Hox genes, a family of transcription factors, are major regulators of animal development. Unlike most genes, however, the order of Hox genes in the genome actually holds meaning.
The Hox genes are a set of transcription factor genes that exhibit an unusual property: They provide a glimpse of one way in which gene expression is translated into the many different forms that animals (metazoans) exhibit. For the most part, the genome seems to be a welter of various genes scattered about randomly, with no order present in their arrangement on a chromosome—the order only becomes apparent in their expression through the process of development. The Hox genes, in contrast, seem like an island of comprehensible structure. These are genes that specify segment identity—whether a segment of the embryo will form part of the head, thorax, or abdomen, for instance—and they are all clustered together in one (usually) tidy spot. Within that cluster, there is even further evidence of order.
Hox Genes in Drosphila
To better understand the arrangement and role of Hox genes, take a look at the Drosophila portion of Figure 1. As shown, in Drosophila there are eight Hox genes in a row, and the genes' order within that row reflects their order of expression in the fly body. The gene found on the left or 3' end of the DNA strand, denoted lab (labial), is expressed in the head; on the other hand, the gene at the right end of the DNA strand, Abd-B (Abdominal-B), is expressed at the end of the fly's abdomen.
Knocking out individual Hox genes in Drosophila causes homeotic transformations—in other words, one body part develops into another. A famous example is the Antennapedia mutant, in which legs develop on the fly's head instead of antennae. The Hox genes are early actors in the cascade of interactions that enable the development of morphologically distinct regions in a segmented animal. Indeed, the activation of a Hox gene from the 3' end is one of the earliest triggers that lead the segment to develop into part of the head.