This causes that only one of the alternative genes can be active in a particular cell. A linkage of such reaction with pattern formation can cause a stable space-dependent activation of genes separated by sharp borders. Many classical experiments concerning the initiation of legs and wings can be accounted for by the assumption that the intersections of two such borders can become the organizing regions. They have from the beginning an internal structure. The model account for the generation of legs etc.
Segmentation is a very important building principle in development.
Segments consist of periodic structures with an internal polarity. We have shown that such a pattern emerges if cell states locally exclude each other but mutually activate each other on long range. A stable situation is reached when the correct cell types are situated side by side. This leads to a controlled neighbourhood of structures. In agreement with the experimental observation, the neighbourhood becomes restored after an experimental perturbation.