What is the difference between penetrance and expressivity?

Penetrance is whether people who carry a disease-causing variant show the disorder at all, expressed as the percentage of carriers who are affected. Expressivity is how the disorder shows up in those who are affected, meaning its severity and which features appear. Incomplete penetrance means some carriers are completely unaffected, while variable expressivity means affected carriers differ from one another.

What is an example of incomplete penetrance?

Harmful BRCA1 and BRCA2 variants are the classic example: according to the National Cancer Institute, more than 60% of women with such a variant develop breast cancer during their lifetime, but some carriers never develop cancer. Huntington disease alleles with 36 to 39 CAG repeats are another example, because people who have them may or may not develop symptoms.

Why is NF1 an example of variable expressivity?

People with the same NF1 variant, even within one family, can look very different: one may have only café-au-lait macules and freckling, while another has many neurofibromas, an optic glioma or scoliosis. NF1 penetrance, by contrast, is nearly complete after childhood, so almost every carrier shows some features. That combination of near-complete penetrance and wide variability makes NF1 the textbook example of variable expressivity.

Why can an autosomal dominant disease appear to skip a generation?

Reduced penetrance is the usual explanation: a person inherits the variant but never develops the disorder, then passes it to a child who is affected. On a pedigree the unaffected carrier looks normal, so the condition can mimic recessive inheritance. Before calling a carrier nonpenetrant, check their age, because disorders such as Huntington disease usually appear only in the thirties or forties.

What is the difference between pleiotropy and variable expressivity?

Pleiotropy means one gene affects several seemingly unrelated traits in the same person, as when an FBN1 variant in Marfan syndrome affects the lens, the aorta and the skeleton. Variable expressivity means people with the same variant differ in how severely or in which ways they are affected. Marfan syndrome shows both.

How do you tell reduced penetrance from anticipation?

Reduced penetrance is suggested by a carrier of a known variant who has no disease. Anticipation is suggested by disease that starts earlier or is more severe in each successive generation, classically because a trinucleotide repeat expands, as in Huntington disease, myotonic dystrophy and fragile X syndrome. A single younger diagnosis in a family with a non-repeat disorder, such as BRCA1-related cancer, does not by itself indicate anticipation.

Penetrance vs Expressivity: BRCA, NF1 & Marfan Examples

Penetrance asks whether a genotype shows up at all; expressivity asks how it shows up. Incomplete (reduced) penetrance means some people who carry a disease-causing variant have no signs of the disorder; BRCA1 and BRCA2 carriers who never develop cancer are the classic example. Variable expressivity means people with the same variant are affected to different degrees or with different features: in NF1 one relative may have a few café-au-lait macules while another has many neurofibromas, and in Marfan syndrome features range from a tall, slender build to aortic dissection. Pleiotropy, anticipation, mosaicism and imprinting are separate ideas that stems use as distractors.

The cheat sheet

	Incomplete penetrance	Variable expressivity
Question it answers	Does a carrier show the phenotype at all?	How severe, and which features?
The individual carrier	Can be completely unaffected	Has the disorder, mild or severe
Measured as	% of carriers with any phenotype	Range of severity and features among affected carriers
Pedigree clue	Unaffected obligate carrier; a dominant disorder "skips" a generation	Every carrier affected, but relatives look very different
Classic examples	BRCA1/BRCA2; HTT alleles with 36 to 39 CAG repeats	NF1; Marfan syndrome
Stem wording	"carries the same variant but has no findings"	"same variant, different manifestations"

Visual hook: picture a row of lamps wired identically (same genotype). Lamps that never switch on = incomplete penetrance (a light switch). Lamps that are on but glow at different brightness = variable expressivity (a dimmer). Both often occur in the same disorder; stems test whichever one the clue points to.

Penetrance: a yes/no in each person, a percentage in populations

$\text{penetrance} = \frac{\text{carriers who show the phenotype}}{\text{all carriers of the genotype}} \times 100\%$

Complete penetrance (100%) means every carrier is affected. NF1 is close: GeneReviews notes that its penetrance is nearly complete after childhood, although molecular testing has documented reduced penetrance in a small number of people.
BRCA1/BRCA2 (incomplete): per NCI, more than 60% of women who inherit a harmful BRCA1 or BRCA2 change develop breast cancer during their lifetime (about 13% in the general population). Over a lifetime, ovarian cancer affects about 39% to 58% of women with a harmful BRCA1 change and 13% to 29% with a BRCA2 change (about 1.1% in the general population). Some carriers never develop cancer.
Huntington disease: people with 36 to 39 CAG repeats in HTT may or may not develop symptoms (a reduced-penetrance range); people with 40 or more almost always do.
Why penetrance is incomplete: MedlinePlus Genetics attributes both reduced penetrance and variable expressivity to a combination of genetic, environmental and lifestyle factors, many of them unknown.

Pedigree math. For an autosomal dominant disorder, the chance that a child of a heterozygous parent is affected is the chance of inheriting the variant times the penetrance:

$P(\text{affected child}) = \tfrac{1}{2} \times \text{penetrance}$

With 80% penetrance, that is $0.5 \times 0.8 = 0.4$ , or 40%, not the 50% you would quote for a fully penetrant disorder.

Age-dependent penetrance trap: before calling a carrier nonpenetrant, check the age. Adult-onset Huntington disease usually appears in a person's thirties or forties, so a symptom-free 25-year-old carrier is simply too young to judge. BRCA-related cancer risks likewise accumulate over a lifetime.

Pedigree trap: an unaffected carrier makes a dominant disorder look as if it skips a generation, which can mimic autosomal recessive inheritance. An unaffected person with an affected parent and an affected child is an obligate carrier. Testing separates this from germline mosaicism: a nonpenetrant carrier has the variant in blood, whereas a parent whose variant is confined to eggs or sperm usually does not.

Variable expressivity: same variant, different disease

NF1 (NF1, neurofibromin, a tumor suppressor): café-au-lait macules, axillary and groin freckling, cutaneous neurofibromas, Lisch nodules, optic glioma, scoliosis. GeneReviews describes NF1 as extremely variable: severity can differ among family members with the same pathogenic variant, and an affected child may be considerably more (or less) severe than the affected parent. About half of cases are new (de novo) variants.
Marfan syndrome (FBN1, fibrillin-1): tall, slender build with long fingers, ectopia lentis, mitral valve prolapse, aortic aneurysm and dissection, scoliosis and chest wall deformity. Signs vary widely in severity, timing of onset and rate of progression; at least 25% of cases result from a new FBN1 mutation.

Classic stem: a father has numerous café-au-lait macules and cutaneous neurofibromas; his sister has only a few café-au-lait macules and mild scoliosis; both carry the same NF1 variant. That is variable expressivity. If 100% of carriers show something, penetrance is complete, however different they look.

NF1 caveat: because NF1 penetrance is nearly complete after childhood, an adult who carries a family's germline NF1 variant yet has no findings is unusual; "unaffected carrier" questions work best with disorders such as BRCA-related cancer. Features confined to one body segment point instead to localized mosaic NF1 (the variant arose after conception, so only some cells carry it); GeneReviews notes that mosaic NF1 is usually milder than typical NF1 and that some adults with mosaic NF1 have no clinical features at all.

Don't confuse: pleiotropy, anticipation, mosaicism, imprinting, heterogeneity

Concept	Definition	Classic example	Stem clue
Pleiotropy	One gene contributes to multiple, seemingly unrelated traits	Marfan (lens, aorta, skeleton); untreated PKU (intellectual disability, eczema, lighter skin)	One variant, several organ systems in the same patient
Anticipation	Earlier onset or greater severity in each generation	Repeat expansions: Huntington (CAG), myotonic dystrophy type 1 (CTG), fragile X (CGG)	Grandparent affected in their 60s, parent in their 40s, child in their teens
Somatic mosaicism	A variant that arises after conception, so only some cells carry it	McCune-Albright (GNAS, not inherited): fibrous dysplasia, irregular "coast of Maine" café-au-lait spots, early puberty; localized mosaic NF1	Patchy or milder-than-expected disease in one person
Germline (gonadal) mosaicism	Only a proportion of eggs or sperm carry the variant	Two children with the same dominant disorder, both parents unaffected	Recurrence despite an unaffected parent with negative blood testing
Genomic imprinting	Only one parent's copy of a gene is active	Prader-Willi (loss of paternal chromosome 15 genes); Angelman (loss of the maternal UBE3A copy, also on chromosome 15)	Phenotype depends on which parent transmitted it
Locus heterogeneity	Different genes cause the same phenotype	Hereditary breast cancer: BRCA1, BRCA2, TP53, PTEN, CDH1, STK11 and others	Same disease, different genes in different families

Pleiotropy vs variable expressivity: pleiotropy is breadth within one patient (many systems from one gene); expressivity is variation between patients. Marfan shows both.
Anticipation mechanism: the repeat often lengthens as it is passed on, and longer repeats bring earlier onset. Fragile X premutations (55 to 200 CGG repeats) can expand to full mutations (more than 200) in a mother's egg-forming cells. Review the parent-of-origin details in the trinucleotide repeat disorders guide.
Imprinting mechanisms: Prader-Willi comes from a paternal deletion (about 70%) or maternal uniparental disomy (about 25%); Angelman from a maternal deletion (about 70%) or, less often, paternal uniparental disomy. See the uniparental disomy guide.

Practice question

Vignette. A 72-year-old woman comes for genetic counseling after her daughter was diagnosed with breast cancer at age 41 and found to carry a pathogenic BRCA1 variant. The patient's mother died of ovarian cancer at 52. The patient has never had breast or ovarian cancer, and her most recent mammogram was normal. Testing shows that she carries the same BRCA1 variant as her daughter. Which of the following best explains why this patient has not developed cancer?

A. Variable expressivity B. Incomplete penetrance C. Anticipation D. Locus heterogeneity E. Genomic imprinting

Answer: B. Incomplete penetrance. She has the genotype but not the phenotype. BRCA1 penetrance is high but incomplete: some carriers never develop cancer. Her age matters: at 72 she has lived through decades of risk, whereas a cancer-free 25-year-old carrier would tell you little.

Why not the others?

Variable expressivity describes differences among affected carriers, for example breast cancer in one and ovarian cancer in another. This patient has no phenotype at all.
Anticipation is tempting because the daughter was diagnosed younger, but it means earlier or more severe disease in each generation from an expanding repeat (Huntington, myotonic dystrophy, fragile X). A BRCA1 variant does not expand, and anticipation cannot explain a carrier with no disease.
Locus heterogeneity means different genes cause the same disease in different families (BRCA2, TP53, PTEN and others for breast cancer). Everyone here shares one variant.
Genomic imprinting is a parent-of-origin effect, as in Prader-Willi vs Angelman syndrome. Nothing in this stem depends on which parent transmitted the variant; the key finding is a carrier with no disease.

Q-bank tactics: name the concept in five questions

Does any carrier, old enough to judge, have zero findings? Incomplete penetrance.
Is every carrier affected, but to different degrees? Variable expressivity.
Does one gene hit several unrelated systems in the same patient? Pleiotropy.
Is disease earlier or worse in each generation, with a repeat disorder? Anticipation.
Does the answer change with the parent of origin (imprinting), a patchy distribution or unaffected parents of two affected children (mosaicism), or different genes in different families (heterogeneity)? Name that mechanism instead.

Key takeaways

Incomplete penetrance = genotype without phenotype in some carriers (BRCA1/BRCA2; HTT 36 to 39 CAG repeats). It can make a dominant disorder appear to skip a generation.
Variable expressivity = same genotype, different severity or features among affected carriers (NF1, Marfan).
Penetrance is a percentage of carriers; the risk that a child of a heterozygous parent is affected is $\tfrac{1}{2} \times$ penetrance.
Check the carrier's age before calling them nonpenetrant (Huntington disease, BRCA-related cancer).
NF1 penetrance is nearly complete after childhood; NF1 features confined to one body segment suggest localized mosaic NF1.
Distractors: pleiotropy (one gene, many systems), anticipation (repeat expansion), imprinting (parent of origin), mosaicism (some cells), locus heterogeneity (many genes, one disease).