Epigenetics literally means "above" or "on top of" genetics. It refers to external modifications to DNA that turn genes "on" or "off." These modifications do not change the DNA sequence, but instead, they affect how cells "read" genes.
Examples of epigenetics
Epigenetic changes alter the physical structure of DNA. One example of an epigenetic change is DNA methylation — the addition of a methyl group, or a "chemical cap," to part of the DNA molecule, which prevents certain genes from being expressed.
Another example is histone modification. Histones are proteins that DNA wraps around. (Without histones, DNA would be too long to fit inside cells.) If histones squeeze DNA tightly, the DNA cannot be "read" by the cell. Modifications that relax the histones can make the DNA accessible to proteins that "read" genes.
Epigenetics is the reason why a skin cell looks different from a brain cell or a muscle cell. All three cells contain the same DNA, but their genes are expressed differently (turned "on" or "off"), which creates the different cell types.
It may be possible to pass down epigenetic changes to future generations if the changes occur in sperm or egg cells. Most epigenetic changes that occur in sperm and egg cells get erased when the two combine to form a fertilized egg, in a process called "reprogramming." This reprogramming allows the cells of the fetus to "start from scratch" and make their own epigenetic changes. But scientists think some of the epigenetic changes in parents' sperm and egg cells may avoid the reprogramming process, and make it through to the next generation. If this is true, things like the food a person eats before they conceive could affect their future child. However, this has not been proven in people.
Epigenetics and cancer
Scientists now think epigenetics can play a role in the development of some cancers. For instance, an epigenetic change that silences a tumor suppressor gene — such as a gene that keeps the growth of the cell in check — could lead to uncontrolled cellular growth. Another example might be an epigenetic change that "turns off" genes that help repair damaged DNA, leading to an increase in DNA damage, which in turn, increases cancer risk.