Here’s a simply analogy: Decades ago, television stations used to broadcast a circular test pattern after regular programming had ended for the day. Lets say that our genes are like that test pattern that is being broadcast. The model of epigenetics, then allows for control through all the dials and buttons on the television set. I can turn the television on and off. I can raise volume. I can change the color, the tint, the contrast, the horizontal, the vertical – I can change every one of those, yet in doing so, I did not change the original broadcast. No, I only changed the readout of the broadcast, and those variations are potentially unlimited.
The distinction between what is controlled by our genes and what is controlled by epigenetic selection is defined early in human development. Human development is divided up into two phases–the first is the embryo phase, followed by the fetal phase. The fetal phase is recognized When the embryo acquires human characteristics and looks like a human it is then called a fetus. The embryological stage extends from the single fertilized egg cell through all the early little morphings and shape changes until it reaches the fetal stage.
Our genes are the fundamental programmers that take the fertilized egg to the stage to when looks like a human. By then, the gene program has laid out of the human body plan with two arms, two legs and nose, eyes, etc. From the fetal stage on, the modifications are now epigenetically controlled, meaning the environment influences them.
For example, once the human form is made epigenetic changes can control whether development leads to big strong muscles and arms for fighting to a bigger brain for thinking. Those decisions are based upon environmental information at the time that the fetus is developing. Sperm and eggs are generic. They form a human but they don’t determine if that human is going to be born in Bosnia or Zimbabwe or Iowa. Each of those environments requires a different physiology to survive in. For instance, if survival is threatened then the physiology of the body changes to create a body that will withstand that threat. Information from the environment is very critical in shaping the expression of the genes that have already been used in the construction of the human body plan.