This description by David Shenk may help you better understand the impact of epigenetics on genetics. Genes are not like robot actors who always say the same lines in the exact same way. It turns out that they interact with their surroundings and can say different things depending on whom they are talking to. This obliterates the long-standing metaphor of genes as blueprints with elaborate predesigned instructions for eye color, thumb size, mathematical quickness, musical sensitivity, etc. Now we can come up with a more accurate metaphor. Rather than finished blueprints, genes—all 22,000 of them—are more like volume knobs and switches. Think of a giant control board inside every cell of your body. Many of those knobs and switches can be turned up/down/on/off at any time—by another gene or by any miniscule environmental input. This flipping and turning takes place constantly. It begins a moment a child is conceived and doesn’t stop until she takes her last breath. Rather than giving us hardwired instructions on how a trait must be expressed, this process of gene-environment interaction drives a unique developmental path for every unique individual.
(Shenk, David. The Genius in All of Us. P 16. NY:Doubleday, 2010)