The big idea: Your DNA sequence is fixed for life — yet which genes are actually switched on or off can change. The environment is one of the things that does the switching.
What you eat, the stress you meet, toxins you are exposed to and even the temperature you grow in can add or remove epigenetic marks — chemical tags (like methyl groups) sitting on top of the DNA.
Those marks change gene expression without changing a single letter of the DNA. So the same genotype can give a different phenotype depending on the environment and on a cell's history.
An environmental signal (diet, stress, a toxin, temperature) can add methyl (–CH₃) marks to the DNA, packing it tightly so RNA polymerase cannot bind — the gene is switched OFF. Compare the two panels: the DNA SEQUENCE is identical in both, only the marks on top of it differ. That is the whole idea of epigenetics — the environment changes expression WITHOUT changing the genotype.
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- Gene expression
- Whether a gene is switched on (transcribed and used) or off (silenced) in a particular cell.
- Epigenetic mark
- A chemical tag added on top of the DNA (e.g. a methyl group) or its histones that changes expression without changing the base sequence.
- Methylation
- Adding methyl (–CH₃) groups to DNA. This usually packs the DNA tightly and switches the gene OFF.
- Genotype
- The DNA base sequence an organism carries — its actual genes.
- Phenotype
- The observable characteristics an organism shows, set by which genes are expressed.
- Environment
- The external and internal conditions a cell meets — diet, stress, toxins, temperature — that can alter epigenetic marks.
Marks sit ON the DNA, not IN it: Picture the DNA sequence as the text of a book and the epigenetic marks as sticky notes and highlighter on the pages.
The words never change — but the notes decide which pages get read and which get skipped.
That is why epigenetics changes expression without changing the genotype.
Read this as a chain of cause and effect: an environmental signal reaches a cell, the cell adds or removes epigenetic marks, and those marks change which genes are expressed — giving a different trait, all from the same DNA.
Environment → epigenetic mark → changed expression
- An environmental factor acts on the cell — for example a change in diet, a burst of stress, a toxin, or the temperature during development.
- The cell responds by adding or removing epigenetic marks (e.g. methylating a stretch of DNA, or modifying its histones).
- Those marks change how tightly the DNA is packed, so they switch a gene on or off — they change gene expression.
- The result is a different phenotype even though the DNA sequence is unchanged — the genome has interacted with the environment.
| Environmental factor | Example of how it acts | Epigenetic outcome |
|---|---|---|
| Diet | A nutrient supplies the methyl groups used to mark DNA | More (or fewer) genes silenced by methylation → a changed trait |
| Stress | Stress hormones reach cells and alter which marks are placed | Genes for the stress response are turned up or down |
| Toxins | A pollutant or chemical changes the pattern of marks | A gene is wrongly silenced or switched on |
| Temperature | The growth temperature sets the pattern of marks early in development | A temperature-dependent trait (e.g. coat colour, sex in some reptiles) |
Epigenetic marks are inherited — through mitosis: When a cell divides by mitosis, it copies its epigenetic marks onto both daughter cells along with the DNA.
This is how a differentiated state is maintained: once a liver cell has switched the right genes on and off, all its descendants stay liver cells — they inherit the same pattern of marks.
So epigenetic marks are heritable through cell division, which is exactly why a single environmental event early on can have a lasting effect on a tissue.
Sometimes across generations — but always reversible: Most marks are wiped and reset when gametes form, but some escape the reset and can be passed to offspring — so an environmental effect can occasionally show up in the next generation.
Crucially, epigenetic changes are reversible: a mark can be removed as easily as it was added, and the gene goes back to its original state.
And they never change the DNA base sequence. This is the line that separates epigenetics from a mutation — a mutation rewrites the letters of the DNA and is normally permanent.
Real examples to quote
- Identical twins. Twins start with the same DNA but diverge over time — by middle age their pattern of methylation differs, because they ate, stressed and lived differently. Same genotype, different epigenome → different traits and disease risks.
- Diet sets coat colour in mice. In a well-known mouse strain, mothers fed a methyl-rich diet have pups whose coat-colour gene is methylated (silenced) — the pups are brown and lean; genetically identical pups on a plain diet are yellow and obese. Same genes, the diet flipped the switch.
- Temperature sets traits. In some plants and reptiles the temperature during development sets the epigenetic marks, fixing a trait such as flowering time or even sex — the same genome gives different outcomes at different temperatures.
The one-line summary: Epigenetics explains how one genotype can give many phenotypes — the environment and a cell's history decide which genes are on, the marks are inherited through mitosis (and sometimes further), yet they are reversible and leave the DNA sequence untouched.
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How this is tested: The classic HL question asks you to distinguish an epigenetic change from a mutation — so always hit the three contrasts: epigenetics changes the marks (not the base sequence), is reversible, and is environmentally influenced, whereas a mutation rewrites the DNA sequence and is usually permanent.
You may also be asked to explain how the environment changes a phenotype without changing the genotype (environment → marks → expression → phenotype), or to use a named example (identical twins diverging, the diet-and-coat-colour mice, a temperature-set trait).
A frequent trap is calling an epigenetic change a mutation — examiners penalise this. Say the base sequence does not change.
IB-style question — epigenetic change versus mutation
Two laboratory mice are genetically identical, but one has a yellow coat and one has a brown coat. The difference is caused by the diet the mothers were fed during pregnancy. Explain, using ideas about epigenetics, how the mice can differ when their DNA is the same — and distinguish this from a mutation. [6]
How to score all six marks
- Same genotype. The two mice have the same DNA base sequence (same genotype), so a difference in gene expression must explain the different coats.
- Environment acts. The diet of the mothers is an environmental factor that altered the epigenetic marks on the pups' DNA.
- The mark. A methyl-rich diet causes methylation of the coat-colour gene, which packs the DNA tightly and switches that gene off (silences it) in the brown mouse.
- Expression → phenotype. Because the gene is expressed differently (on in one mouse, off in the other), the two mice show a different phenotype — yellow vs brown — from the same genes.
- Not a mutation. This is NOT a mutation: the DNA base sequence is unchanged. Only the marks on top of the DNA differ.
- Reversible / heritable. The change is reversible (the marks can be removed) and is inherited through mitosis to the cell's descendants — unlike a mutation, which rewrites the sequence and is usually permanent. (Award 1 mark per distinct point, up to 6.)
Final answer
The mice share the same DNA (genotype), so the coat difference is a difference in gene expression, not in the genes. The mothers' diet (an environmental factor) added methyl marks to the coat-colour gene in one pup, silencing it, so that pup expresses the gene differently and shows a different phenotype. This is not a mutation: the base sequence is unchanged — only the epigenetic marks differ. The change is reversible and is inherited through mitosis, whereas a mutation changes the DNA sequence and is usually permanent.
✓ Why this scores full marks: It separates genotype (the DNA, identical) from phenotype (the coat, different), names the environmental trigger (diet) and the mark (methylation silencing the gene), and nails the distinction from a mutation — the base sequence does not change, the change is reversible, and it is inherited through mitosis.
The easiest mark to drop is the last one: if you forget to say the DNA sequence is unchanged, the answer reads like a mutation and loses the key contrast.
| Epigenetic change | Mutation | |
|---|---|---|
| What changes | The MARKS on the DNA (e.g. methyl groups) or on the histones — not the letters themselves | The DNA BASE SEQUENCE itself (the actual A, T, C, G letters) |
| Does the base sequence change? | NO — the genotype is untouched | YES — the genotype is permanently altered |
| Reversible? | YES — marks can be added and removed | Usually NO — the change is permanent (unless repaired/reverted) |
| Triggered by the environment? | Often — diet, stress, toxins and temperature can add or remove marks | Sometimes (mutagens), but many mutations are random copying errors |
| Heritable? | Through MITOSIS to daughter cells (keeps a cell's identity); SOMETIMES across generations | Through mitosis, and through meiosis to offspring if it is in a gamete |
| Effect | Same DNA gives a DIFFERENT phenotype (a gene is switched on or off) | A new or altered DNA sequence → possibly a new allele / protein |