All the children of a couple have the same DNA, half comes from the mother and half comes from the father and once they combine a new DNA structure is generated. It should be clear that the structure of the genetic code is fixed from conception, but the activity levels of each gene can be raised or lowered in the early stages of embryonic development and epigenetics is the science that studies how these levels are modified.
One of the greatest discoveries has been that genes are not independent but can influence each other and, furthermore, the environment is an important factor in genetic activity. The clearest example of this influence occurred in what we know as “the winter of hunger” in Holland. Almost at the end of the Second World War, the scarcity and famine due to the conflict reached levels in which pregnant women were fed with a quarter of the daily requirements. This famine told the fetuses that the world was a place of limited resources, so the genes that regulated the height of babies were reduced and those that regulated the ability to transform food into fat were raised, due to this that generation of Dutch had a lower height and greater obesity problems, But what is even more impressive is that the children and grandchildren of that generation have continued to develop these conditions 70 years later, even though the mothers have had completely healthy pregnancies.
As we can see, the structure of the genes was not altered, but extreme changes in the environment modified the levels of genetic activity that were even inherited from generation to generation.