There's a gene for that!

Many people say the human body and mind are so complex, there is no way to understand them in terms of simple causes and effects. But there are two common examples where a single genetic "switch" can control complex behavior or development:

• Broadcast commands (with hormones)
• Master genes

Broadcast commands (with hormones)

One way to send a signal throughout the body is to release a hormone into the bloodstream. Throughout the body, the "troops" (i.e. hormone receptors) are listening and waiting.  They are trained to respond to this "command", like soldiers with radio receivers waiting for an order.

Releasing the hormone (often done by the brain) is a singular act. The complex part (having receptors pre-arranged throughout the body that are specially trained to respond to the hormone) was established well in advance, like soldiers who are trained and strategically positioned across a battlefield. They listen for commands (with their ears or "signal detectors"), and ignore any other commands (i.e. other hormones) for which they were not trained to respond. A single command (or signal) such as "Division 1, charge!" sets in motion a complex, coordinated behavior.

Setting up a complex system of receptors (ahead of time) in diverse locations allows a simple signal to trigger a complex response.

Master genes

Some genes are the generals of our development as well. The SRY gene, for example, issues a single command for the developing human body to develop into a male. This command is broadcast for a few hours only - before birth (as studied in mice) - then switches off forever.

Of course, the SRY master switch triggers a cascade of other genetic activity throughout the body, the same way that a general's order may have far-reaching consequences. In other words, the simplicity of the command is made possible by the prior "training" of the troups. Even though male development (or commanding an army) is an extremely complex process, it can be conducted using simple commands.

The strategic distribution of the receptors (soldiers), their pre-existing ability to recognize and respond to the signals, and the resulting cascade of activity once activated, are the truly complex aspects of this. But the general (or hormone, or master gene) who issues the simple command is the one who gets the glory of launching the battle.