NCI Dictionary of Genetics Terms

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The NCI Dictionary of Genetics Terms contains technical definitions for more than 200 terms related to genetics. These definitions were developed by the PDQ® Cancer Genetics Editorial Board to support the evidence-based, peer-reviewed PDQ cancer genetics information summaries.

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20 results found for: P
pathogenic variant
(PA-thoh-JEH-nik VAYR-ee-unt)
A genetic alteration that increases an individual’s susceptibility or predisposition to a certain disease or disorder. When such a variant (or mutation) is inherited, development of symptoms is more likely, but not certain. Also called deleterious mutation, disease-causing mutation, predisposing mutation, and susceptibility gene.
pathognomonic
(PA-thog-noh-MAH-nik)
Findings that are distinctive or characteristic of a particular disease or condition and make the diagnosis.
PCR
A procedure that produces millions of copies of a short segment of DNA through repeated cycles of: (1) denaturation, (2) annealing, and (3) elongation. PCR is a very common procedure in molecular genetic testing and may be used to generate a sufficient quantity of DNA to perform a test (e.g., allele-specific amplification, trinucleotide repeat quantification). Also called polymerase chain reaction.
pedigree
(PEH-dih-gree)
A graphic illustration of family history.
penetrance
(PEH-neh-trunts)
A characteristic of a genotype; it refers to the likelihood that a clinical condition will occur when a particular genotype is present.
phenocopy
(FEE-noh-KAH-pee)
A phenotypic trait or disease that resembles the trait expressed by a particular genotype, but in an individual who is not a carrier of that genotype. For example, breast cancer in a hereditary breast/ovarian cancer syndrome family member who does not carry the family’s BRCA1 or BRCA2 mutation would be considered a phenocopy. Such an individual does not have the family’s cancer-related mutation and therefore, they do not have the associated cancer risk from that specific mutation.
phenotype
(FEE-noh-tipe)
The observable characteristics in an individual resulting from the expression of genes; the clinical presentation of an individual with a particular genotype.
point mutation
(poynt myoo-TAY-shun)
An alteration in a DNA sequence caused by the substitution of a single nucleotide for another nucleotide.
point variant
(poynt VAYR-ee-unt)
A genetic alteration caused by the substitution of a single nucleotide for another nucleotide. Also called point mutation.
polymerase chain reaction
(puh-LIH-meh-rays chayn ree-AK-shun)
A procedure that produces millions of copies of a short segment of DNA through repeated cycles of: (1) denaturation, (2) annealing, and (3) elongation. Polymerase chain reaction is a very common procedure in molecular genetic testing and may be used to generate a sufficient quantity of DNA to perform a test (e.g., allele-specific amplification, trinucleotide repeat quantification). Also called PCR.
polymorphism
(PAH-lee-MOR-fih-zum)
A common mutation. “Common” is typically defined as an allele frequency of at least 1%. All genes occur in pairs, except when x and y chromosomes are paired in males; thus a polymorphism with an allele frequency of 1% would be found in about 2% of the population, with most carriers having one copy of the polymorphism and one copy of the normal allele.
population risk
(PAH-pyoo-LAY-shun …)
The proportion of individuals in the general population who are affected with a particular disorder or who carry a certain gene; often discussed in the genetic counseling process as a comparison to the patient’s personal risk given his or her family history or other circumstances.
positive predictive value
(PAH-zih-tiv preh-DIK-tiv VAL-yoo)
The likelihood that an individual with a positive test result truly has the particular gene and/or disease in question. Also called PPV.
PPV
The likelihood that an individual with a positive test result truly has the particular gene and/or disease in question. Also called positive predictive value.
predisposing mutation
(PREE-dih-SPOH-zing myoo-TAY-shun)
A germline mutation that increases an individual’s susceptibility or predisposition to a certain disease or disorder. When such a mutation is inherited, development of symptoms is more likely, but not certain. Also called susceptibility gene.
presymptomatic testing
(PREE-simp-toh-MA-tik TES-ting)
Genetic analysis of an asymptomatic or unaffected individual who is at risk of a specific genetic disorder.
proband
(PROH-band)
The individual through whom a family with a genetic disorder is ascertained. In males this is called a propositus, and in females it is called a proposita.
proposita
(proh-PAH-zih-tuh)
The female individual through whom a family with a genetic disorder is ascertained. In males this is called a propositus.
propositus
(proh-PAH-zih-tus)
The male individual through whom a family with a genetic disorder is ascertained. In females this is called a proposita.
pseudogene
(SOO-doh-jeen)
A DNA sequence that resembles a gene but has been mutated into an inactive form over the course of evolution. It often lacks introns and other essential DNA sequences necessary for function. Though genetically similar to the original functional gene, pseudogenes do not result in functional proteins, although some may have regulatory effects.