Figure 1. Waddington's “Epigenetic Landscape”.

These figures (adapted and redrawn from [30]) represent the indirect relationship of genotype to phenotype. An organism, represented by the ball, moves through development over a landscape with valleys representing various possible phenotypic states. The shape of this landscape is determined by an individual's overall genotype, which may have dramatic effects on the relative likelihood of different end points (for example, schizophrenic or not, or left- or right-handed). The diagrams represent two “runs” of the developmental process in two individuals (A and B) with the same starting genotype (as in monozygotic twins, for example). These two individuals therefore inherit the same probability of developing a certain phenotype but may have different actual phenotypic end points, determined by chance events and environmental effects, especially at critical points.