The word 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. It has also been described as heritable changes in gene function that occur without a change in the nucleotide sequence. Epigenetic modifications can be passed from one cell generation to the next (mitotic inheritance) and between generations of a species (meiotic inheritance).
In plants, it is well established that epigenetic modifications can be inherited from one generation to the next. 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. In parallel to the term “genome” that defines the complete set of genetic information contained in the DNA of an organism, “epigenome” generally refers to the complete set of characteristics of epigenetic pathways in an organism. The epigenome can be thought of as a second code overlaid on top of the DNA sequence code of the genome.
Epigenetic Mechanisms Researchers have identified four types of epigenetic pathways: DNA methylation, histone modification, nucleosome remodelling, and non-coding RNA-mediated pathways. These epigenetic pathways intertwine with each other to regulate expression of genes, and it is likely that other pathways beyond these four known ones will be discovered in the future.