The French flag model: a morphogen (Bicoid in Drosophila) forms an exponential gradient c(x) = c₀ exp(−x/λ). Cells read their position from local concentration and adopt one of three fates at threshold concentrations T₁ and T₂.
Wolpert's positional information (1969): cells use morphogen concentration as a coordinate. Scaling: if λ ∝ L (embryo length), boundary positions remain at fixed fractional positions despite size variation. Bicoid mRNA is deposited at the anterior pole; the protein diffuses and is degraded with length constant λ ≈ 0.2 embryo lengths. Noise in gradient sensing limits positional precision to ~1% of body length.