Two people can carry the very same gene variant and end up with very different stories. One never gets sick, another has a mild case, and a third has a severe one. Genetics uses two words for these two kinds of difference: penetrance for whether a condition appears at all, and expressivity for how it shows when it does.1MedlinePlus. What are reduced penetrance and variable expressivity? Both help explain why a positive genetic result is usually a statement about chances rather than a forecast of one person’s future.
Penetrance Counts Who Shows It, Expressivity Measures How Much
Penetrance is the share of people with a particular variant who show any signs or symptoms of the condition.2MedlinePlus. What are reduced penetrance and variable expressivity? Each carrier either shows the expected condition or does not, so penetrance treats the outcome as a yes-or-no question.3PubMed Central. Incomplete Penetrance and Variable Expressivity: From Clinical Studies to Population Cohorts If every carrier shows signs by a given age, the variant is called fully penetrant. If fewer do, it has reduced or incomplete penetrance, meaning some carriers never show signs.4PubMed Central. Incomplete Penetrance and Variable Expressivity: From Clinical Studies to Population Cohorts Because of that, penetrance is often tied to an age, as in “by 65” or “by 80.”5PubMed Central. Incomplete Penetrance and Variable Expressivity: From Clinical Studies to Population Cohorts
Penetrance describes a group of carriers, not one person. When a report says a variant has 80 percent penetrance, it means about 80 of every 100 carriers would be expected to show signs, and it cannot say which group you belong to.
Expressivity is the degree to which the trait differs between the individuals who have it.6Nature Education. Phenotype Variability: Penetrance and Expressivity Variable expressivity, where severity differs between people, means the same variant can produce anything from mild to severe symptoms across a spectrum.7PubMed Central. Incomplete Penetrance and Variable Expressivity: From Clinical Studies to Population Cohorts Expressivity can be described for a single person, such as mild, moderate or severe, because it is about that person’s own signs.
Take 100 carriers of a variant. If 80 show signs, penetrance is 80 percent. Among those 80, some may have mild signs and others severe ones, and that spread is expressivity. The 20 with no signs fall under penetrance, not expressivity. Knowing one number tells you nothing about the other.
A mild case is not “partial penetrance”: the person shows signs, so it counts as penetrant, and mildness belongs under expressivity. A quick test is to ask “whether” for penetrance and “how much” for expressivity.
Examples of Penetrance in Real Conditions
Inherited retinoblastoma, an eye cancer of childhood, is passed down when one parent carries the variant.8PubMed. Retinoblastoma caused by an RB1 variant with unusually low penetrance in a Danish family Its penetrance is usually above 90 percent.9PubMed. Retinoblastoma caused by an RB1 variant with unusually low penetrance in a Danish family Some families have much lower penetrance, though, and the members who do develop tumors tend to have them in only one eye.10PubMed. Retinoblastoma caused by an RB1 variant with unusually low penetrance in a Danish family In one family, nine relatives carried the same variant as the affected child but never developed the cancer.11PubMed. Retinoblastoma caused by an RB1 variant with unusually low penetrance in a Danish family
A family tree can show this. A grandparent and grandchild are affected, but the parent in between is not. With incomplete penetrance, that parent can carry and pass on the variant without symptoms, which can make an inherited condition look as if it appeared from nowhere.
Huntington disease is caused by a repeated stretch of DNA, and a longer stretch raises the chance of symptoms by a given age.12PubMed. A new model for prediction of the age of onset and penetrance for Huntington’s disease based on CAG length A 40-year-old with 42 repeats has about a 91 percent chance of symptoms starting by age 65.13PubMed. A new model for prediction of the age of onset and penetrance for Huntington’s disease based on CAG length But carriers with 36 to 38 repeats often remain symptom-free into old age, and this borderline range is more common in the general population than once thought.14PubMed Central. Huntington disease reduced penetrance alleles occur at high frequency in the general population
For the C9orf72 variant, which can cause ALS or dementia, symptoms typically start around age 58.15PubMed Central. Age-related penetrance of the C9orf72 repeat expansion By age 83, nearly all carriers (99.5 percent) have symptoms.16PubMed Central. Age-related penetrance of the C9orf72 repeat expansion A symptom-free carrier at 40 therefore says little about what will happen later. A variant can look low-penetrance in young people and high-penetrance in older ones, so for any percentage, ask by what age, and among whom.
Examples of Variable Expressivity
Neurofibromatosis type 1 (NF1) is a common inherited condition that affects the skin and nerves, among other parts of the body.17PubMed Central. Unravelling the genetic basis of variable clinical expression in neurofibromatosis 1 It varies a great deal between families and also within the same family.18PubMed Central. Unravelling the genetic basis of variable clinical expression in neurofibromatosis 1 For that reason NF1 is treated as a model condition for studying why expressivity varies.19American Journal of Medical Genetics. Neurofibromatosis type 1: A model condition for the study of the molecular basis of variable expressivity in human disorders
Expressivity covers which signs appear, not only how strong they are. One NF1 carrier may have mainly skin spots while another develops nerve tumors, so doctors look for the whole range of possible features.
Marfan syndrome is a rare inherited connective-tissue disorder caused by changes in the FBN1 gene.20PubMed Central. Association of modifiers and other genetic factors explain Marfan syndrome clinical variability It varies in when it starts, how severe it is, and how many body systems it affects.21PubMed Central. Association of modifiers and other genetic factors explain Marfan syndrome clinical variability Its most serious risk is a tear in the body’s main artery after it has widened.22PubMed Central. Association of modifiers and other genetic factors explain Marfan syndrome clinical variability Even within one family, doctors cannot yet predict how severe it will be in a particular child, which makes testing before birth hard to interpret.23European Journal of Human Genetics. Clinical utility gene card for: Marfan syndrome type 1 and related phenotypes FBN1 A mildly affected parent can have a child who is much more seriously affected, even though both carry the same variant.
Why the Same Variant Acts Differently
People with the same variant can have a severe form, a mild form, or no symptoms at all, and modifier genes (other genes that change a variant’s effect) help explain this.24PubMed Central. Identifying modifier genes of monogenic disease: strategies and difficulties A modifier usually has no disease effect by itself in a person who lacks the main variant. It matters only in combination, where it can lessen or worsen the effect of the main variant. In some conditions, full penetrance appears to need extra variants in other genes.25PubMed Central. Where genotype is not predictive of phenotype: towards an understanding of the molecular basis of reduced penetrance in human inherited disease The total load of a person’s other common and rare variants can also push them over the threshold into disease or keep them below it.26PubMed Central. Genetic modifiers of rare variants in monogenic developmental disorder loci Affected relatives share the main variant but, depending on how closely related they are, only some of their other genes, which is part of why their outcomes can still differ.
Doctors can also sum many small genetic differences into a polygenic score that measures background risk.27PubMed Central. Polygenic background modifies penetrance of monogenic variants for tier 1 genomic conditions Among people carrying a variant linked to breast cancer, the chance of breast cancer by age 75 ranged from 13 to 76 percent depending on their score.28PubMed Central. Polygenic background modifies penetrance of monogenic variants for tier 1 genomic conditions For familial high cholesterol variants, the chance of heart disease by 75 ranged from 17 to 78 percent, and for Lynch syndrome colon cancer, from 11 to 80 percent.29PubMed Central. Polygenic background modifies penetrance of monogenic variants for tier 1 genomic conditions Carriers of a common kidney-disease variant with a high polygenic score had a 54-fold higher risk of chronic kidney disease, compared with 3-fold for those with a low score.30PubMed Central. Polygenic risk alters the penetrance of monogenic kidney disease These scores are still being developed, and they add to family history and the variant rather than replacing them.
Environment matters too. In PKU, an inherited condition in which the body can’t break down phenylalanine (a building block of protein), the diet decides the outcome.31PubMed Central. Phenylketonuria in Children and Mothers: Genes, Environments, Behavior When phenylalanine levels stay below about 6.6 mg/dL, predicted IQ is around normal, and above that it falls about 4.7 points for each 1 mg/dL rise.32PubMed Central. Phenylketonuria in Children and Mothers: Genes, Environments, Behavior Some diabetes risk genes, meanwhile, show their effect mainly in people whose lifestyle already puts them at risk.33PubMed Central. Dissecting gene–environment contributions to Type 2 diabetes
Chance and gene switching play a part as well. Chemical marks that switch genes on or off can change whether a trait appears, and these marks can respond to the environment.34PubMed Central. Where genotype is not predictive of phenotype: towards an understanding of the molecular basis of reduced penetrance in human inherited disease Even genetically identical cells in identical conditions differ, because the chemistry of gene activity is partly random.35PubMed Central. Noise in gene expression: origins, consequences, and control In genetically identical lab animals, random variation in cells can help explain why some get a condition and others don’t.36bioRxiv. How do stochastic processes and genetic threshold effects explain incomplete penetrance and inform causal disease mechanisms?
One way to tie these causes together is a risk “bucket” that has to fill past a line before disease appears. The main variant adds a lot, while other genes, environment and chance add or subtract. Crossing the line is penetrance, and how far past it you go relates to expressivity. A risk factor that pushes a carrier over the line raises penetrance, since more carriers then show signs. Once someone is affected, the same factors can push the condition toward the milder or more severe end. So a gene, a diet or a lifestyle does not belong to one measure only; the two terms describe different outcomes, not different causes.
How Penetrance Is Measured and Why Old Numbers Ran High
Penetrance was mostly estimated from people who were already ill and their relatives who were referred for testing.37PubMed. Guidance for estimating penetrance of monogenic disease-causing variants in population cohorts This approach substantially overestimates penetrance, because families with many affected members are the ones that get noticed.38PubMed. Guidance for estimating penetrance of monogenic disease-causing variants in population cohorts Families with a variant but few or no affected members rarely get tested, so early figures leave out milder cases and look worse than the truth for the average carrier.
Large population studies can now test people first and check their health afterwards.39PubMed. Guidance for estimating penetrance of monogenic disease-causing variants in population cohorts People give DNA and their medical records are followed, so researchers can find carriers without knowing who is sick, then count how many have the condition. That gives a more representative penetrance, though the method has limits: volunteers in such databases tend to be healthier than average, and records may be incomplete.
The results are striking. Across thousands of variants labelled as disease-causing, the average penetrance in large health databases was only about 7 percent.40PubMed Central. Population-Based Penetrance of Deleterious Clinical Variants For variants linked to dilated cardiomyopathy (an enlarged, weakened heart), at most about 30 percent of carriers showed even subtle heart signs.41PubMed Central. The Frequency, Penetrance and Variable Expressivity of Dilated Cardiomyopathy-Associated Putative Pathogenic Gene Variants in UK Biobank Participants About 2.8 percent of people in one large database carried a variant linked to disease.42PubMed Central. The Developmental-Genetics of Canalization
So if you see a frightening percentage online, check whether the figure came from affected families or from the general population. Family-based figures often fit people with a strong family history better, and without such a history your own risk may be lower. A genetic counselor can say which estimate applies to you.
Figures also keep changing. Huntington estimates were revised once borderline carriers were found among healthy older adults, and older people are sometimes never diagnosed, which shifts estimates too.43PubMed Central. Huntington disease reduced penetrance alleles occur at high frequency in the general population
What These Ideas Mean If You Carry a Variant
Some variants, such as R117H in the cystic fibrosis gene, combine reduced penetrance with variable expressivity, so outcomes range from no symptoms to disease.44PubMed. Developing a primer for communicating about penetrance, expressivity, and genotype-phenotype correlation of R117H and the poly-T tract in CFTR Specialists have built guides that give the likely range of outcomes for these variants rather than a single answer.45PubMed. Developing a primer for communicating about penetrance, expressivity, and genotype-phenotype correlation of R117H and the poly-T tract in CFTR Variants found during pregnancy need especially careful counseling for the same reasons.46PubMed. Ten points to consider when providing genetic counseling for variants of incomplete penetrance and variable expressivity detected in a prenatal setting
It also helps to know what kind of variant your report describes. A variant of uncertain significance means scientists do not yet know whether it causes disease at all. A reduced-penetrance variant is known to raise risk, but not every carrier will be affected. The two call for different responses.
A few questions are worth bringing to an appointment: what is my chance of symptoms, by what age, and how severe might they be? Share your family history, because it can change which estimate fits you. And ask whether screening or lifestyle changes can lower your risk, as diet does in PKU.
A session with a genetic counselor usually starts with a family tree going back several generations. The counselor explains both the chance of symptoms and the range of possible severity, and discusses whether relatives might want testing. They may recommend regular checks, such as heart scans for Marfan syndrome.
Having no symptoms now is not a clean all-clear. With a reduced-penetrance variant, it does not guarantee that none will come later, especially if the figure is tied to a later age. It also does not mean you will be affected, since many carriers stay well but some do not. Follow-up plans such as screening schedules are often still sensible for carriers who feel well.