Polygenic inheritance

Polygenic inheritance describes traits shaped by two or more genes, often hundreds or thousands, rather than by a single gene. Each variant nudges the trait up or down by a small amount, and the effects largely add together. Height, skin color, blood pressure and body weight are familiar human examples.

Because so many combinations of alleles are possible, polygenic traits vary smoothly instead of falling into a few distinct classes, and in a large population their values usually form a bell-shaped curve. They do not give the simple ratios of Mendelian inheritance, even though every contributing gene is passed on by Mendel's rules. Most such traits are also affected by the environment, for example by diet or exercise, and are then described as multifactorial.

Many common conditions, including type 2 diabetes, coronary heart disease and some cancers, have a large polygenic component. Genome-wide association studies find the variants involved, and their small effects can be added up into a polygenic risk score. Such a score estimates a person's genetic risk relative to other people, but it does not account for lifestyle or environment and cannot predict an individual outcome with certainty.