HOX genes are homologous control genes that direct the head to feet (or tail) development of the body, otherwise termed anteroposterior identity.
Homeobox genes are, according to Merriam Webster, a short usually highly conserved DNA sequence in various eukayotic genes and especially homeotic genes that encodes a DNA-binding amino acid domain of some proteins.
Importance to evolutionary theoryEdit
These genes are important to understanding the theory that all mammals are decendents from a common ancestor. The fact that these are homologous genes means that a HOX gene that's function is to direct the placement of the arm in one bilateral mammal is the same as in another seemingly unrelated bilateral animal. This has been proven in labratories where genes have been transferred from one species to another and the correct body part is formed in the correct location.
Some people use the term HOX gene and Homeobox interchangably but they shouldn't. All Hox genes are Homeobox genes but not all Homeobox genes are HOX genes. Homeobox genes direct the develoment and placement of many more mammalian features than just the anteroposterior identity.
Implications in Evolution--Hox genes in actionEdit
There is little argument that snakes evolved from some kind of carnivorous tetrapod lizard; in fact, modern pythons and boas still possess rudimentary hind limbs consisting of a pelvic girdle and truncated femur that develop at the junction between rib-bearing vertebrae and lymphapophysis-bearing vertebrae at cloacal level (Cohn & Tickle, 1999). Limblessness has elvolved many times in vertebrate evolution and is often accompanied by elongation and loss of regional differences in the axial skeleton (Carroll, 1988). Hox genes are central to the specification of structures along the anterior-posterior body axis and have been implicated in the evolution of limblessness in snakes. Cohn and Tickle (1999) show that Hox gene expression domains are expanded along the body axis in python embryos, and that this can account for the absence of forelimbs and the expansion of thorasic identity in the axial skeleton. Di-Poi et al (2010) compare expression levels of posterior Hox genes in the corn snake and whiptail lizard, two squamates with different axial skeletons. Major differences were obseved during somitogenesis (sectioning of the body)
Harrison, Jennifer. The difference between homeobox and hox genes.
Genetics Home Reference
Evolution 101 - Berkeley University
Genetic Toolkit - PBS Evolution Library
Carroll, R. (1988). Vertebrate Paleontology and Evolution. Freeman: New York.
Cohn, M. J., & Tickle, C. (1999). Developmental basis of limblessness and axial patterning in snakes. Nature, 399, 474–479.
Di-Poï, N., Montoya-Burgos, J. I., Miller, H., Pourquié, O., Milinkovitch, M. C., & Duboule, D. (2010). Changes in Hox genes’ structure and function during the evolution of the squamate body plan. Nature, 464(7285), 99–103. doi:10.1038/nature08789