NCI Dictionary of Genetics Terms

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Welcome to the NCI Dictionary of Genetics Terms, which contains technical definitions for more than 150 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.

17 results found for: P
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.
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.