Epigenetics is the study of how the environment and other factors can change the way that genes are expressed. While epigenetic changes do not alter the sequence of a person's genetic code, they can play an important role in development. Scientists who work in epigenetics explore the mechanisms that affect the activity of genes.

SOMETHING about EPIGENETICS here, this is taken from:
https://www.niehs.nih.gov/health/topics/science/epigenetics/index.cfm
For decades, scientists have known the basic structure of our DNA, the building blocks that make up our genes. Although nearly every cell in the human body has the same set of genes, why is it that different types of cells, such as those from brain or skin, look and behave so differently?

The answer is epigenetics, a rapidly growing area of science that focuses on the processes that help direct when individual genes are turned on or off. While the cell’s DNA provides the instruction manual, genes also need specific instructions. In essence, epigenetic processes tell the cell to read specific pages of the instruction manual at distinct times.

Some epigenetic changes are stable and last a lifetime, and some may be passed on from one generation to the next, without changing the genes.

Several epigenetic processes involve chemical compounds that attach, or bind, to DNA or to proteins that package the DNA within cells called histones. When a chemical compound binds to DNA, certain genes switch on or off, selecting which proteins are made.

For example, the epigenetic process of DNA methylation involves the binding of a chemical compound called a methyl group to certain locations on the DNA. This binding changes the structure of DNA, making genes more or less active in their role of making proteins.

Another process called histone modification involves chemical compounds that bind to histone proteins. Ribonucleic acids, or RNAs, are also present in cells and can participate in epigenetic processes that regulate the activity of genes.

DNA methylation and histone modification are normal processes within cells and play a role in development, by instructing stem cells, or cells capable of turning into more specialized cells, like brain or skin cells.

Epigenetic processes are particularly important in early life when cells are first receiving the instructions that will dictate their future development and specialization. These processes can also be initiated or disrupted by environmental factors, such as diet, stress, aging, and pollutants.

In 2005, a team of Italian researchers provided the first concrete evidence for the role of environmental epigenetics in explaining why twins with the same genetic background can have vastly different disease susceptibilities.1  The researchers showed that, at birth, pairs of identical twins have similar epigenetic patterns, including DNA methylation and histone modifications.

However, over time, the epigenetic patterns of individuals become different, even in twins. Since identical twins are the same genetically, the differences are thought to result from a combination of different environmental influences that each individual experiences over a lifetime.