Most characteristics are a result of multiple genes interacting, rather…

The idea

This statement contrasts polygenic inheritance with single-gene inheritance. A single gene can produce clear-cut categories, such as the ABO blood group (which has three alleles producing four distinct blood groups). However, most characteristics, such as human height and mass, are polygenic, meaning they are controlled by multiple genes interacting. Several genes each contribute small effects, and their alleles interact so the phenotype forms a continuous spectrum rather than distinct classes. Environmental factors also heavily influence many of these features. In an exam, you may be asked to explain why a characteristic shows a continuous range of values, linking it to multiple genes and environmental effects.

What earns the marks

• Most characteristics are influenced by multiple genes interacting, rather than a single gene.
• Polygenic inheritance typically produces continuous variation, resulting in a range of phenotypes.
• Single-gene inheritance usually produces discontinuous variation with clear-cut, distinct categories.
• Environmental factors often interact with genetics to affect polygenic characteristics such as height or mass.
• Examples of polygenic features include human height, skin colour, and body mass.

Common mistakes and how to put them right

• Thinking every characteristic is controlled by exactly one gene.

  Put it right by stating that most characteristics are polygenic and involve multiple interacting genes.

• Assuming continuous variation proves that only one gene is involved.

  Put it right by linking continuous variation to many genes each having small effects.

• Ignoring the environment when explaining polygenic features.

  Put it right by noting that the environment also contributes significantly to characteristics such as height.

Exam tips

• Use the phrase 'multiple genes interacting' when explaining continuous variation.
• Contrast a named single-gene example, such as cystic fibrosis, with a named polygenic example, such as human height.