Ubx mutation transforms:
Ultrabithorax protein, part of Bithorax complex, normally represses wing-promoting genes and promotes haltere developmental program in third thoracic segment. Haltere discs require Ubx to suppress wing margin and activate haltere-specific genes. Loss-of-function mutation derepresses wingless, vestigial, and other wing genes in T3 imaginal discs, converting halters into second pair of wings creating four-winged flies observed by Bateson. Transformation does not affect legs or antennae, indicating segment-specific target repertoire. Ubx illustrates Hox principle where relatively small regulatory change rewires entire organ identity, maintaining segment count while altering appendage morphology and function essential for flight stability.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 20: Ubx mutation transforms halteres into wings - Bithorax complex function.