[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Epidemiology](https://durgical.com/en/epidemiology/) › [Study designs](https://durgical.com/en/epidemiology/areas/designs/) › Family, twin and adoption studies

# Family, twin and adoption studies

- Study design

To weigh the role of genes, researchers compare relatives, identical with fraternal twins, and adoptees with both their families.

Updated Nov 13, 2026

## Key points

- Family aggregation: risk in relatives of cases exceeds that in relatives of controls; it does not separate genes from shared environment.
- Monozygotic twins share almost all their DNA; dizygotic twins share half on average.
- Higher concordance in monozygotic than dizygotic twins suggests a genetic influence; concordance below 100% in monozygotic twins shows environment.
- Heritability (Falconer's formula): h² = 2 × (r MZ − r DZ), with r = the correlation of the trait between twins.
- Adoption: if adoptees resemble their biological family more than their adoptive one, genes weigh more.
- Heritability applies to one population in one environment; it does not tell how much of one person's disease is genetic.

On this page

- The basics Key points
- What it is
- How it is built
- Worked example
- Markers
- When to use it
- Common pitfalls
- Memory aid
- Elsewhere in DurgiPedia

- Clinical Designs that separate genes and environment

- Study Test yourself

- Connections Connections

## What it is

Genetic epidemiology starts by asking whether a disease clusters in families more than expected. Because families share both genes and environment, twin studies (monozygotic against dizygotic) and adoption studies try to separate the two influences. They yield heritability, the share of the variation of a trait in a population that is attributed to genetic differences.

## How it is built

- Start from index cases (probands) and study their relatives with the same case definition used for controls' relatives.
- Twin zygosity is confirmed with DNA markers, not only by physical likeness.
- Adoption studies need records that identify the biological parents and their health.

## Worked example

- Twin register with at least one twin affected by a disease: 100 monozygotic pairs and 100 dizygotic pairs.
- Monozygotic: 30 concordant pairs (both ill) and 70 discordant. Pairwise concordance = 30/100 = 30%.
- Dizygotic: 10 concordant and 90 discordant. Concordance = 10/100 = 10%.
- Concordance is 3 times higher in monozygotic pairs (30/10): there is a genetic component.
- But 70% of identical pairs are discordant: environment or chance weighs heavily.
- For blood pressure, r MZ = 0.60 and r DZ = 0.35: h² = 2 × (0.60 − 0.35) = 0.50.
- Reading: half of the variation in blood pressure in that population is linked to genes; not half of each person's.

## Designs that separate genes and environment

| Design | What it compares | Strength | Limitation |
| --- | --- | --- | --- |
| Family aggregation | Risk in relatives of cases and of controls | Simple; first step | Does not separate genes from shared environment |
| Twins | Concordance in monozygotic and dizygotic pairs | Estimates heritability | Assumes equal environments for both types; few pairs |
| Adoption | Adoptees against biological and adoptive families | Separates genes from upbringing | Selective placement, prenatal environment, few cases |
| Twins reared apart | Monozygotic twins separated at birth | Very informative | Extremely rare; small samples |

## Markers

- MZ
- DZ
- h²
- Concordance

## When to use it

- Used when a hereditary component is suspected in a complex disease (type 2 diabetes, schizophrenia, hypertension).
- They are the step before searching for specific genes with linkage or association studies.

## Common pitfalls

- Reading heritability as ‘50% of the disease is genetic’ in one person.
- Confusing familial with hereditary: language clusters in families and is not genetic.
- Subtracting concordances directly to get h²: Falconer's formula applies to correlations, not to concordance percentages.
- Forgetting that monozygotic twins may share more environment (treatment, placenta) than dizygotic twins.

## Memory aid

Identical twins who differ tell you what the environment adds; identical twins who match more than fraternal ones tell you what genes add.

## Test yourself

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

## Concordance is 30% in monozygotic and 10% in dizygotic twins. What does it suggest?

A genetic component (more concordance when more genes are shared) and a large role for environment, since 70% of identical pairs are discordant.

## r MZ = 0.70 and r DZ = 0.45. What is the heritability by Falconer's formula?

h² = 2 × (0.70 − 0.45) = 0.50.

## What does an adoption study add that family aggregation does not?

It separates genes (biological family) from upbringing and shared environment (adoptive family).

## Why does high heritability not mean a trait cannot change?

Because it measures variation in a given population and environment; changing the environment can change the trait and the heritability itself.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/epidemiology/family-twin-and-adoption-studies/)

## Elsewhere in DurgiPedia

- [Multifactorial (complex) disorders](https://durgical.com/en/general-pathology/multifactorial-disorders/)
- [Population genetics: Hardy–Weinberg and carrier frequency](https://durgical.com/en/biochemistry/topics/population-genetics-hardy-weinberg-and-carrier-frequency/)

## Connections

## Related 5

- [Linkage and genetic association studies (GWAS)](https://durgical.com/en/epidemiology/linkage-and-genetic-association-studies/)
- [Gene-environment interaction](https://durgical.com/en/epidemiology/gene-environment-interaction/)
- [Migrant studies](https://durgical.com/en/epidemiology/migrant-studies/)
- [Case-control study](https://durgical.com/en/epidemiology/case-control-study/)
- [Correlation](https://durgical.com/en/epidemiology/correlation/)

[← 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: [What is heritability?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/inheritance/heritability/) (National Library of Medicine (MedlinePlus), 2021); [Genetic Epidemiology](https://archive.cdc.gov/www_cdc_gov/genomics/resources/books/genepi2.htm) (Centers for Disease Control and Prevention, 1997); [Genetics of Gastric Cancer (PDQ®)–Health Professional Version](https://www.cancer.gov/types/stomach/hp/gastric-genetics-pdq) (National Cancer Institute, 2025).

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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/family-twin-and-adoption-studies/