Wolpert's French flag model — morphogen gradients specifying cell identity
Lewis Wolpert's positional information hypothesis (1969) proposes that cells acquire identity based on morphogen concentration — their "positional value." The French flag model is the canonical illustration: a single morphogen gradient emanating from a localized source is interpreted by two thresholds to divide a field into three distinct territories (blue, white, red). This mechanism underlies patterning of the Drosophila embryo (Bicoid gradient), vertebrate limb axes, and neural tube dorsoventral identity. The gradient is maintained by source production balanced against diffusion and degradation (Robin boundary conditions).