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NCI Dictionary of Genetics Terms

NCI Dictionary of Genetics Terms


17 results found for: P



  • pathognomonic  listen  (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  listen  (PEH-dih-gree)
    A graphic illustration of family history.

  • penetrance  listen  (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  listen  (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  listen  (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.