Conservation and colinearity of homeotic genes

The hypothetical progenitor organism has a chromosome with several homeotic genes that are similar but not identical. Additional gene duplication and selection events led to new speciation events and eventually led to the evolution of Drosophila, having the two clusters of Antennapedia and bithorax genes, and the mouse having four clusters of Hox genes located on four different chromosomes. The order of expression of the genes is correlated with the specification of the anterior to posterior segments of each organism, a phenomenon called colinearity.