complex trait

A complex trait is one whose pattern of inheritance refuses to boil down to a single gene with neat ratios — it emerges instead from many genes and many environmental influences tangled together. The word complex here is doing honest work: it signals that no simple rule maps genotype to phenotype, and that prediction has to be statistical rather than deterministic.

Most of the traits people actually care about are complex in this sense, from height and blood pressure to the risk of common diseases and aspects of behaviour. Each of perhaps hundreds or thousands of genetic variants contributes a small effect, those effects combine in ways that need not be a simple sum, and the environment layers on top — so the same starting genotype can land in many different end states.

Studying complex traits is the domain of genome-wide association studies, polygenic scores, and careful heritability analysis, all of which trade the certainty of single-gene genetics for probabilities and population averages. The honest limit is that even when hundreds of contributing variants are found, they often explain only part of the heritability, and predicting any one individual's outcome stays far from exact.