or discontinuous, with separate categories. The type of variation helps show how genes and the environment affect a characteristic, and determines which graph to draw.
How to recognise continuous variation
A characteristic shows continuous variation if it can take any value between two extremes. There are no gaps in the range. For example, people can have many different heights, rather than belonging to a small set of height categories. Height, mass and leaf length are examples. A histogram has values along the bottom and the number of people or items up the side. Its bars show a continuous range, and the overall pattern is often a bell-shaped curve: most values are near the middle, with fewer at either end.
What causes continuous variation
Continuous characteristics are affected by many genes. The environment usually has a large effect too. Height, for example, depends on many genes and on diet, so people can have a wide range of heights. Other examples of continuous characteristics include skin colour and body mass.
How to recognise discontinuous variation
A characteristic shows discontinuous variation if it has distinct categories, with nothing in between. Blood group has four categories: A, B, AB and O. Nobody has a blood group between A and B. Blood group, tongue rolling and seed shape are examples. Use a bar chart with gaps between the bars. The gaps show that the categories are separate.
What causes discontinuous variation
Discontinuous characteristics are usually controlled by one gene, or a very small number of genes, and usually have no environmental effect. Diet, for example, does not change a person's blood group. Variation can also be genetic only, environmental only, or caused by both genes and the environment. Blood group is an example of genetic variation. A scar, an accent, or a plant stunted by shade are examples of environmental variation. Height is affected by both genes and the environment.
Compare the two types and choose a graph
Check whether the values form a range or separate categories. Then choose the graph to match. The graph shape is a quick way to identify the type of variation, and drawing the right one can earn a mark. Continuous: a range with no gaps; many genes contribute; the environment usually has a large effect; use a histogram, often with a bell-shaped curve. Discontinuous: distinct categories; usually one gene or very few; usually no environmental effect; use a bar chart with gaps. If a characteristic can take any value between two extremes, it is continuous.