The "French Flag"
problem & Positional Information
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
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.