Sample of a meaty metaphor found at www.edge.org/3rd_culture/marcus03/marcus_index.html "A gene is really not a dictator, but an opportunity, because each gene actually has two parts. Everybody knows a gene constructs a protein, but not everybody realizes that the other half of every gene is essentially what's called a regulatory region. It's essentially like an "if" in a computer program. Each gene is really like an if/then statement. There's a "then" that says, build this particular protein. It could be insulin in the pancreas, it could be hemoglobin in precursors to red blood cells, or it could be a particular protein for building a neuron in the brain. But when it does that, it is controlled by the "if" part of each gene. So there's an "if" and a "then." This seemingly very simple idea, a tiny little twist on the usual idea of thinking of a gene as coding for a particular protein, means that every gene has some kind of way it can respond to the environment, either inside the cell or outside the cell. So the "if" that controls whether a gene is turned on or not is responsive to chemical signals that are around a particular cell, and those chemical signals can be used for things like telling the cell where it is in the growing body so if it moves around it can adopt a new plan according to its new location. It also means that the external environment can, in principle, modify gene expression. Each gene becomes like a switch."