French flag model
A term which refers to the ability of a morphogen–as described by Lewis Wolpert–which he defined as a signalling molecule that acts directly on cells (not by serial induction) to evoke specific cellular responses dependent on the morphogen’s local concentration.
Examples, morphogens Decapentaplegic, TGF-beta, hedgehog, sonic hedgehog, wingless, wnt, epidermal growth factor, fibroblast growth factor.
During early development, morphogen gradients generate different cell types in distinct spatial order. In this model, the sharp defining boundaries of the French flag are used to illustrate the effect of a morphogen on cell differentiation: a morphogen affects cell states based on concentration, these states are represented by the different colours of the French flag: high concentrations activate a “blue” gene, lower concentrations activate a “white” gene, with “red” serving as the default state in cells exposed to a concentration of morphogen below the necessary threshold