For any one individual, it is impossible to specify the relative roles of genes and environment in the expression of a complex trait. For example, in an individual 66 inches tall it would be meaningless to attribute 33 inches of height to parentage (genes) and 33 inches of height to nutrition (environment). This kind of partitioning makes no sense because genes and environment act together so intimately in each individual that their effects are inseparable. To attempt to separate them for any individual would be like asking to what extent it is breathing or oxygen that keeps us alive. Both are important, and both must take place together if we are to live.

It is nevertheless possible to separate genes and environment in regard to their effects on the differences, or variation, among individuals within a particular population. For some traits, the variation seen among individuals is due largely if not exclusively to differences in the environment. For other traits, the variation is due mainly to genetic differences. In the case of human height, roughly 80% of the variation among individuals of the same sex is due to genetic differences, and the remaining 20% to differences in their environment, primarily differences in nutrition during their years of growth.