The proteins that organize DNA, called histones, can be chemically modified in a variety of ways. The modifications have the effect of turning on/off stretches of the DNA simply by modifying its physical structure (the way the dna is arranged and packed around the histones). There are tons more mods that can happen which have a variety of effects (for example, tune the amount that a gene is expressed). This is called epigenetics if you want to google more.

If you've heard of those studies where if your grandparents experienced a famine it makes some difference in your genetics, one hypothesis is that its caused by this.

This doesn't sound "commented" at all. It sounds like you are talking about conditional statements/loops.