A fine-grained pattern by a set of nested pattern forming reactionInitially, a relatively crude pattern is formed that controls the formation of the next finer pattern, and so on. Imagine that under the influence of the maternal positional information a primary basic subdivision into relative coarse regions is achieved by regional activation of the gap genes. To avoid confusion by the strange and arbitrary names (such as 'krüppel', 'knirps'…) they should be called I, II, III and IV. These regions would be in itself more or less homogeneous. It may appear straightforward to assume that each of these regions become further subdivided into two or three sub-regions. However, the polarity must be transmitted throughout the cascade in order to make sure that the final segments have the correct polarity. Each of the region regions I, II, III... has per se no polarity but the borders between the regions have. For instance, II is anterior to III). For this reason, I have proposed that not the regions itself but the borders between these regions, resulting for instance from an overlap of the signals produced in the regions I and II, II and III... act as a scaffold and organize the subsequent pattern. This has led to the prediction (Meinhardt, 1985) [PDF] that the region in which a gap gene is transcribed is only half as large as the gap in the sequence of segments that is caused by the corresponding mutation. For instance, if the gene responsible for region III is broken, neither the II/III border nor the III/IV border is present. Thus, in addition to the III region, half of the II region and half of the IV region would be lost too. The resulting gap would be twice as large as the III region itself. This prediction has been shown to be correct. The Krüppel gene, for instance, is expressed in a region of about 3.5 future segments  while in a Krüppel mutant at least 7 segments are missing.
Pair rule patterns: repetition of at least four elementsAs mentioned on the previous page, a periodic structure with polarity requires the repetition of at least three elements. In the pair rule mutants, the remaining pattern still maintains the original polarity. For this reason I have proposed that the pair rule pattern results from the repetition of at least four basic building blocks. If one of the four elements is missing, three elements can remain and the polarity is maintained. Although more than 8 pair rule genes are known, it has turned out that only a mutation in four of them leads to a change in the gene expression of the remaining ones , indicating that these genes generate the primary pair rule pattern.