brennan
Argumentative Brit
Open floor.
Hox Genes:
Hox genes control how cells differentiate themselves into different tissue depending on their location. For instance, they control the way a cell in an insect's thorax knows to start growing like a leg as opposed to an antenna or wing.
The activation order is critical to establish the proper gradient for that part of the body; mess around with order, and you may get a leg where there should be an antenna.