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late or variable onset
The state in which a genetic trait is expressed later in life or is expressed at no fixed time in a life history.
linkage
The tendency for genes or segments of DNA closely positioned along a chromosome to segregate together at meiosis, and therefore be inherited together.
linkage analysis
A gene-hunting technique that traces patterns of disease in high-risk families. It attempts to locate a disease-causing gene by identifying genetic markers of known chromosomal location that are co-inherited with the trait of interest. (NCI Thesaurus)
locus
The physical site or location of a specific gene on a chromosome. (NCI Thesaurus)
locus heterogeneity
The situation in which mutations in genes at different chromosomal loci result in the same phenotype.
LOD score
Logarithm of the odd score. A statistical estimate of whether two genetic loci are physically near enough to each other (or "linked") on a particular chromosome that they are likely to be inherited together. A LOD score of three or more is generally considered statistically significant evidence of linkage. Also called logarithm of the odd score. (NCI Thesaurus)
logarithm of the odd score
LOD score. A statistical estimate of whether two genetic loci are physically near enough to each other (or "linked") on a particular chromosome that they are likely to be inherited together. A LOD score of three or more is generally considered statistically significant evidence of linkage. Also called LOD score. (NCI Thesaurus)
LOH
Loss of heterozygosity. If there is one normal and one abnormal allele at a particular locus, as might be seen in an inherited autosomal dominant cancer susceptibility disorder, loss of the normal allele produces a locus with no normal function. When the loss of heterozygosity involves the normal allele, it creates a cell that is more likely to show malignant growth if the altered gene is a tumor suppressor gene. Also called loss of heterozygosity. (NCI Thesaurus)
loss of heterozygosity
LOH. If there is one normal and one abnormal allele at a particular locus, as might be seen in an inherited autosomal dominant cancer susceptibility disorder, loss of the normal allele produces a locus with no normal function. When the loss of heterozygosity involves the normal allele, it creates a cell that is more likely to show malignant growth if the altered gene is a tumor suppressor gene. Also called LOH. (NCI Thesaurus)
microsatellite
Repetitive segments of DNA scattered throughout the genome in noncoding regions between genes or within genes (introns). They are often used as markers for linkage analysis because of their naturally occurring high variability in repeat number between individuals. These regions are inherently genetically unstable and susceptible to mutations. (NCI Thesaurus)
microsatellite instability
MSI. A characteristic of cells that contain an abnormality in DNA mismatch repair (see microsatellite). For example, the presence of MSI in colorectal tumor tissue may be used as a marker for germline mutations in one of the DNA mismatch repair genes associated with HNPCC. MSI can also occur sporadically, and in these cases is related to gene hypermethylation. This is an issue in the differential diagnosis of HNPCC. Also called MSI.
missense mutation
A single base pair substitution that alters the genetic code in a way that produces an amino acid that is different from the normal amino acid at that position. (NCI Thesaurus)
mode of inheritance
The manner in which a genetic trait or disorder is passed from one generation to the next. Autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, multifactorial, and mitochondrial inheritance are examples. Each mode of inheritance results in a characteristic pattern of affected and unaffected family members. (NCI Thesaurus)
mosaicism
The occurrence of 2 or more cell lines with different genetic or chromosomal make-up, within a single individual or tissue.
MSI
Microsatellite instability. A characteristic of cells that contain an abnormality in DNA mismatch repair (see microsatellite). For example, the presence of MSI in colorectal tumor tissue may be used as a marker for germline mutations in one of the DNA mismatch repair genes associated with HNPCC. MSI can also occur sporadically, and in these cases is related to gene hypermethylation. This is an issue in the differential diagnosis of HNPCC. Also called microsatellite instability.
mutation
A change in the usual DNA sequence at a particular gene locus. Mutations (including polymorphisms) can be harmful, beneficial, or neutral in their effect on cell function. (NCI Thesaurus)
mutation analysis
Germline genetic testing method targeted to detect a specific mutation (such as a deleterious MSH2 mutation previously identified in a family), panel of mutations (such as the 3 BRCA mutations comprising the founder mutation panel for individuals of Ashkenazi Jewish ancestry) or type of mutation (such as a large deletions or insertions in the BRCA1 gene). This type of testing is distinct from complete gene sequencing or mutation scanning. The latter are designed to detect most mutations in the region being tested. Current usage also applies this term to any genetic test. (NCI Thesaurus)
negative predictive value
NPV. The likelihood that an individual with a negative test result is truly unaffected and/or does not have the particular gene mutation in question. Also called NPV. (NCI Thesaurus)
new mutation
An alteration in a gene that is present for the first time in one family member as a result of a mutation in a germ cell (egg or sperm) of one of the parents, or a mutation that arises in the fertilized egg itself during early embryogenesis. Also called de novo mutation. (NCI Thesaurus)
noncarrier
An individual who does not carry a mutation previously identified in his or her family.
nonpenetrance
The state in which a genetic trait, although present in the appropriate genotype, fails to manifest itself in the phenotype (e.g., a woman with a BRCA1 mutation who lives to be elderly and never develops breast or ovarian cancer).
novel mutation
A newly discovered, distinct gene alteration; NOT the same as new or de novo mutation.
NPV
Negative predictive value. The likelihood that an individual with a negative test result is truly unaffected and/or does not have the particular gene mutation in question. Also called negative predictive value. (NCI Thesaurus)
nucleotide
A molecule consisting of a nitrogen-containing base (adenine, guanine, thymine, or cytosine in DNA; adenine, guanine, uracil, or cytosine in RNA), a phosphate group, and a sugar (deoxyribose in DNA; ribose in RNA). DNA and RNA are polymers comprised of many nucleotides, strung together like beads in a necklace. (NCI Thesaurus)
null allele
A mutation that results in either no gene product or the absence of function at the phenotypic level.
No entries.
PCR
Polymerase chain reaction. 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. (NCI Thesaurus)
pedigree
A graphic illustration of family history. (NCI Thesaurus)
penetrance
A characteristic of a genotype; it refers to the likelihood that a clinical condition will occur when a particular genotype is present. (NCI Thesaurus)
phenocopy
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
The observable characteristics in an individual resulting from the expression of genes; the clinical presentation of an individual with a particular genotype. (NCI Thesaurus)
point mutation
An alteration in a DNA sequence caused by the substitution of a single nucleotide for another nucleotide.
polymerase chain reaction
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 PCR. (NCI Thesaurus)
polymorphism
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. (NCI Thesaurus)
population risk
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
PPV. The likelihood that an individual with a positive test result truly has the particular gene and/or disease in question. Also called PPV. (NCI Thesaurus)
PPV
Positive predictive value. 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. (NCI Thesaurus)
predisposing mutation
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. (NCI Thesaurus)
presymptomatic testing
Genetic analysis of an asymptomatic or unaffected individual who is at risk of a specific genetic disorder.
proband
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
The female individual through whom a family with a genetic disorder is ascertained. In males this is called a propositus.
propositus
The male individual through whom a family with a genetic disorder is ascertained. In females this is called a proposita.
No entries.
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