Two genes may interact to activate a third, producing a tri-partite pattern such as that seen in the French flag (right). In this model, Gene B is expressed on a decreasing Anterior/Posterior gradient, Gene R on an increasing gradient over the same field. In the anterior compartment, where Gene B is expressed and Gene R is not active, a "blue" phenotype is produced. In the posterior compartment, where Gene B is inactive and Gene R is expressed, a "red" phenotype is produced. In the middle compartment, the simultaneous activities of both genes activates Gene P, which produces a "white" phenotype. That is, cells in the middle compartment "read" positional information. The B and R fields may be mRNA gradients from maternal genes that trigger a P zygotic gene.

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