"General purpose" control genes are important elements in
building complicated organisms like flies. Some "control" genes
are common to many organisms (they
inherited from our common ancestor). For example, Hox
genes help lay out the basic body forms of many animals, including
humans, flies, and worms. They set up the head-to-tail organization. You
can think of them as directing instructions as an embryo develops: "Put the
head here! Legs go over there!"
They are general purpose in the sense that they are similar in many organisms;
it doesn't matter if it's a mouse's head or a fly's head that is being
built, the same gene directs the process. Small changes in such powerful
regulatory genes, or changes in the genes turned on by them, could represent a major
source of evolutionary change.