Practice question

Maternal morphogen gradients directly regulate first class of zygotic genes called gap genes which establish broad regional subdivisions along anterior-posterior axis. Gap genes including hunchback, Krüppel, giant, knirps encode transcription factors with overlapping expression domains set by threshold responses to Bicoid, Caudal and Terminal signals. Their mutual repression refines boundaries and they activate pair-rule genes such as even-skipped, fushi-tarazu which divide embryo into parasegments. Segment polarity genes like engrailed act later. Thus gap genes represent initial zygotic interpreters of maternal positional information linking maternal pattern to segmentation cascade.