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

The idea

Although simple monohybrid examples such as pea colour or earlobe shape are useful for learning inheritance, most characteristics are polygenic, meaning they are controlled by several genes acting together. Each gene may have multiple alleles, and the combined effects produce a continuous range of phenotypes rather than a few distinct categories. Human height, skin colour, body mass and many crop yields are examples. Environmental factors also influence these characteristics, so the final phenotype depends on both genotype and environment. Because several genes contribute, inheritance patterns are more complex than a single dominant or recessive pair, and outcomes are often shown as a normal distribution. Understanding polygenic inheritance explains why most people do not fall neatly into two groups and why predictions for individuals are less certain than in simple monohybrid crosses.

What earns the marks

• Most characteristics are controlled by multiple genes rather than one gene, which is called polygenic inheritance.
• Each gene involved may have several alleles, and their effects combine to influence the characteristic.
• Polygenic inheritance usually produces continuous variation, giving a range of phenotypes rather than distinct categories.
• Environmental factors interact with multiple genes, so the phenotype is not determined by genotype alone.
• Examples include human height, skin colour and body mass, which show a spread of values in a population.
• Simple single-gene examples are useful models but do not represent most inherited characteristics.

Common mistakes and how to put them right

• Assuming every characteristic follows a simple dominant or recessive pattern.

  Put it right: Most characteristics are polygenic and show continuous variation.

• Believing continuous variation means the characteristic is not inherited.

  Put it right: Many genes still contribute, alongside environmental effects.

• Confusing multiple alleles with multiple genes.

  Put it right: Polygenic inheritance involves several different genes interacting.

Exam tips

• Use the phrase continuous variation when describing the outcome of polygenic inheritance, and give a named example such as human height.
• If a question asks why a characteristic shows a range of values, refer to multiple genes and environmental influence together.
• Avoid claiming that a single gene fully determines a polygenic characteristic; instead describe the combined effect of many genes.