[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Complex traits: familial risk, twins, heritability and polygenic scores

# Complex traits: familial risk, twins, heritability and polygenic scores

Common diseases run in families without a simple pattern; twin studies, heritability and polygenic scores measure how much genes contribute.

Updated Nov 13, 2026

## Key facts

- Multifactorial (threshold) model: many genes and environmental factors add up to a liability; disease appears above a threshold.
- Recurrence risk rises with the number of affected relatives, closeness of relationship, severity in the affected person, and when the affected person is of the less commonly affected sex.
- Twin studies: identical twins share almost all their DNA and fraternal twins about half; a much higher concordance in identical twins points to genes, and concordance below 100 % shows the environment's share.
- Heritability is the proportion of variation in a trait within a population explained by genetic differences; it describes a population in a given environment, not an individual.
- A polygenic risk score adds up many common variants of small effect found by genome-wide association studies; it can identify people at several-fold higher risk of, for example, coronary disease.

## In detail

- Gene–environment interactions: factor V Leiden with oral contraceptives raises clot risk much more than either alone; APOE ε4 raises Alzheimer risk.
- Some multifactorial disorders have strong single loci: HLA-DQ2/DQ8 in coeliac disease, HLA in type 1 diabetes, RET variants in Hirschsprung disease.
- Polygenic scores work less well in people whose ancestry is under-represented in the studies that built them, which risks widening health inequalities.
- Adoption studies separate shared genes from shared upbringing.

**Also called**
Multifactorial inheritance, Polygenic risk score

## Clinical notes

- For a couple with one child with a multifactorial birth defect (cleft lip, neural tube defect), empirical recurrence risks from population studies guide counselling.
- Polygenic scores are not yet routine care; family history remains the practical tool.

## Test yourself

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

## What does a heritability of 80 % for height mean?

In that population and environment, 80 % of the variation in height between people is explained by genetic differences; it says nothing about one person.

## What do twin concordance studies show?

Higher concordance in identical than in fraternal twins points to a genetic contribution; concordance below 100 % in identical twins shows environmental influence.

## Why do polygenic scores perform worse in some populations?

They were built mostly from European-ancestry data, so their variants and weights transfer poorly to other ancestries.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/biochemistry/topics/complex-traits-familial-risk-twins-heritability-and-polygenic-scores/)

## Connections

## Related conditions 6

- [Alzheimer disease](https://durgical.com/en/conditions/alzheimer-disease/)
- [Type 1 diabetes mellitus](https://durgical.com/en/conditions/type-1-diabetes-mellitus/)
- [Coeliac disease](https://durgical.com/en/conditions/coeliac-disease/)
- [Inherited thrombophilia](https://durgical.com/en/conditions/inherited-thrombophilia/)
- [Cleft lip and palate](https://durgical.com/en/conditions/cleft-lip-and-palate/)
- [Hirschsprung disease](https://durgical.com/en/conditions/hirschsprung-disease/)

## Related 4

- [Population genetics: Hardy–Weinberg and carrier frequency](https://durgical.com/en/biochemistry/topics/population-genetics-hardy-weinberg-and-carrier-frequency/)
- [DNA polymorphisms, linkage and DNA fingerprinting](https://durgical.com/en/biochemistry/topics/dna-polymorphisms-linkage-and-dna-fingerprinting/)
- [How disease genes are found: linkage, association and sequencing](https://durgical.com/en/biochemistry/topics/how-disease-genes-are-found-linkage-association-and-sequencing/)
- [Apolipoprotein E (ApoE)](https://durgical.com/en/biochemistry/enzymes/apolipoprotein-e/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [What is heritability?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/inheritance/heritability/) (MedlinePlus, US National Library of Medicine, 2026); [What are genome-wide association studies?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/genomicresearch/gwastudies/) (MedlinePlus, US National Library of Medicine, 2026); [Polygenic Risk Score (PRS)](https://www.genome.gov/genetics-glossary/Polygenic-Risk-Score) (National Human Genome Research Institute (NHGRI), 2026).

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Source: https://durgical.com/en/biochemistry/topics/complex-traits-familial-risk-twins-heritability-and-polygenic-scores/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/topics/complex-traits-familial-risk-twins-heritability-and-polygenic-scores/