Continuous variation
Continuous variation shows a complete range of phenotypes between two extremes, with no distinct categories. Examples include height, body mass, hand span, leaf length and foot size. Any value within the range is possible.
Continuous variation is typically controlled by many genes (polygenic inheritance) and is strongly influenced by environmental factors. For example, height is influenced by many genes but also by nutrition during growth. When data for a continuous variable is plotted, it produces a normal distribution curve (bell-shaped curve) with most individuals near the mean and fewer at the extremes.
Discontinuous variation
Discontinuous variation has distinct, separate categories with no intermediates. Examples include blood group (A, B, AB or O), tongue rolling (can or cannot), sex (male or female) and eye colour in simplified genetics (though real eye colour is more complex).
Discontinuous variation is usually controlled by a single gene or a small number of genes, with little or no environmental influence. There is no continuous spectrum between categories: a person is either blood group A or B, not somewhere in between.
Choosing the correct graph
| Feature | Continuous | Discontinuous |
|---|---|---|
| Data type | Measured (quantitative, a range) | Categoric (distinct groups) |
| Graph type | Histogram or line graph | Bar chart (gaps between bars) |
| Shape | Normal distribution curve | No specific pattern |
In a histogram, the bars touch because the data is continuous. In a bar chart for discontinuous data, the bars have gaps because the categories are separate. This distinction is tested in Paper 6 data-handling questions.
Genetic and environmental causes
Most continuous variation is caused by a combination of genetic and environmental factors. For example, identical twins (same genes) raised in different environments may have different heights due to differences in diet and health. This shows the environmental contribution.
Most discontinuous variation is caused primarily by genes with minimal environmental influence. Blood group is determined by a single gene and is not affected by diet, exercise or any environmental factor. Understanding this distinction helps answer questions about the causes of variation.