The term epigenetics has over time been used in various senses, in part because the Greek prefix epi- has at least six meanings in English (including 'on', 'after' and 'in addition'), but also because various theories of epigenetic development, inheritance, and evolution have been proposed (see Historical notes below).
In biology today, epigenetics has two closely related meanings:
In both cases, the object of study includes how gene regulatory information that is not expressed in DNA sequences is transmitted from one generation (of cells or organisms) to the next - that is (harking back to the Greek prefix), 'in addition to' the genetic information encoded in the DNA.
In recent years, there has been rapid progress in understanding epigenetic mechanisms, which include differences in DNA methylation, as well as difference in chromatin structure. Another possibility involves the genomes of cytoplasmic elements (chloroplasts and mitochondria). Other mechanisms have also been proposed. See epigenetic inheritance for a more detailed discussion.
The epigenome is the overall epigenetic state of a cell. As one embryo can generate a multitude of cell fates during development, one genome could be said to give rise to many epigenomes.
Some biologists at one time believed that genetics, which seemed to postulate a one-to-one correspondence between genotype and phenotype, could not explain cell differentiation. They developed a theory that each undifferentiated cell underwent a crisis that determined its fate, which was not inherent in its genes, and was therefore (borrowing from the Greek) epigenetic.
The psychologist Erik Erikson developed an epigenetic theory of human development which focuses on psycho-social crises. In Erikson's view, each individual goes through several developmental stages, the transition between each of which is marked by a crisis. According to the theory, although the stages are largely predetermined by genetics, the manner in which the crises are resolved is not; by analogy with the epigenetic theory of cell differentiation, the process was said to be epigenetic.
The biologist C.H. Waddington is sometimes credited with coining the term epigenetics in 1942, when he defined it as “the branch of biology which studies the causal interactions between genes and their products which bring the phenotype into being”. However the term goes back at least to 1896 (see References).