concept

Pleiotropy

When a single gene performs several completely unrelated jobs throughout development. The Sonic hedgehog gene, for example, is needed for limbs to grow out but is also essential for patterning the brain, the spinal cord, and the teeth. This many-rolled nature is exactly what makes mutating a master gene's coding sequence dangerous - fix one thing and you break several at once - and it is why evolution usually edits regulatory switches rather than the gene itself.