What is epigenetic control of gene expression? How does it occur?

Strictly speaking, it is the heritable changes in the function of your genes that do not involve a change in your DNA's base sequence. Simply put, it is the way in which we regulate whether a gene is transcribed or not, or how often it is transcribed. There's two overall ways we can do this:1) Methylate the DNA. This is usually done by adding a methyl group (-CH3) to a cytosine in your DNA base sequence. This results in a silencing effect i.e. the gene is less expressed.2) Histone acetylation. Histones are positively charged proteins around which the negatively charged sugar-phosphate backbone of the DNA wraps around, resulting in compaction of the DNA. The more positively charged the histone is, the stronger the DNA binds, and the less the gene is expressed. If we add acetyl groups to the histones, it reduced their positive charge, thus reducing how tightly the DNA binds. This results in a more frequently transcribed gene.