Three Classes of Zygotic Genes

The identity of 12 segments is shown at left. Segments are recognizable at this stage by the pattern of hair-like denticles on the ventral surface, shown as dark trapezoids on each segment. Mutant forms of various genes inhibit development of various segments, as indicated by the pink block(s): the resulting phenotype is shown to the right. Three classes are recognizable.

(1) Gap Genes affect large numbers of contiguous segments at either the anterior or posterior poles. For example, Krüppel ["cripple"] eliminates everything forward of  A6, and knirps ["little guy"] eliminates abdominal segments

(2) Pair-Rule Genes are expressed in an iterated pattern in alternate segments (like a picket fence) so as to produce a seven-striped patterns. There are two types, primary that are induced by Gap genes, and secondary that are induced by primary Pair-Rule genes or other secondary Pair-Rule genes. For example, Even-skipped  is a Primary-Rule gene that leaves segments 1, 3, 5, 7, 9, & 11. Odd-skipped leaves segments 2, 4, 6, 8, 10, & 12.

(3) Segment polarity Genes act within each of the segments, to determine anterior and posterior compartments. For example, Gooseberry reverses the posterior compartment in each segment, and leaves the denticles (on the anterior compartment) intact.