In eukaryotic cells, changes in chromatin structure that affect gene expression (discussed in Section 12.3) are at least partly responsible for the phenomena of epigenetics, which we discussed in Chapters 4 and 8.

The term epigenetics comes from the Greek root epi, which means “over” or “above”; the term has come to represent the inheritance of variation above and beyond differences in DNA sequence. Epigenetics usually refers to phenotypes and processes that are transmitted to other cells and sometimes to future generations, but are not the result of differences in the DNA base sequence. Many epigenetic effects are caused by changes in gene expression that result from alterations to chromatin structure or other aspects of DNA structure, such as DNA methylation. Some have broadened the definition of epigenetics to refer to any alteration of chromatin or DNA structure that affects gene expression. Here, we will use the term epigenetics to refer to changes in gene expression or phenotype that are potentially heritable without alteration of the underlying DNA base sequence.

Many epigenetic changes are stable, persisting across cell divisions or even generations. However, epigenetic alterations can also be influenced by environmental factors. The fact that epigenetic traits may be induced by environmental effects and transmitted to future generations has been interpreted by some to mean that through epigenetics, genes have memory—that environmental factors acting on individuals can have effects that are transmitted to future generations. Epigenetics has been called “inheritance, but not as we know it.” TRY PROBLEM 37