(homeotic genes) are a family of genes that control basic body plan formation.
They are responsible for the location of body parts and explains why so many organisms share similar body types. For example many organisms display bilateral symmetry (left and right) with a head two forelimbs two hind limbs and a tail.
This suggests that many hox genes evolved very early since so many organisms share similar basic body plans.
in these genes can be observed in fruit flies when legs are put on the head where antennae belong.
It also suggests a possible mechanism for the evolution of snakes (and many other species) as their four legged ancestors experienced increasing numbers of vertebrae resulting in selection against legs that eventually served no purpose.
A lot of information the student can go to whatever depth he/she needs on this website: http://www.nature.com/scitable/topicpage/hox-genes-in-development-the-hox-code-41402

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