Practice question

Pair-rule genes initiate periodic pattern where each gene's striped pattern covers every other parasegment; loss removes those parasegments causing deletion of alternate segments observed as missing denticle belts every second segment. Even-skipped mutants delete odd bands, hairy deletes complementary set. Continuous deletion characterizes gap mutants, anterior restriction characterizes head gaps. Pair-rule class links double-segment periodicity to single-segment pattern via segment polarity activation. Mechanism involves combinatorial regulation where gap proteins repress pair-rule stripes in interstripe intervals, creating precise periodic output essential for segmentation clock and metameric organization conserved across arthropod lineages and developmental timing.