Figure 2S.

Hoxd gene duplications. Consider cells expressing Hoxd9 and Hoxd10. (a) Posterior gene duplications. If Hoxd11 is duplicated (left), F increases above the minimum value F2; therefore, Hoxd10 can be expressed ectopically in more anterior locations (right). (b) Anterior gene duplication. If Hoxd9 is duplicated, the pulling force increases (left) but the increase is not sufficient to reach F3 for the extrusion of three genes; therefore, P must increase for activation of the third gene (Hoxd10), whose expression is then shifted posteriorly (right).