Same code, different readers
Almost every cell in your body carries the same DNA and the same genotype. A liver cell and a brain cell are reading the identical instruction book. Yet they look different, make different proteins, and do completely different jobs. The reason is that each cell type switches a different set of genes on and off — and remembers that choice as it divides.
Epigenetics is the study of this second layer: heritable changes in gene expression that do not alter the DNA sequence itself. If the genome is the text, the epigenome is the set of highlighter marks, sticky notes, and folded-down corners that tell the cell which pages to read now and which to skip.
What counts as an epigenetic mark
An epigenetic mark is a chemical tag added to DNA or to the proteins it wraps around, which changes how easily a gene can be read — without touching the A, T, G, C sequence. Two big families dominate: DNA methylation (a small chemical group placed on the DNA itself) and histone modification (tags on the histone proteins that DNA coils around). Together, across the whole genome, these marks form the epigenome.
The decisive test is reversibility plus heritability. A true epigenetic change is stable enough to be copied when a cell divides (so a liver cell makes liver cells), yet reversible in principle — unlike a mutation, which permanently rewrites the sequence. This is why the same fixed genotype can produce many different cell-level phenotypes.
GENOME (the text) epi-MARKS (the annotations) ...A T G C G C A T... methyl group on a C -> page "closed" ...A T G C G C A T... histone tag (active) -> page "open" Same sequence in liver and neuron. Different marks -> different genes read -> different cell.