Segment Identity

Gap genes target homeotic gene complexes.

Drosophila have two homeotic gene clusters: ANT-C (Antennapedia complex) responsible for segmental identity in head and anterior thorax, and BX-C ( Bithorax complex) which is responsible for segmental identity in the posterior thorax and abdomen.

Homeosis: also known as homeotic transformation, is the development of one body part with the phenotype of another.

Homeosis has revealed a lot about how segment identity is established.

Homeodomain proteins

The homeotic genes encode transcription factors of a class called homeodomain proteins. These proteins interact with the regulatory regions of the homeotic genes such that their expression patterns are mutually exclusive. They also regulate downstream target genes responsible for assigning different functions and identities on each segment.