The term epigenetics was introduced in the 1940s by Conrad Waddington, who defined it as: ‘‘the branch of biology which studies the causal interactions between genes and their products which bring the phenotype into being.’’ However, with the growth and evolution of genetics, the term gained a narrower and more specific sense.

Today, epigenetics is widely known and referred to as:

‘‘the study of changes in gene function that are mitotically and/or meiotically heritable and that do not entail a change in DNA sequence.’’

Common epigenetic modifications include: (the most common being methylation)

1. Histone variants
2. Posttranslational modifications of amino acids on the amino-terminal tail of histones
3. Covalent modifications of DNA bases

Besides covalent modifications of DNA, histones and their posttranslational modifications have also been implicated in the organization of chromatin structure and regulation of gene transcription.

Short article about epigenetics and the controversy surrounding its definition here.

Sources:

Dupont, Armant and Brenner. 2009. Semin Reprod Med. Epigenetics: Definition, Mechanisms and Clinical Perspective. 27 (5): 351-7.

Wu and Morris. 2001. Science. Genes, genetics and epigenetics: A correspondence. 293 (5532): 1103-1105.