Variable expressivity

Variable expressivity is variation in the severity or form of a phenotype among individuals with the same genotype. It contrasts with penetrance, which concerns whether a phenotype appears at all.

Genetic modifiers: Genetic variants that alter the phenotypic effect of another variant. Modifier variants can amplify or reduce the severity produced by the genotype under study.
Genotype: The genetic constitution of an organism or cell. Expressivity compares phenotypes among individuals who share a relevant genotype.
Neurofibromatosis type 1: An autosomal dominant disorder caused by pathogenic variants in the NF1 gene. People with the same familial NF1 variant can differ widely in tumor burden and other features.
Locus heterogeneity: A condition in which variants in different genes cause similar phenotypes. Similar clinical presentations can arise from different genotypes, unlike expressivity among carriers of one genotype.
Epigenetics: Heritable changes in gene activity that do not alter the underlying DNA sequence. Epigenetic differences can change gene activity and produce different phenotypes among carriers.
Phenotype: An organism's observable traits, shaped by genotype and environment. Expressivity describes variation in the traits produced by a genotype.
Marfan syndrome: A connective-tissue disorder commonly caused by pathogenic variants in FBN1. Clinical severity ranges from mild skeletal findings to serious cardiovascular complications.
Allelic heterogeneity: A condition in which different variants in one gene cause the same disorder. Different causal variants can explain variation across patients without demonstrating expressivity of one shared genotype.
Gene–environment interaction: A change in a genetic effect depending on environmental conditions. Different exposures can change the severity or form of a phenotype among people with the same genotype.
Penetrance: The proportion of individuals with a genotype who show its associated phenotype. Penetrance asks whether a phenotype appears; expressivity concerns its severity or form.

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