epigenetics
/ ep-ih-jeh-NEH-tiks /
Two identical twins share the same DNA letter for letter, yet one may develop a disease the other never does, and a caterpillar and the butterfly it becomes carry the same genome while looking nothing alike. Something beyond the DNA sequence is shaping how those genes behave. That something is the subject of epigenetics: heritable changes in how genes are expressed that do not involve any change in the DNA sequence itself.
The 'epi-' means 'on top of' — epigenetics is the layer of marks and states sitting on top of the DNA that tells the cell which genes to keep on and which to keep off, without rewriting the genes. The main molecular carriers are DNA methylation, where small methyl groups are added to cytosine bases (especially at CpG sites) and tend to silence genes, and the covalent histone modifications of the histone code, which open or compact chromatin. 'Heritable' here has two senses, and they should be kept apart. The clear, well-established sense is mitotic heritability: when a cell divides, it copies not only its DNA but also many of these marks, so a liver cell's daughters stay liver cells — the cell remembers its identity. The marks are laid down by writer enzymes, maintained through division by maintenance enzymes, and read by reader proteins, and they can be reversed, which is what makes them a flexible memory rather than a permanent edit.
Epigenetics matters because it is how one genome produces and maintains many cell types, how cells remember decisions made during development, and how environment can leave a lasting mark on gene activity within a lifetime. But the field is also surrounded by hype, so honesty is essential. Most epigenetic marks are erased and reset between generations; true inheritance of acquired epigenetic states from parent to offspring through the germ line is rare in mammals and must be demonstrated case by case, not assumed. 'Epigenetic' does not mean 'inherited from your ancestors' lifestyle' — it usually means 'a reversible mark on chromatin that controls a gene', most of which is set and reset within an organism, not passed down.
A liver cell and a neuron carry identical DNA but very different patterns of DNA methylation and histone marks; when each divides, it copies its own pattern, so liver cells beget liver cells and neurons stay neurons — cell identity is an epigenetic memory.
Marks on top of the DNA tell a cell what to be — and the cell copies them when it divides.
Beware the popular overreach. Most epigenetic marks are reset each generation; genuine transgenerational inheritance through the germ line is the exception in mammals, not the rule. 'Epigenetic' mostly means a reversible mark controlling a gene within a body, not a memory of an ancestor's experience.