Discuss the role of hox genes in animal development.

Segmentation is a key process in animal development defined as the serial repetition of tissue pattern along the axis of the body and allows differentiation of the developing organism. This differentiation in particular is mediated through Hox Genes. This essay will discuss their role and possible mechanisms for how they work.
What are the hox genes?
The hox genes are a set of genes that determine a segments identity. In the human there are 4 sets of 13 genes. These contain a homeobox domain which allows them to trigger the expression of various genes throughout the tissue. The exact nature of their expression is unknown however their role in patterning can be shown throughout various experiments.
Experiments in flies
The hox genes were originally discovered in the drosophila by Edward B lewis when he noticed that in some mutant drosphila some segments took the identity of segments above them. For example the drosophila expressed a pair of wings where it should have expressed a pair of legs.
The nature of the expression.
Draw expression with 3’ genes expressed anteriorly and 5’ and 3’ genes being expressed posteriorly. The identity of the segment appears to be determined by the most caudal segment and knock out will anteriorise it.
Furthermore inappropriate expression of the hox gene will cause a caudal phenotype.
Experiments in rats would cause the production sacral ribs etc.
Consider in somite formation. Do mechanism for molecular segmentation clock. The hox expression is what differentiates each somite. How is this determined. There is a retinoic acid gradient (opposite to the FGF gradient). The Hox genes have a retinoic acid response element and hence can be affected by them. This also means they are sensitive to this and hence retinoic acid is a teratogen for humans.
What other evidence for hox genes are there?
Hox genes seem to control the formation of limb buds. This is through the expression of hox genes causing tbx4/5 causing FGF8 which induces limb bud formation. Furthermore in limb formation the patterning of the cartilage is determined by hox genes. However in this case the activator of it is well studied. AER, ZPA, ectoderm activate the hox genes via FGF8, shh, and wnt. Differential expressions of each cause differential hox expression.


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