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Hox genes are highly conserved across most animal species and are DNA sequences that specify the anterior-posterior axis and segment identity of organisms as they develop. Hox is an abbreviation of homeobox, the name given to the region of DNA of 180 base pairs that codes for a protein domain called homeodomin. These are transcription factors that bind to DNA and regulate the transcription of genes. They are involved in a cascade of events that can turn genes "on" or "off."
A homeodomain protein binds to DNA sequences known as gene enhancers which can activate or repress a gene's actions. So for example, a Hox gene product may activate the gene inside a developing insect that will specify structures on its thoracic region, or a Hox gene product may repress a gene involved in the development of the antenna.
Hox genes are also sometimes referred to as homeotic selector genes.
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First Demonstration of Hox Genes
The function of Hox genes was first demonstrated in plants in the latter part of the 19th century when mutations caused stamens to grow in the place of petals in some flowering plants observed by William Bateson. He described the phenomenon in his book Materials for the Study of Variation. "The case of the modification of the antenna of the insect into a foot, of the eye of a crustacean into an antenna, or a petal into a stamen, and the like, are examples of the same kind."
In the 1940s Edward Lewis was working with the biologist's favourite friend - the fruit fly Drosophila melanogaster - and he demonstrated that mutations in the Antennapedia hox gene causes legs to develop on the head of a fly in the place of the antenna.
That Hox genes are highly conserved across different animal species has been demonstrated by the fact that a fly can function normally with the homeotic selector protein of a chicken (Lutz, B et al (1996). Rescue of Drosophila labial null mutant by the chicken ortholog Hoxb-1 demonstrates that the function of Hox genes is phylogenetically conserved. Genes and Development 10: 176-184).
Hox genes are classified differently in different phyla; humans typically contain four clusters of Hox genes; Hox a, Hox b, Hox c, and Hox d.
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In summary what are Hox genes? They are those genes that play a pivotal role in laying down body plans. If you like they are the biological equivalents of the building site foreman, dictating which part of the structure goes where. Evidence of their importance comes from the fact that they are so highly conserved, as mutations can be disastrous, sometimes fatal for an organism.
One astounding fact about Hox genes is that many are positioned on a chromosome in the same order that they are spatially expressed. So for example if you were able to observe Hox genes on a DNA sequence and look from left to right, you would see that genes on the left are for the head regions, genes in the middle are for the abdomen and genes on the right are for the lower parts of an organism's body.