Discontinuous Variation

What is Discontinuous Variation?

Variation in which the phenotypes of a species fall into a limited number of distinct categories with no intermediates between them. Each individual belongs to exactly one category, the data are counted rather than measured, and in the standard Cambridge O Level Biology examples the categories are controlled mainly by genes with little or no environmental influence. ABO blood group, pea seed shape and pea seed colour are the required examples, and the correct graph is a bar chart with separated bars and frequency on the vertical axis.

This definition is part of the Inheritance chapter in Cambridge O Level Biology.

Concepts related to Discontinuous Variation

• bar chart
• abo blood group
• Allele
• categorical data

Common mistakes with Discontinuous Variation

• “Discontinuous variation cannot be inherited.” The reverse: the required examples are controlled mainly by genes and are inherited very reliably. Your blood group was fixed at fertilisation. Write “Discontinuous variation in these examples is controlled mainly by genes, with little environmental effect.”