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."