[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Epidemiology](https://durgical.com/en/epidemiology/) › [Bias, confounding and causation](https://durgical.com/en/epidemiology/areas/bias/) › Gene-environment interaction

# Gene-environment interaction

- Concept

An exposure harms people with a certain gene variant far more, or a gene only causes disease when an exposure is present.

Updated Nov 13, 2026

## Key points

- Classic examples: phenylketonuria and dietary phenylalanine; alpha-1 antitrypsin deficiency and smoking; G6PD deficiency and oxidants (fava beans, some drugs).
- Analysed with a 2 × 4 table: genotype (yes/no) × exposure (yes/no), with a reference group that has neither.
- Case-only design: estimates multiplicative interaction if genotype and exposure are independent in the population.
- Needs large samples and good exposure measurement; many findings have not been replicated.
- A strong interaction lets prevention (avoiding the exposure) target carriers of the genotype.

On this page

- The basics Key points
- What it is
- Worked example
- Markers
- In practice
- Common pitfalls
- Memory aid
- Elsewhere in DurgiPedia

- Safety Examples of gene-environment interaction

- Study Test yourself

- Connections Connections

## What it is

Gene-environment interaction exists when the effect of an exposure on disease risk depends on genotype, or the effect of a genotype depends on exposure. It is a form of effect modification assessed with a 2 × 4 table (genotype × exposure) and read on the multiplicative or additive scale. It explains why the same exposure affects people differently and underlies the idea of personalised prevention.

## Worked example

- Case-control study, reference with neither variant nor exposure: 50 cases and 200 controls.
- Variant only: 20 cases and 80 controls: OR = (20/80)/(50/200) = 0.25/0.25 = 1.0.
- Exposure only: 60 cases and 120 controls: OR = 0.5/0.25 = 2.0.
- Variant and exposure: 60 cases and 40 controls: OR = 1.5/0.25 = 6.0.
- Expected with no multiplicative interaction: 1.0 × 2.0 = 2.0. Observed 6.0: three times more.
- Additive scale: RERI ≈ 6.0 − 1.0 − 2.0 + 1 = 4.0, also positive.
- Reading: the exposure triples its effect in carriers; avoiding it is a priority for them.

## Examples of gene-environment interaction

| Genotype | Exposure | Result |
| --- | --- | --- |
| Phenylketonuria | Dietary phenylalanine | Neurological damage that a restricted diet prevents |
| Alpha-1 antitrypsin deficiency | Smoking | Early and severe emphysema |
| G6PD deficiency | Fava beans and oxidant drugs | Haemolysis |
| Slow acetylator (NAT2) | Aromatic amines | Higher risk of bladder cancer |

## Markers

- G × E
- 2 × 4 table
- Case-only

## In practice

- Newborn screening for phenylketonuria is the success story: the genotype is detected and the environment (diet) is changed.
- Pharmacogenetics applies the same idea: genotype changes the effect or toxicity of a drug.
- For most common diseases, known interactions do not yet change recommendations.

## Common pitfalls

- Testing many gene-exposure combinations without correcting for multiple comparisons.
- Measuring exposure poorly: misclassification dilutes the interaction.
- Using the case-only design when genotype influences the chance of being exposed.
- Reporting interaction without stating the scale (additive or multiplicative).

## Memory aid

Genes load the gun and the environment pulls the trigger: without one of them, nothing fires.

## Test yourself

Think of the answer, then tap the question to see it.

## OR for the variant alone 1.0, exposure alone 2.0 and both 6.0. Is there multiplicative interaction?

Yes: the expected value would be 1.0 × 2.0 = 2.0 and 6.0 is observed.

## What assumption does the case-only design need?

That genotype and exposure are independent in the population the cases come from.

## Which gene-environment interaction has an effective prevention?

Phenylketonuria: newborn screening finds the genotype and a low-phenylalanine diet prevents the damage.

## Why do many gene-environment findings fail to replicate?

Small samples, poorly measured exposure and many tests without correction for multiple comparisons.

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## Elsewhere in DurgiPedia

- [Multifactorial (complex) disorders](https://durgical.com/en/general-pathology/multifactorial-disorders/)
- [Xenobiotic biotransformation: phases I, II and III](https://durgical.com/en/biochemistry/topics/xenobiotic-biotransformation-phases-i-ii-and-iii/)

## Connections

## Related 6

- [Additive and multiplicative interaction](https://durgical.com/en/epidemiology/additive-and-multiplicative-interaction/)
- [Effect modification (interaction)](https://durgical.com/en/epidemiology/effect-modification/)
- [Linkage and genetic association studies (GWAS)](https://durgical.com/en/epidemiology/linkage-and-genetic-association-studies/)
- [Family, twin and adoption studies](https://durgical.com/en/epidemiology/family-twin-and-adoption-studies/)
- [Case-control study](https://durgical.com/en/epidemiology/case-control-study/)
- [Causal models: necessary, sufficient and component causes](https://durgical.com/en/epidemiology/causal-models-necessary-sufficient-and-component-causes/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.

Sources: [Genetic Epidemiology](https://archive.cdc.gov/www_cdc_gov/genomics/resources/books/genepi2.htm) (Centers for Disease Control and Prevention, 1997); [What is heritability?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/inheritance/heritability/) (National Library of Medicine (MedlinePlus), 2021).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/gene-environment-interaction/