Homeobox - Hox Genes

Hox Genes

Hox genes are essential metazoan genes as they determine the identity of embryonic regions along the anterio-posterior axis. The first vertebrate Hox gene was isolated in Xenopus by Eddy De Robertis and colleagues in 1984, marking the beginning of the young science of Evo-devo.

In vertebrates, the four paralog clusters are partially redundant in function, but have also acquired several derived functions. In particular, HoxA and HoxD specify segment identity along the limb axis.

The main interest in this set of genes stems from their unique behaviour. They are typically found in an organized cluster. The linear order of the genes within a cluster is directly correlated to the order of the regions they affect as well as the timing in which they are affected. This phenomenon is called colinearity. Due to this linear relationship, changes in the gene cluster due to mutations generally result in similar changes in the affected regions.

For example, when one gene is lost the segment develops into a more anterior one, while a mutation that leads to a gain of function causes a segment to develop into a more posterior one. This is called ectopia. Famous examples are Antennapedia and bithorax in Drosophila, which can cause the development of legs instead of antennae and the development of a duplicated thorax, respectively.

Molecular evidence shows that some limited number of Hox genes have existed in the Cnidaria since before the earliest true Bilatera, making these genes pre-Paleozoic.