Epigenetics refers to the study of how environmental factors influence gene expression in a manner  independent from the nucleotide sequence.   

Examples of epigenetic modifications to the genome include: DNA methylation, phosphorylation, acetylation, ubiquitylation, and alteration of the chromatin structure [1].  

These changes can both positively and negatively impact the expression of targetted genes.  The best understood mechanism of epigentic gene regulation involves the addition or removal of methyl groups on consecutive cytosine residues by methyltransferases, where their addition often inhibits the recruitment of transcriptional machinery.    

Epigenetic mechanisms

Epignetic Mechanisms

Similiarly, the addition or removal of acetyl groups to the tails of histone octomers alters the llevel of DNA compaction in the chromatin structure.  This can either block the recognition sequences of  transcription factors to repress transcription of a specific gene, or increase their exposure to enhance gene transcription.

In some instances, these changes are heritable and can be retained for various periods of time ranging from several rounds of cell division within an organism to multiple generations[2].