Hox genes are a series of genes that controls the differentiation of cells and tissues in an embryo by turn on and off other genes. If Hox genes express incorrectly, a major changes in the morphology of the individual can be seen.
Let’s understand this with an example: The Ultrabithorax, a Hox gene determines the Third thoracic segment in <em>Drosophila melanogaster</em>. Third thoracic segment displays a pair of legs and a pair of reduced wings used for balancing. In the mutant<em> Drosophila</em> the functional Ultrabithorax protein is absent. Due to this mutation, the third thoracic segment now expresses the same structures found on the second thoracic segment, which contains a pair of legs and a pair of wings which are fully developed .
