Epigenetics is the study of or the actual influence that occurs to effect a change in a cell which is not a message from our bodies DNA.
It literally means over-genetics.
"stably heritable phenotype resulting from changes in a chromosome without alterations in the DNA sequence"
To make that more digestible here is an imagined example.
Identical twins are separated at birth. They meet again in their fifties. (identical twins have exactly the same DNA)
Twin A was brought up in poverty but had worked as a labourer most of his life while Twin B was brought up in a suburban middle class and has an office job.
Poorer Twin A is in good physical shape but because he did not eat well, particularly as a child, his height has been affected. Suburban Twin B has been at his desk job for 20 years and eats a lot of fast food so he might be taller but is probably over weight.
Despite the fact that they now look quite different a geneticist will find that their DNA is still identical.
Imagine you gave two children exactly the same amount and shaped lego bricks and asked them to build a person. While the bricks are identical each child will create a different type of person.
Epigenetics doesn't change your DNA but it decides how much or whether some genes are expressed as the different cells in your body. The study of Epigenetics looks at the cells over the course of your life to see whether those changes might be passed down to your children or grandchildren.
Your DNA carries the instructions telling the body which cells should become what different parts of the body. But your DNA alone does not make these decisions and this is where epigenetics comes in.
Methyl groups, made of carbon and hydrogen atoms, attach themselves to your cell's DNA activating the switches within the DNA which basically tells them which type of cell it is supposed to be i.e. a liver cell as opposed to a heart cell etc.
Epigenetic influences are also controlled by histones. These are proteins around which your DNA is wrapped. If the DNA strands are loosely wrapped around the histones your gene expression is given more freedom as opposed to the constricted gene expression of tightly wrapped DNA.
For example, if the genes governing* the production of melanin (the natural pigment that decides skin tone, hair and eye colour) was tightly wrapped because of an epigenetic influence you would have an iris that was genetically supposed to be for example brown but instead is for example blue because the melanin never was produced.
Taking DNA as the set of lego bricks we talked about would mean that the children are the Methyl Groups and Histones because they decide which brick is to be used for which part of your body.
But these epigenetic markers can change throughout your life. For instances during puberty and pregnancy the epigenetic markers burst into activity and so allow the body to react in a suitable way.
Epigenetics can also change because of environmental influences. Our lifestyles, our nutrition intake (or lack thereof), whether we smoke and even our stress levels can all affect a person on an epigenetic level. What happens on a cellular level is that any of these lifestyle factors can cause a methyl group bind to the wrong place on our DNA giving the wrong instructions which can lead to an overproduction of cells that were never meant to be created e.g. Cancer.
Hereditary Epigenetics has shown to us that the lifestyle decisions we make can alter the way our genes operate and abnormalities can be passed down to our children and grandchildren. Now that Geneticists have more of an understanding of Epigenetics they are working on ways of switching off or turning down these effects that we have inherited from our ancestors.
*Eye colour is influenced by more than one gene. The actual number of genes that contribute to eye colour is currently unknown. There is evidence that as many as 16 different genes could be responsible for eye colour in humans; however, the main two genes associated with eye colour variation are OCA2 and HERC2, and both are localized in Chromosome 15.