Discontinuous variation

In the syllabus and the exam

Discontinuous variation describes a characteristic that falls into a small number of distinct, separate categories with no intermediate forms, and it is typically controlled by one gene, or a very small number of genes, with little or no environmental influence on which category an individual falls into. Candidates should be able to name standard examples, such as ABO blood group, the ability to roll the tongue, and the presence or absence of attached earlobes, and to represent such data correctly using a bar chart with clearly separated bars, reflecting the fact that the categories on the x-axis are discrete rather than points along a continuous numerical scale. A common error is drawing a discontinuous variation bar chart with the bars touching, which visually blurs the distinction from a continuous variation histogram that the exam is specifically testing candidates' ability to recognise. Because discontinuous variation is usually controlled by one or very few genes, it follows recognisable inheritance patterns similar to those studied in monohybrid inheritance, and higher-order questions sometimes ask candidates to apply genetic cross reasoning to a discontinuous human characteristic, connecting this term directly back to earlier work on genotype and phenotype. Candidates should also practise sorting a mixed list of human characteristics into continuous and discontinuous categories, since exam questions frequently combine both types in a single table to test whether the underlying distinction, rather than memorised examples alone, has been understood.

Example

Human ABO blood group falls into exactly four distinct categories, A, B, AB and O, with no individual showing an intermediate blood group between them, a clear example of discontinuous variation.

Related terms

Continuous variationVariation

Source:SRC-DSKP-EN