Introduction

Abstract

What is epigenetics? This book emphasizes epigenetic control of gene expression and development, so a useful definition is: The study of mitotically and/or meiotically heritable changes in gene function that cannot be explained by changes in DNA sequence. In both plant and mammalian tissue culture, for example, it is known that a significant percentage of clonally derived variants are not true mutations because they can be efficiently reverted by agents that inhibit enzymatic DNA methylation (see Riggs and Jones 1983; Harris 1986; Meins). Stable epigenetic control of gene function also is an essential component of normal development of most, and perhaps all, complex organisms, especially mammals and higher plants, which have unending division of differentiated cells. The epigenetic changes that take place during differentiation are normally erased in the germ line, but there are examples in both plants and transgenic mice of epigenetic variants that are transmitted through meiosis (see McClintock 1965; Sapienza 1990; Chaillet 1994; and the paper by Brink et al. reprinted at the end of this volume). This book provides a survey through early 1996 of many of the epigenetic systems now being studied.

Most research since the advent of genetics – especially in the post-Watson and Crick era – has focused on the study of (and use of) true mutations. A search of the National Library of Medicine literature database revealed that in the late 1960s and early 1970s only about 3 papers per year used the term epigenetics. However, there has been an accelerating increase in papers...