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The Complexity of Inheritance

Gregor Mendel famously used pea plants to show how single genes control characteristics, but in humans and most other organisms, inheritance is rarely that simple. The vast majority of an organism's observable features, known as its phenotype, are the result of multiple genes interacting rather than the inheritance of a single gene.

Monogenic vs Polygenic Inheritance

Monogenic inheritance occurs when a characteristic is determined by a single gene.
This type of inheritance usually results in discontinuous variation, where individuals fall into distinct, non-overlapping categories (e.g., human blood groups, or whether you have cystic fibrosis).
Polygenic inheritance occurs when a characteristic is determined by the interaction of multiple genes.
In polygenic inheritance, each gene involved contributes a small, additive effect to the overall phenotype.

Features of Polygenic Traits

Because polygenic traits rely on instructions across multiple genes, they typically result in continuous variation.
This means the trait shows a wide range of possible phenotypes that often follow a normal distribution or bell curve across a population.
Classic examples of polygenic traits include human height (controlled by over 400 genes), eye colour, skin pigmentation, and body mass.
Features controlled by multiple genes are also much more likely to be influenced by environmental factors, such as diet and nutrition affecting a person's final height.

Modelling Polygenic Inheritance

All genetic variants (also known as alleles) arise from mutations, but most phenotypic features are the result of the cumulative effect of many such variants. Because a student's genotype for a trait like eye colour involves several different genes, it is too complex to be modelled using a single Punnett square. Punnett squares are exclusively used to model single gene (monogenic) inheritance. Note that you are not expected to describe the exact mechanisms of how these multiple genes interact (such as one gene masking another, which is a process you do not need to know).

Students often state that "every feature is controlled by multiple genes" — be careful, as a few features (like blood groups) are controlled by a single gene, so always state that "most" features are polygenic rather than "all".

In multiple-choice questions, examiners expect you to recall that single-gene inheritance (like Mendel's peas) is the exception rather than the rule for most human traits.

Never try to draw a single Punnett square to model a polygenic trait like height or eye colour; always explain that there are too many possible allele combinations to model it this way.

The observable characteristics of an organism, resulting from the interaction of its genotype and its environment.

The combination of alleles (genetic variants) an organism has for a particular gene or genes.

The determination of a characteristic by the inheritance of a single gene.

The determination of a characteristic by the interaction of multiple genes.

Variation within a population in which a characteristic has a wide range of values that flow into each other with no distinct categories.

Variation that produces distinct, discrete categories for a characteristic with no intermediate values.

Different versions of a gene, also known as alleles, which arise from mutations.

Changes in the DNA sequence that lead to the creation of new genetic variants.

Different versions of the same gene (used interchangeably with genetic variants).