Epigenetics
Epigenetics is the layer of marks on the DNA and its packaging that switches genes on or off without changing the sequence itself. DNA methylation and histone modifications are the main marks. They respond to environment and age and can differ even between identical twins.
Marks on the genome
The most common mark is DNA methylation: small chemical tags on certain bases that usually turn a gene down or off entirely. In addition, the packaging of DNA around histone proteins determines whether a section is open and readable or tightly closed. Both mechanisms change how strongly a gene is used without altering a single letter.
Response to environment and age
Unlike the sequence, the epigenetic pattern is changeable. Diet, smoking, stress and above all age leave traces. From methylation patterns one can even read a biological age quite accurately, a so-called epigenetic clock. Identical twins start with an identical sequence but increasingly differ in their marks over the course of life.
What genotyping does not see
Genome determines the sequence, that is which bases are present. It does not read the methylation and histone marks; that needs other methods. One can picture it like this: the sequence is the script, the epigenetics is the direction that decides which scenes are played. Genome shows the script.
What Genome measures. Genome reads the DNA sequence, not its epigenetic marks. This article explains a layer that genotyping does not capture.
Related topics
Sources
- 1Jaenisch & Bird, 2003 Epigenetic regulation of gene expression: how the genome integrates intrinsic and environmental signals. Nature Genetics 33:245–254. doi.org/10.1038/ng1089
- 2Fraga et al., 2005 Epigenetic differences arise during the lifetime of monozygotic twins. PNAS 102:10604–10609. doi.org/10.1073/pnas.0500398102