Types of Variation (SQA National 5 Biology): Revision Note

Types of variation

• Variation describes the differences in characteristics between individuals of the same species

• Variation is the result of genes from two parents combining, giving rise to a unique offspring

• It can be discrete or continuous, depending on how the trait is inherited

Discrete variation

• Discrete variation is controlled by one gene (single gene inheritance)

• Characteristics determined by one gene alone are unaffected by environmental influences

• The phenotypes produced form distinct categories, meaning that individuals either have one form of a characteristic or another

  • Examples of discrete variation include:

    • blood groups (A, B, AB or O)

    • tongue rolling ability

    • flower colour in pea plants (purple or white)

• Discrete data is represented in bar charts with no intermediate values

Continuous variation

• Continuous variation is controlled by many genes (polygenic inheritance)

• The more genes that are involved, the more intermediate values exist in the phenotype

• Many characteristics that demonstrate continuous variation are impacted by environmental factors

  • For example, a person's height is determined by their genetics, but without the proper diet, they may not reach their full growth potential, and so will not be as tall as expected

• The phenotypes show a continuous range of values with no distinct categories

  • Examples of continuous variation include:

    • height

    • body mass

    • leaf length in plants

    • milk yield in cows

• Continuous data is shown as a bell-shaped curve on a histogram