[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Inheritance concepts, genotype–phenotype and the pedigree

# Inheritance concepts, genotype–phenotype and the pedigree

The words geneticists use to link a genotype to what a person shows, and how a three-generation family tree reveals the pattern.

Updated Nov 13, 2026

## Key facts

- Genotype is the alleles at a locus; phenotype is what can be observed. Homozygous: two identical alleles; heterozygous: two different; hemizygous: one copy (X-linked genes in males); compound heterozygous: two different pathogenic alleles.
- Penetrance is the share of people with the genotype who show the disease; it can be incomplete or rise with age (Huntington disease, BRCA1/2).
- Expressivity is how severe the disease is in those affected; it varies widely within one family in neurofibromatosis type 1.
- Pleiotropy: one gene affects several organs (Marfan syndrome: eye, skeleton, aorta).
- Heterogeneity: many variants in one gene (allelic, as in CFTR), many genes for one phenotype (locus, as in hearing loss), or different disorders from one gene (phenotypic).

## In detail

- Loss-of-function variants usually act recessively; when half the normal product is not enough (haploinsufficiency), loss of one copy is dominant.
- Gain-of-function variants (achondroplasia, Huntington disease) and dominant-negative variants, in which an abnormal protein spoils the normal one (collagen in osteogenesis imperfecta, fibrillin in Marfan), act dominantly.
- Pedigree symbols: squares for males, circles for females, filled shapes for affected people, an arrow for the proband, a double line for a consanguineous couple.
- A family history should cover three generations, ancestry, ages at diagnosis and death, pregnancy losses and consanguinity.
- A dominant disorder with unaffected parents suggests a new mutation, germline mosaicism, reduced penetrance or misattributed parentage.

**Also called**
Penetrance, expressivity and pleiotropy, Family history and pedigree drawing

## Clinical notes

- Red flags for a hereditary condition: early onset, several affected relatives, bilateral or multiple tumours, rare tumours, consanguinity and repeated pregnancy losses.
- Genotype–phenotype correlation guides prognosis: in cystic fibrosis, some CFTR variants preserve pancreatic function.

## Test yourself

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

## What is the difference between penetrance and expressivity?

Penetrance is whether a person with the genotype shows the disease at all; expressivity is how severe it is when present.

## How does a dominant-negative variant act?

Its abnormal protein interferes with the normal protein from the other allele, so function falls below half.

## What should a basic genetic family history include?

Three generations, ancestry, ages at diagnosis and death, pregnancy losses and any consanguinity.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/biochemistry/topics/inheritance-concepts-genotype-phenotype-and-the-pedigree/)

## Connections

## Related conditions 5

- [Marfan syndrome](https://durgical.com/en/conditions/marfan-syndrome/)
- [Neurofibromatosis (types 1 and 2)](https://durgical.com/en/conditions/neurofibromatosis/)
- [Cystic fibrosis](https://durgical.com/en/conditions/cystic-fibrosis/)
- [Achondroplasia](https://durgical.com/en/conditions/achondroplasia/)
- [Huntington disease](https://durgical.com/en/conditions/huntington-disease/)

## Related 4

- [Types of mutations](https://durgical.com/en/biochemistry/topics/types-of-mutations/)
- [Population genetics: Hardy–Weinberg and carrier frequency](https://durgical.com/en/biochemistry/topics/population-genetics-hardy-weinberg-and-carrier-frequency/)
- [Where new mutations come from: de novo variants, rates and parental age](https://durgical.com/en/biochemistry/topics/where-new-mutations-come-from-de-novo-variants-rates-and-parental-age/)
- [Trinucleotide repeat expansion disorders](https://durgical.com/en/biochemistry/topics/trinucleotide-repeat-expansion-disorders/)

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

Sources: [Pedigree](https://www.genome.gov/genetics-glossary/Pedigree) (National Human Genome Research Institute (NHGRI), 2026); [Mendelian Inheritance](https://www.genome.gov/genetics-glossary/Mendelian-Inheritance) (National Human Genome Research Institute (NHGRI), 2026); [What are reduced penetrance and variable expressivity?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/inheritance/penetranceexpressivity/) (MedlinePlus, US National Library of Medicine, 2026); [Why is it important to know my family health history?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/inheritance/familyhistory/) (MedlinePlus, US National Library of Medicine, 2026).

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

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/biochemistry/topics/inheritance-concepts-genotype-phenotype-and-the-pedigree/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/topics/inheritance-concepts-genotype-phenotype-and-the-pedigree/