Question

the bithorax mutation clearly reflects the "posterior specification rule" of Hox gene expression. The Antennapedia 738 mutation is clearly different, because. Ant is required to specify identify in the thorax. What mechanism might generate the famous Ant73B phenotype?

Answer 1

Drosophila melanogaster, the homeotic  gene Antennapedia 738 is expressed in the thoracic embryonic epidermis and is required for proper development of the thorax and legs . Loss-of-function mutations in the embryo result in transformation of the second and third thoracic segments toward the prothoracic segment. Ectopic expression of the Antennapedia 738 gene leads to distinct phenotypes, such as the transformation of antennae into second legs and eye reduction

A deletion mutation in the Antennapedia Complex of Drosophila melanogaster, Df(3R)SCBXL2, induces both dominant and recessive loss-of-function phenotypes. The deletion is associated with diminished function of proboscipedia (pb), a homeotic gene required for mouthparts formation. Df(3R)SCBXL2 also has associated dominant thoracic defects related to diminished expression of the homeotic Antennapedia (Antp) gene copy on the homologous chromosome. This is shown to be a consequence of ectopic pb expression in the thorax. Newly juxtaposed Antp sequences provide the pb gene on the deletion bearing chromosome with a second promoter, Antp , in addition to its own. Ectopic pb protein expression occurs under Antp control, by alternate splicing, and results in diminished accumulation of Antp protein in the imaginal disc cells where Antp is normally expressed. The analysis of this mutant chromosome thus demonstrates that pb protein is capable of participating in the negative regulation of a more posteriorly expressed homeotic gene, as well as serving a homeotic "selector" function in the head.