Cells at different distances from the zone of polarizing activity are exposed to different concentrations of the morphogens and respond differently. Figure 15.14 illustrates the effects of the ZPA on wing development. A gene cascade controls segmentation and pattern formation in Drosophila melanogaster . The product of one gene promotes or represses the expression of another gene, and so on. During development of D. melanogaster, activation of segmentation genes leads to activation of homeotic genes in different segments.
Homeotic genes define the functional characteristics of the segments.
Homeotic genes contain the homeobox , which encodes an amino acid sequence that is part of many transcription factors. The homeobox is a 180 base pair sequence and is found in many species, even distantly related species. The homeobox is found in key genes of distantly related species suggesting that numerous regulatory mechanisms may trace back to a single evolutionary precursor. Homebox genes code for a 60 amino acid sequence called the homeodomain that is part of some proteins including transcription factors. Thus they appear to help regulate development.