Free explainer3 min read
Epigenetics
Understand the model, recognise it in a stem, separate the look-alikes, then apply it.
Start with the mental model
Epigenetics matters because it explains how environment and experience change which genes are switched on or off, without changing the underlying DNA sequence itself. Epigenetic changes are chemical modifications, such as DNA methylation (adding a methyl chemical tag to DNA) and histone modification (chemically altering the proteins DNA is wrapped around), that switch genes on or off. For example, imagine identical twins who share the exact same DNA sequence but develop different psychiatric outcomes after very different life experiences: differing epigenetic marks likely explain part of that difference. In practice, this gene-environment interaction is a major reason two people with identical genetic risk can have very different outcomes.
Visual explanationUse the diagram to rebuild the concept from memory.
01Core model
DNA methylation typically silences a gene by adding methyl chemical tags to it, while histone modification changes how tightly DNA is packaged, making genes more or less accessible for reading. Neither process alters the underlying DNA sequence, unlike a genetic mutation. Genomic imprinting is a specific epigenetic phenomenon in which only one copy of a gene, inherited from either the mother or the father, is normally active, while the other is epigenetically silenced. A deletion on chromosome 15 causes Prader-Willi syndrome when the father's copy is lost, since only the paternal copy is normally active there. The same deletion causes the clinically very different Angelman syndrome when the mother's copy is lost instead. Say two children share an identical chromosome 15 deletion: one inherited it paternally and has Prader-Willi syndrome, while the other inherited it maternally and has Angelman syndrome. In practice, which parent a gene comes from, not just which gene is affected, can determine the entire clinical picture.
02Recognise it in an SBA
• In the rat studies of maternal care, what changed in pups that received more licking and grooming?
• What did studies of people conceived during a wartime famine demonstrate?
• Why is caution needed before claiming that an environmental exposure has transgenerational effects in humans?
03Separate the look-alikes
| Epigenetic change | alters gene expression without changing the DNA sequence, unlike a genetic mutation which changes the sequence itself. |
|---|---|
| DNA methylation | typically silences a gene, whereas histone modification changes how accessible DNA is for reading more generally. |
| Prader-Willi syndrome | results from losing the paternal chromosome 15 copy, unlike Angelman syndrome which results from losing the maternal copy. |
| Genomic imprinting | only one parent's gene copy is normally active, rather than both copies contributing equally as with most genes. |