The relationship between the structure of the Hox gene complexes and the phenotypes of Hox gene mutants was illuminated by the molecular characterization of genes. Molecular cloning of the sequences encompassing each Hox locus provided the means to analyse where in the developing animal each gene is expressed.
To understand the genetic control of development, the spatial aspects of gene expression and gene regulation are very important. In the case of Hox genes, the visualization of gene and protein expression was important to understand the relation among gene organization, gene function, and mutant phenotypes.
In the developing embryo, the Hox genes are expressed in spatially restricted sometimes overlapping domains within the embryo. The genes are also expressed in the larval and pupal tissues, which give rise to the adult body parts. The pattern of Hox-gene expression correlates with the regions of the animal affected by mutations.
In Drosophila embryo, the Hox gene is expressed in the posterior thorax and most of the abdominal segments of the embryo. In the Ubx mutants, the development of these segments is altered. Ubx is expressed in the developing hind wing. It is not expressed in developing forewing.
It is very important to distinguish the role of Hox genes in determining the identity of a structure from that governing its formation. In the absence of function of all Hox genes, there is formation of segments, but all the segments that formed have same identity. Limbs also form, but they have antennal identity. Likewise wings have forewing identity.