Figure 19.13 A Gene Cascade Controls Pattern Formation in the Drosophila Embryo (A) Maternal effect genes induce gap, pair rule, and segment polarity genes—collectively referred to as segmentation genes. (B) Expression of two gap genes, hunchback (orange) and Krüppel (green), overlaps; both genes are transcribed in the yellow area. (C) The pair rule gene fushi tarazu is transcribed in the dark blue areas. (D) The segment polarity gene engrailed (bright green) is seen here at a slightly more advanced stage than is depicted in (A). By the end of this cascade, a group of nuclei at the anterior of the embryo, for example, is determined to become the first head segment in the adult fly.