Genetics Terms Starting With V
Genetics Glossary: V
Jump to Genetics Term
Variable Expressivity
/ VAIR-ee-uh-bul ek-SPRES-ih-vih-tee / · Latin: variabilis + expressio
Variable Expressivity is the phenomenon in which individuals carrying the same disease-causing genotype show a range of phenotypic severity or form, from mild to severe, due to the influence of modifier genes, environment, and developmental chance.
Expressivity varies because the phenotypic outcome of any genotype is shaped by the genetic background, environmental exposures, stochastic developmental events, and modifier genes acting on the same pathway. In conditions with variable expressivity, two affected individuals in the same family carrying the identical mutation may present with very different clinical severity. Neurofibromatosis type 1 illustrates this range strikingly: some individuals with an NF1 mutation have only a few cafe-au-lait spots, while others develop hundreds of neurofibromas and serious neurological complications.
Variable expressivity limits the ability to predict phenotype from genotype alone, a challenge that genetic counselors address directly when advising families about inherited conditions.
Identical twins, who share the same genotype, sometimes differ in the severity of a condition showing variable expressivity, demonstrating that non-genetic factors including epigenetic modifications and random developmental variation contribute to phenotypic outcome independently of DNA sequence.
Variable expressivity is not the same as incomplete penetrance. Penetrance describes whether a phenotype appears at all in a carrier, while expressivity describes the degree of severity among those individuals who do express the phenotype.
Marfan syndrome shows variable expressivity: some individuals with an FBN1 mutation have only mild tall stature and long fingers, while others develop aortic root dilation exceeding 5 centimeters that requires surgical intervention, and still others experience lens dislocation. All of these outcomes can occur within a single family sharing the same pathogenic variant.
Variation
/ vair-ee-AY-shun / · Latin: variatio (difference)
Variation is genetic difference in DNA sequences among individuals within a population, arising from mutations, recombination, and the reshuffling of alleles during reproduction.
Variation is the raw material on which natural selection and genetic drift act, and without it evolution cannot proceed. Mutation creates novel alleles, while recombination shuffles existing alleles into new combinations during meiosis, generating far more genotypic diversity than mutation alone could produce. Population geneticists quantify variation using measures such as nucleotide diversity, allelic richness, and heterozygosity; in humans, nucleotide diversity is approximately 0.1 percent, meaning roughly one nucleotide in every thousand differs between any two people.
Populations that have passed through severe bottlenecks, such as the cheetah (Acinonyx jubatus), show dramatically reduced genetic variation, leaving them vulnerable to infectious disease and environmental change.
A 2015 analysis of more than 60,000 human exomes by the Exome Aggregation Consortium found that the average person carries roughly 54 variants predicted to cause loss of function in protein-coding genes, yet most people show no obvious disease from these variants because the second allele remains functional.
Genetic variation is not the same as mutation in everyday usage. Variation encompasses all heritable differences between individuals, including common single-nucleotide polymorphisms present in millions of people, while mutation typically refers to a change from a defined reference sequence that may be rare or novel.
Dog breeds illustrate the power of selection on standing genetic variation: the size difference between a Chihuahua and a Great Dane, a roughly 40-fold difference in body mass, was produced by artificial selection acting on alleles already present in ancestral wolf (Canis lupus) populations over fewer than 15,000 years. Much of this size variation maps to a small number of loci, including the IGF1 gene on chromosome 15.
Vector
/ VEK-ter / · Latin: vector (carrier)
Vector is a DNA molecule used as a vehicle to carry foreign genetic material into a host cell for cloning, expression, or gene therapy purposes.
Cloning vectors such as plasmids and bacteriophages amplify specific DNA sequences in bacterial host cells, producing large quantities of a target gene. Expression vectors carry regulatory sequences that drive transcription and translation of the inserted gene, enabling production of recombinant proteins such as human insulin. Viral vectors, including adeno-associated viruses, lentiviruses, and adenoviruses, are engineered to deliver therapeutic genes into human cells for gene therapy.
Each vector type is chosen based on the size of the insert it can carry, the host cell it targets, and whether stable or transient expression is needed.
The first recombinant protein produced using a vector, human insulin expressed from a plasmid in Escherichia coli, entered clinical use in 1982 and has since replaced animal-derived insulin for diabetes treatment.
Recombinant Proteins →A vector is the same thing as a vaccine. While some vaccines use viral vectors to deliver antigens, the term vector in molecular biology refers to any DNA vehicle for gene delivery, not specifically a vaccine component.
Immune System Fun Facts →The AAV9 viral vector used in the gene therapy Zolgensma efficiently crosses the blood-brain barrier when given intravenously to infants, delivering the SMN1 gene to motor neurons throughout the spinal cord. A single intravenous dose of approximately 1.1 x 10^14 vector genomes per kilogram of body weight is used in patients under two years of age, making it one of the highest-dose gene therapy products approved to date.
Fun Facts About the Nervous System →Voretigene Neparvovec
/ vor-ET-ih-jeen neh-PAR-voh-vec / · Coined name: vore (delivery) + tigene (gene) + neparvovec (AAV2 vector)
Voretigene Neparvovec is a gene therapy that delivers a functional copy of the RPE65 gene into the retinal cells of patients with inherited retinal dystrophy, making it the first FDA-approved gene therapy for an inherited disease.
The therapy is administered by subretinal injection directly beneath the photoreceptor layer, placing the AAV2-RPE65 vector in close proximity to the retinal pigment epithelium cells that need the corrected gene. Patients with biallelic RPE65 mutations who received voretigene neparvovec in clinical trials showed significant improvements in light sensitivity, visual field, and the ability to navigate in low light. Its 2017 FDA approval marked a historic milestone, demonstrating that gene therapy could safely and durably correct an inherited blinding condition.
In the pivotal Phase 3 trial published in The Lancet in 2017, participants who received voretigene neparvovec improved their multi-luminance mobility test scores by a mean of 1.6 log units compared to controls, a difference that persisted at one-year follow-up.
Voretigene neparvovec does not restore completely normal vision. Significant improvements in functional vision occur in patients with RPE65 mutations, but photoreceptor cells that have already died before treatment cannot be recovered by the therapy.
Cell Death →A teenage patient who could previously navigate only in bright daylight received voretigene neparvovec and within weeks could detect light at levels equivalent to a moonlit night, transforming daily independence. The Phase 3 trial enrolled 31 participants aged 4 to 44 years, and 13 of 21 treated patients achieved the maximum possible score on the mobility test by one year, compared to none of the 10 control patients.