In my story about the elevator, I wrote that understanding "switches" is crucial to understanding the interaction of nature with nurture. Our nature -- including the configuration of our body, the connections between our nerves, and the specific location of hormone receptors throughout our tissue -- is designed to respond to specific experiences, or nurture. In other words, we're designed like an elevator, waiting for our buttons to get pushed before we act. We are designed to respond to specific types of experiences, and we ignore the rest, as an elevator reacts to a pushed button but ignores strong smells.
Pushing a button is a momentary act, yet the elevator has special circuitry that "remembers" the decision, and also ignores us if we accidentally push the button twice in a row. In electronics, that "memory" circuitry is known as a switch, or latch. A single experience has a lasting effect. In the human body, permanent changes in genetic expression can result after receiving the right stimulus (i.e. for which they were designed to be affected).
Some human genes act as switches or latches. A single experience can be "remembered" by temporarily or permanently switching off a gene, which may set in motion a complex, one-way, irreversible developmental process. In other words, a latched gate swings closed behind you, cutting off your option to return.
That's another reason genetically identical twins aren't exactly the same in appearance or mannerisms. One twin may have an experience for which a genetic latch (which they both share) was designed to be permanently closed. If the other twin does not have that same experience, his latch remains open. In the same manner, two elevators may have an identical design (nature), but are sent to opposite floors because different buttons were pressed (nuture). In this case, the changes are not permanent, however -- it just depends on the design.