PhowaBoy is definitely right here.

Another example; puberty. During the preteen years a gene is (naturally) activated that stimulates the production of various hormones (testosterone, estrogen, etc). This is an event that takes place naturally, but can be 'triggered' by certain outside stimuli.

It's not as if rabbits consciously activate brown fur, or humans decide to go into puberty. But it's still an example of activation/deactivation of genetic code.

EDIT: to be clear, the genetic material for such events is present at birth. When the gene is 'activated', this means that the DNA begins to be translated into RNA and then transcribed into proteins, which in turn do the work.