Practice question

Drosophila embryonic segmentation proceeds hierarchically: maternal gradients activate gap genes then pair-rule then segment polarity. Gap genes like hunchback, Krüppel, knirps, giant are transcription factors expressed in broad overlapping domains along anterior-posterior axis, each controlling large contiguous blocks of segments. Mutations delete several adjacent segments creating gaps in larval cuticle pattern, hence name. They regulate pair-rule gene stripe expression. Segment polarity genes control subsegments and polarity within each segment via Wg and En, homeotic genes confer identity not segment number. Therefore gap genes establish coarse regionalization specifying large segmental domains upstream of finer patterning cascades in early embryogenesis.