Genetics glossary

From WikiMD's Wellness Encyclopedia

This is a glossary of terms commonly used in the study of genetics and related disciplines in biology. It is intended as introductory material for novices; for more specific and technical detail, please see the article corresponding to each term. Introductory articles in the field include:

Dictionary of genetic terms


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A[edit | edit source]

A small, benign skin growth that may have a stalk (peduncle). Acrochordons most commonly appear on the neck, axillary, groin, and inframammary regions. Also called skin tag.

A method used to identify genetic variants associated with traits and/or diseases in ethnic groups whose genomes resulted from a recent mixture of two or more geographically distinct ancestral populations.

Individuals in a pedigree who exhibit the specific phenotype under study.

One of two or more DNA sequences occurring at a particular gene locus. Typically one allele (“normal” DNA sequence) is common, and other alleles (mutations) are rare.

Different mutations in the same gene that cause different phenotypic manifestations or severity of disease.

The occurrence of one or more extra or missing chromosomes leading to an unbalanced chromosome complement, or any chromosome number that is not an exact multiple of the haploid number (which is 23).

A type of gene that regulates cell growth. When an antioncogene is mutated, uncontrolled cell growth may occur. This may contribute to the development of cancer. Also called tumor suppressor gene.

A term used to describe collectively a number of noncoital methods of conception that are used to treat infertility with donor or nondonor eggs and sperm including in vitro fertilization (IVF), gamete intrafallopian transfer (GIFT), and zygote intrafallopian transfer (ZIFT). Also called assisted reproductive technology.

One of two major ancestral groups of Jewish individuals, comprised of those whose ancestors lived in Central and Eastern Europe (e.g., Germany, Poland, Russia). The other group is designated Sephardic Jews and includes those whose ancestors lived in North Africa, the Middle East, and Spain. Most Jews living in the United States are of Ashkenazi descent.

A term used to describe collectively a number of noncoital methods of conception that are used to treat infertility with donor or nondonor eggs and sperm including in vitro fertilization (IVF), gamete intrafallopian transfer (GIFT), and zygote intrafallopian transfer (ZIFT). Also called ART.

Proportion of a disease in exposed individuals that can be attributed to an exposure. In the context of genetic studies, the "exposure" is the frequency of a specific genetic variant.

Refers to any of the chromosomes numbered 1-22 or the genes on chromosomes 1-22. This term excludes the sex-determining chromosomes, X and Y.

Autosomal dominant inheritance refers to genetic conditions that occur when a mutation is present in one copy of a given gene (i.e., the person is heterozygous).

Autosomal recessive inheritance refers to genetic conditions that occur only when mutations are present in both copies of a given gene (i.e., the person is homozygous for a mutation, or carries two different mutations of the same gene, a state referred to as compound heterozygosity).

B[edit | edit source]

When chromosomes at a particular stage in cell division are stained using one of several laboratory techniques, a specific pattern of light and dark stripes (bands) appears when the chromosomes are viewed through a microscope; the banding pattern assists in assigning each chromosome its particular number and evaluating its structure.

Two nitrogen-containing bases pair together between double-stranded DNA; only specific combinations of these bases (e.g., adenine with thymine; guanine with cytosine) are possible, a fact which facilitates accurate DNA replication; when quantified (e.g., 8 base pairs, or bp), this term refers to the actual number of base pairs in a sequence of nucleotides.

Of or pertaining to both alleles of a single gene (paternal and maternal). For example, biallelic mutation carriers have a mutation (not necessarily the same mutation) in both copies of a particular gene (a paternal and a maternal mutation).

C[edit | edit source]

In classical genetics, an individual who carries one deleterious allele for an autosomal recessive disorder. In clinical discussions, may refer to an individual who carries a deleterious allele that predisposes to disease.

The proportion of individuals in a population who have a single copy of a specific recessive gene mutation; also sometimes applied to the prevalence of mutations in dominantly acting genes such as BRCA1 and BRCA2. Also called carrier rate.

The proportion of individuals in a population who have a single copy of a specific recessive gene mutation; also sometimes applied to the prevalence of mutations in dominantly acting genes such as BRCA1 and BRCA2. Also called carrier frequency.

A systematic process for the identification of individuals at risk for a hereditary condition. The process begins with the identification of an individual with the condition and/or a pathogenic variant associated with the condition and then extending genetic testing to his/her at-risk biological relatives. This process is repeated as more affected individuals or pathogenic variant carriers are identified.

A genomic imbalance that occurs when a cell has an abnormal number of chromosomes. This can be caused by unexpected chromosomal crossover or by the presence of small, extra-chromosomal pieces of DNA.

Discrete physical structures inside a cell nucleus that consist of proteins and DNA organized into genes.

An identical copy of a DNA sequence or entire gene; one or more cells derived from and identical to a single ancestor cell OR to isolate a gene or specific sequence of DNA.

Refers to the genetic trait involving the number of copies of a particular gene present in the genome of an individual. Genetic variants, including insertions, deletions, and duplications of segments of DNA, are also collectively referred to as CNVs. CNVs account for a significant proportion of the genetic variation between individuals. Also called copy number variant.

In DNA or RNA, a sequence of 3 consecutive nucleotides that codes for a specific amino acid or signals the termination of gene translation (stop or termination codon).

The presence of two different mutated alleles at a particular gene locus.

A type of mutation testing in which a segment of DNA is screened for mismatched pairing between normal and mutated base pairs. Also called CSGE.

A condition or trait present at birth. It may be the result of genetic or non-genetic factors.

Genetic relatedness between individuals who are descendants of at least one common ancestor.

A process of information exchange between a clinician and an individual or their legal proxy designed to facilitate autonomous, informed decision making. The informed consent process for genetic testing should include an explanation of the medical and psychosocial risks, benefits, limitations, and potential implications of genetic analysis, a discussion of privacy, confidentiality, the documentation and handling of genetic test results, as well as options for managing the hereditary disease risk. Also called informed consent.

Constitutional DNA refers to tissue derived from reproductive cells (egg or sperm) that become incorporated into the DNA of every cell in the body of the offspring. A germline mutation may be passed from parent to offspring. Also called germline DNA.

An individual who presents for genetic counseling. Also called counselee.

Refers to the genetic trait involving the number of copies of a particular gene present in the genome of an individual. Genetic variants, including insertions, deletions, and duplications of segments of DNA, are also collectively referred to as copy number variants. Copy number variants account for a significant proportion of the genetic variation between individuals. Also called CNV.

The transmission, together, of 2 or more genes on the same chromosome, as a result of their being in very close physical proximity to one another (i.e., linked).

An individual who presents for genetic counseling. Also called consultand.

Discrete aggregates of lymphoid white blood cells, some with germinal centers and surrounding fibrosis, commonly found around some colorectal adenocarcinomas in the absence of a clinical or pathological diagnosis of Crohn disease. Also called Crohn-like reaction.

Discrete aggregates of lymphoid white blood cells, some with germinal centers and surrounding fibrosis, commonly found around some colorectal adenocarcinomas in the absence of a clinical or pathological diagnosis of Crohn disease. Also called Crohn disease-like reaction.

A type of mutation testing in which a segment of DNA is screened for mismatched pairing between normal and mutated base pairs. Also called conformation-sensitive gel electrophoresis.

A benign tumor that arises from smooth muscle tissue in a hair follicle, forming a papule. Cutaneous leiomyomas (or leiomyomata) can be painful in the presence of cold or tactile stimuli.

Itchy papules that are brownish-red in color and typically appear on the shins, thighs, feet, or neck.

The study of the structure, function, and abnormalities of human chromosomes.

D[edit | edit source]

A set of clinical criteria that is used to stratify prostate cancers into three risk categories to estimate cancer growth and spread. The criteria include Gleason score, prostate-specific antigen level, and clinical stage at diagnosis.

A genetic alteration that is present for the first time in one family member as a result of a variant (or mutation) in a germ cell (egg or sperm) of one of the parents, or a variant that arises in the fertilized egg itself during early embryogenesis. Also called de novo variant, new mutation, and new variant.

A genetic alteration that is present for the first time in one family member as a result of a variant (or mutation) in a germ cell (egg or sperm) of one of the parents, or a variant that arises in the fertilized egg itself during early embryogenesis. Also called de novo mutation, new mutation, and new variant.

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 disease-causing mutation, pathogenic variant, predisposing mutation, and susceptibility gene mutation.

A type of genetic change that involves the absence of a segment of DNA. It may be as small as a single base but can vary significantly in size.

The molecular basis of heredity; encodes the genetic information responsible for the development and function of an organism and allows for transmission of that genetic information from one generation to the next. The DNA molecule is structured as a double-stranded helix held together by weak hydrogen bonds between purine-pyrimidine nucleotide base pairs: adenine (A) paired with thymine (T), and guanine (G) paired with cytosine (C). Also called DNA.

Refers to the number of times a nucleotide is read during sequencing. A greater depth of coverage can increase confidence in the final results. Deep coverage aids in differentiating sequencing errors from single nucleotide polymorphisms. This can be specifically useful when a patient has a mosaicism or when a tumor is heterogeneous for a mutation.

Presence of necrotic cellular debris within the lumen of the neoplastic glands in the colorectal mucosa.

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, pathogenic variant, predisposing mutation, and susceptibility gene mutation.

The molecular basis of heredity; encodes the genetic information responsible for the development and function of an organism and allows for transmission of that genetic information from one generation to the next. The DNA molecule is structured as a double-stranded helix held together by weak hydrogen bonds between purine-pyrimidine nucleotide base pairs: adenine (A) paired with thymine (T), and guanine (G) paired with cytosine (C). Also called deoxyribonucleic acid.

A specific physical region or amino acid sequence in a protein which is associated with a particular function or corresponding segment of DNA.

The presence of two different mutated alleles at two separate genetic loci.

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A heritable change that does not affect the DNA sequence but results in a change in gene expression. Examples include promoter methylation and histone modifications. Also called epigenetic variant and epimutation.

A heritable change that does not affect the DNA sequence but results in a change in gene expression. Examples include promoter methylation and histone modifications. Also called epigenetic alteration and epimutation.

The study of heritable changes that do not affect the DNA sequence but influence gene expression.

A heritable change that does not affect the DNA sequence but results in a change in gene expression. Examples include promoter methylation and histone modifications. Also called epigenetic alteration and epigenetic variant.

ER-negative PR-negative HER2/neu-negative breast cancer is defined by a lack of expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2/neu). Also called triple-negative breast cancer.

The sequence of DNA present in mature messenger RNA, some of which encodes the amino acids of a protein. Most genes have multiple exons with introns between them.

F[edit | edit source]

A distinctive facial feature or expression that is characteristic of a specific condition.

A test result that indicates an individual is affected and/or has a certain gene mutation when he or she is actually unaffected and/or does not have the mutation; i.e., a positive test result in a truly unaffected or mutation-negative individual.

A phenotype or trait that occurs with greater frequency in a given family than in the general population; familial traits may have a genetic and/or nongenetic etiology.

The genetic relationships within a family combined with the medical history of individual family members. When represented in diagram form using standardized symbols and terminology, it is usually referred to as a pedigree or family tree. Also called family medical history.

The genetic relationships within a family combined with the medical history of individual family members. When represented in diagram form using standardized symbols and terminology, it is usually referred to as a pedigree or family tree. Also called family history.

The parents, siblings, or children of an individual. Also called first-degree relative.

A benign tumor of the hair follicle that appears as a small, whitish papule. Fibrofolliculomas are typically found on the face, ears, neck, and upper torso. They are pathognomonic for Birt-Hogg-Dubé syndrome, a hereditary condition associated with the development of kidney cancer.

The parents, siblings, or children of an individual. Also called FDR.

A technique used to identify the presence of specific chromosomes or chromosomal regions through hybridization (attachment) of fluorescently-labeled DNA probes to denatured chromosomal DNA. Examination through a microscope under fluorescent lighting detects the presence of the colored hybridized signal (and hence presence of the chromosome material) or absence of the hybridized signal (and hence absence of the chromosome material). Also called fluorescence in situ hybridization.

A technique used to identify the presence of specific chromosomes or chromosomal regions through hybridization (attachment) of fluorescently-labeled DNA probes to denatured chromosomal DNA. Examination through a microscope under fluorescent lighting detects the presence of the colored hybridized signal (and hence presence of the chromosome material) or absence of the hybridized signal (and hence absence of the chromosome material). Also called FISH.

A genetic alteration observed with high frequency in a group that is or was geographically or culturally isolated, in which one or more of the ancestors was a carrier of the altered gene. This phenomenon is often called a founder effect. Also called founder variant.

A genetic alteration observed with high frequency in a group that is or was geographically or culturally isolated, in which one or more of the ancestors was a carrier of the altered gene. This phenomenon is often called a founder effect. Also called founder mutation.

An insertion or deletion involving a number of base pairs that is not a multiple of three, which consequently disrupts the triplet reading frame of a DNA sequence. Such variants (or mutations) usually lead to the creation of a premature termination (stop) codon, and result in a truncated (shorter-than-normal) protein product. Also called frameshift variant.

An insertion or deletion involving a number of base pairs that is not a multiple of three, which consequently disrupts the triplet reading frame of a DNA sequence. Such variants (or mutations) usually lead to the creation of a premature termination (stop) codon, and result in a truncated (shorter-than-normal) protein product. Also called frameshift mutation.

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G[edit | edit source]

The basic unit of heredity that occupies a specific location on a chromosome. Each consists of nucleotides arranged in a linear manner. Most genes code for a specific protein or segment of protein leading to a particular characteristic or function.

A phenomenon in which the signs and symptoms of some genetic conditions tend to become more severe and/or appear at an earlier age as the disorder is passed from one generation to the next. Huntington disease is an example of a genetic disorder in which the biological mechanism for this phenomenon has been well documented. In other cases, it may be due to factors such as increased surveillance or other nongenetic causes.

A communication process that seeks to assist affected or at-risk individuals and families in understanding the natural history, disease risks, and mode of transmission of a genetic disorder; to facilitate informed consent for genetic testing when appropriate; to discuss options for risk management and family planning; and to provide for or refer individuals for psychosocial support as needed.

The production of the same or similar phenotypes (observed biochemical, physiological, and morphological characteristics of a person determined by his/her genotype) by different genetic mechanisms. There are two types: (1) allelic heterogeneity – when different alleles at a locus can produce variable expression of a condition; and (2) locus heterogeneity – the term used to describe disease in which mutations at different loci can produce the same disease phenotype.

An identifiable segment of DNA (e.g., Single Nucleotide Polymorphism [SNP], Restriction Fragment Length Polymorphism [RFLP], Variable Number of Tandem Repeats [VNTR], microsatellite) with enough variation between individuals that its inheritance and co-inheritance with alleles of a given gene can be traced; used in linkage analysis.

Increased likelihood or chance of developing a particular disease due to the presence of one or more gene mutations and/or a family history that indicates an increased risk of the disease. Also called genetic susceptibility.

Genetic testing designed to identify individuals in a given population who are at higher risk of having or developing a particular disorder, or carrying a gene for a particular disorder.

Increased likelihood or chance of developing a particular disease due to the presence of one or more gene mutations and/or a family history that indicates an increased risk of the disease. Also called genetic predisposition.

An alteration in the most common DNA nucleotide sequence. The term variant can be used to describe an alteration that may be benign, pathogenic, or of unknown significance. The term variant is increasingly being used in place of the term mutation.

An inherited syndrome that includes a dermatological (skin) phenotype.

A genome-wide association study (GWAS) is a way for scientists to identify inherited genetic variants associated with risk of disease or a particular trait. This method surveys the entire genome for genetic polymorphisms, typically single nucleotide polymorphisms (SNPs) (pronounced “snips”), that occur more frequently in cases (people with the disease or trait being assessed) than in controls (people without the disease or trait). Also called GWAS.

An epigenetic process resulting in the inactivation of an allele depending on which parent it was inherited from. Genomic imprinting can have clinical relevance because it may affect the expression of a gene mutation (i.e., the phenotype) in the offspring of an affected parent depending on which parent is passing on the mutation.

At its broadest level, genotype includes the entire genetic constitution of an individual. It is often applied more narrowly to the set of alleles present at one or more specific loci.

A laboratory process in which an individual’s germline DNA is analyzed for specific nucleotides or bases to determine whether certain variants are present. Genotyping differs from sequencing in which all of the nucleotides comprising a specific length of DNA are assessed (e.g., within a gene, exome, or genome).

The cells from which eggs or sperm (i.e., gametes) are derived.

Germline DNA refers to tissue derived from reproductive cells (egg or sperm) that become incorporated into the DNA of every cell in the body of the offspring. A germline mutation may be passed from parent to offspring. Also called constitutional DNA.

A gene change in a reproductive cell (egg or sperm) that becomes incorporated into the DNA of every cell in the body of the offspring. A variant contained within the germline can be passed from parent to offspring, and is, therefore, hereditary.

A GWAS (genome-wide association study) is a way for scientists to identify inherited genetic variants associated with risk of disease or a particular trait. This method surveys the entire genome for genetic polymorphisms, typically single nucleotide polymorphisms (SNPs) (pronounced “snips”), that occur more frequently in cases (people with the disease or trait being assessed) than in controls (people without the disease or trait). Also called genome-wide association study.

H[edit | edit source]

The situation that occurs when one copy of a gene is inactivated or deleted and the remaining functional copy of the gene is not adequate to produce the needed gene product to preserve normal function.

A set of closely linked genetic markers present on one chromosome which tend to be inherited together.

Describes an individual who has only one member of a chromosome pair or chromosome segment rather than the usual two. Hemizygosity is often used to describe X-linked genes in males who have only one X chromosome. This term is sometimes used in somatic cell genetics where cancer cell lines are often hemizygous for certain alleles or chromosomal regions.

The proportion of variation in a population trait that can be attributed to inherited genetic factors. Heritability estimates range from 0 to 1 and are often expressed as a percentage. A number close to 1 may be indicative of a highly heritable trait within a population. It should not be used to estimate risk on an individual basis.

A method of detecting sequence differences between normal DNA and the DNA to be tested. It is commonly used as a screening method to detect potential mutations in a gene.

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 heterogeneity logarithm of the odds score is calculated in the presence of locus heterogeneity (when the same phenotype can be caused by mutations in genes at different chromosomal loci). Also called HLOD score.

Occurs when the two alleles at a particular gene locus are different. A heterozygous genotype may include one normal allele and one mutation, or two different mutations. The latter is called a compound heterozygote.

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. An HLOD score is calculated in the presence of locus heterogeneity (when the same phenotype can be caused by mutations in genes at different chromosomal loci). Also called heterogeneity logarithm of the odds score.

Occurs when both alleles at a particular gene locus are the same. A person may be homozygous for the normal allele or for a mutation.

Deficiency of melanin.

I[edit | edit source]

Penetrance refers to the likelihood that a clinical condition will occur when a particular genotype is present. A condition is said to show incomplete penetrance when some individuals who carry the pathogenic variant express the associated trait while others do not.

A negative test result in an individual where a clearly deleterious mutation has not been found in any family members. The genetic risk status of such an individual must be interpreted in the context of his or her personal and family history. Also called indeterminate result and uninformative result.

A negative test result in an individual where a clearly deleterious mutation has not been found in any family members. The genetic risk status of such an individual must be interpreted in the context of his or her personal and family history. Also called inconclusive result and uninformative result.

A clinically affected individual through whom attention is first drawn to a genetic disorder in a family.

In genetic testing, a test result that reveals definitively the presence or absence of the germline genetic alteration associated with the hereditary disorder being assessed. In linkage analysis, the ability to distinguish between maternally inherited and paternally inherited DNA markers (polymorphisms) within or near a given gene of interest.

A process of information exchange between a clinician and an individual or their legal proxy designed to facilitate autonomous, informed decision making. The informed consent process for genetic testing should include an explanation of the medical and psychosocial risks, benefits, limitations, and potential implications of genetic analysis, a discussion of privacy, confidentiality, the documentation and handling of genetic test results, as well as options for managing the hereditary disease risk. Also called consent process.

Describes the clinical manifestations associated with a mutation conferring cancer susceptibility.

A type of genetic change that involves the addition of a segment of DNA. It may be as small as a single base but can vary significantly in size.

The sequence of DNA in between exons that is initially copied into RNA but is cut out of the final RNA transcript and therefore does not change the amino acid code. Some intronic sequences are known to affect gene expression.

A chromosomal defect in which a segment of the chromosome breaks off and reattaches in the reverse direction.

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An extended family.

L[edit | edit source]

The state in which a genetic trait is expressed later in life or is expressed at no fixed time in a life history.

Where alleles (DNA markers) occur together more often than can be accounted for by chance because of their physical proximity on a chromosome. Also called linkage disequilibrium.

The tendency for genes or segments of DNA closely positioned along a chromosome to segregate together at meiosis, and therefore be inherited together.

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.

Where alleles (DNA markers) occur together more often than can be accounted for by chance because of their physical proximity on a chromosome. Also called LD.

The physical site or location of a specific gene on a chromosome.

The same phenotype is caused by mutations in genes at different chromosomal loci.

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 3 or higher is generally understood to mean that two genes are located close to each other on the chromosome. In terms of significance, a LOD score of 3 means the odds are 1,000:1 that the two genes are linked and therefore inherited together. Also called logarithm of the odds 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 logarithm of the odds score of 3 or higher is generally understood to mean that two genes are located close to each other on the chromosome. In terms of significance, a logarithm of the odds score of 3 means the odds are 1,000:1 that the two genes are linked and therefore inherited together. Also called LOD score.

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.

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.

M[edit | edit source]

A high-throughput method used to determine a portion of the nucleotide sequence of an individual’s genome. This technique utilizes DNA sequencing technologies that are capable of processing multiple DNA sequences in parallel. Also called next-generation sequencing and NGS.

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.

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.

A genetic alteration in which a single base pair substitution alters the genetic code in a way that produces an amino acid that is different from the usual amino acid at that position. Some missense variants (or mutations) will alter the function of the protein. Also called missense variant.

A genetic alteration in which a single base pair substitution alters the genetic code in a way that produces an amino acid that is different from the usual amino acid at that position. Some missense variants (or mutations) will alter the function of the protein. Also called missense mutation.

A laboratory method commonly used for the detection of unusual copy number changes (insertions or deletions) of genomic sequences. Also called multiplex ligation-dependent probe amplification.

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.

The occurrence of 2 or more cell lines with different genetic or chromosomal make-up, within a single individual or tissue.

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.

Genetic tests that use next-generation sequencing to test multiple genes simultaneously. Also called multiple-gene panel test and multiple-gene test.

Genetic tests that use next-generation sequencing to test multiple genes simultaneously. Also called multigene test and multiple-gene test.

Genetic tests that use next-generation sequencing to test multiple genes simultaneously. Also called multigene test and multiple-gene panel test.

A method for detecting multiple genetic alterations (i.e., gene mutations or single nucleotide polymorphisms in a single gene or across the genome) simultaneously.

A laboratory method commonly used for the detection of unusual copy number changes (insertions or deletions) of genomic sequences. Also called MLPA.

A change in the usual DNA sequence at a particular gene locus. Although the term often has a negative connotation, mutations (including polymorphisms) can be harmful, beneficial, or neutral in their effect on cell function. The term variant is sometimes used as a synonym for the term mutation.

A germline genetic testing method targeted to detect a specific variant or mutation (such as a deleterious MSH2 variant previously identified in a family), panel of variants (such as the 3 BRCA pathogenic variants comprising the founder mutation panel for individuals of Ashkenazi Jewish ancestry) or type of variant (such as large deletions or insertions in the BRCA1 gene). This type of testing is distinct from complete gene sequencing or variant scanning. The latter are designed to detect most variants in the region being tested. Current usage also applies this term to any genetic test.

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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.

A genetic alteration that is present for the first time in one family member as a result of a variant (or mutation) in a germ cell (egg or sperm) of one of the parents, or a variant that arises in the fertilized egg itself during early embryogenesis. Also called de novo mutation, de novo variant, and new variant.

A genetic alteration that is present for the first time in one family member as a result of a variant (or mutation) in a germ cell (egg or sperm) of one of the parents, or a variant that arises in the fertilized egg itself during early embryogenesis. Also called de novo mutation, de novo variant, and new mutation.

A high-throughput method used to determine a portion of the nucleotide sequence of an individual’s genome. This technique utilizes DNA sequencing technologies that are capable of processing multiple DNA sequences in parallel. Also called massively parallel sequencing and NGS.

A high-throughput method used to determine a portion of the nucleotide sequence of an individual’s genome. This technique utilizes DNA sequencing technologies that are capable of processing multiple DNA sequences in parallel. Also called massively parallel sequencing and next-generation sequencing.

An individual who does not carry a mutation previously identified in his or her family.

A research study that is designed to determine whether one intervention is not worse than another control intervention by a predetermined margin. An intervention that yields outcomes which are equivalent to or better than the control intervention is considered not inferior to the control intervention. Noninferiority studies are often conducted to examine whether a new, experimental treatment is not worse than an established standard of care.

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).

A genetic alteration that causes the premature termination of a protein. The altered protein may be partially or completely inactivated, resulting in a change or loss of protein function. Also called nonsense variant.

A genetic alteration that causes the premature termination of a protein. The altered protein may be partially or completely inactivated, resulting in a change or loss of protein function. Also called nonsense mutation.

A newly discovered, distinct genetic alteration; NOT the same as new or de novo variant (or mutation). Also called novel variant.

A newly discovered, distinct genetic alteration; NOT the same as new or de novo variant (or mutation). Also called novel mutation.

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.

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.

A mutation that results in either no gene product or the absence of function at the phenotypic level.

The study of the interaction of dietary and genetic factors and its effect on metabolism, health status, and risk of disease.

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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 mutation.

Findings that are distinctive or characteristic of a particular disease or condition and make the diagnosis.

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.

A graphic illustration of family history.

Penetrance refers to the likelihood that a clinical condition will occur when a particular genotype is present. For adult-onset diseases, penetrance is usually described by the individual carrier's age, sex, and organ site. For example, the penetrance for breast cancer in female carriers of BRCA1 pathogenic variants is often quoted by age 50 years and by age 70 years.

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.

The observable characteristics in an individual resulting from the expression of genes; the clinical presentation of an individual with a particular genotype.

Irregular patterns of reddish brown pigmentation of the skin associated with sun exposure, aging, or a genetic cause.

A genetic alteration caused by the substitution of a single nucleotide for another nucleotide. Also called point variant.

A genetic alteration caused by the substitution of a single nucleotide for another nucleotide. Also called point mutation.

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.

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.

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.

The likelihood that an individual with a positive test result truly has the particular gene and/or disease in question. Also called 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.

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, pathogenic variant, and susceptibility gene mutation.

Genetic analysis of an asymptomatic or unaffected individual who is at risk of a specific genetic disorder.

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.

The female individual through whom a family with a genetic disorder is ascertained. In males this is called a propositus.

The male individual through whom a family with a genetic disorder is ascertained. In females this is called a proposita.

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.

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In genetics, the likelihood that a hereditary trait or disorder present in one family member will occur again in other family members. This is distinguished from recurrence risk for cancer, which is the chance that a cancer that has been treated will recur.

The quantitative or qualitative assessment of an individual’s risk of carrying a certain gene mutation, or developing a particular disorder, or of having a child with a certain disorder; sometimes done by using mathematical or statistical models incorporating such factors as personal health history, family medical history and ethnic background.

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A low-throughput method used to determine a portion of the nucleotide sequence of an individual’s genome. This technique uses polymerase chain reaction (PCR) amplification of genetic regions of interest followed by sequencing of PCR products.

Clinical evaluation of an asymptomatic individual in the general population aimed at identifying abnormalities which might signal the presence of a specific medical condition. The intent is to find diseases at the earliest possible stage in their development in order to improve the chances for cure or reduce morbidity.

The aunts, uncles, grandparents, grandchildren, nieces, nephews, or half-siblings of an individual. Also called second-degree relative.

The aunts, uncles, grandparents, grandchildren, nieces, nephews, or half-siblings of an individual. Also called SDR.

The process of fitting formal genetic models to data on expressed disease characteristics (phenotype) in biological family members in order to determine the most likely mode of inheritance for the trait or disease under study.

The frequency with which a test yields a true positive result among individuals who actually have the disease or the gene mutation in question. A test with high sensitivity has a low false-negative rate and thus does a good job of correctly identifying affected individuals.

A single, isolated occurrence of an inherited condition in a family. Simplex cases can result from certain inheritance patterns or be caused by de novo mutations or reduced penetrance within the family. It can also result from nongenetic causes such as adoption or alternate paternity.

DNA sequence variations that occur when a single nucleotide (adenine, thymine, cytosine, or guanine) in the genome sequence is altered; usually present in at least 1% of the population. Also called SNP.

A laboratory test used to separate single-stranded nucleic acids based on subtle differences in their DNA sequence, often a single base pair, which results in a different secondary structure and a measurable difference in mobiity through a gel. Also called SSCP analysis.

A type of mutation scanning; the identification of abnormally-migrating single-stranded DNA segments on gel electrophoresis. Also called SSCP.

Mitotic crossover between homologous chromosomes.

A small, benign skin growth that may have a stalk (peduncle). Skin tags most commonly appear on the neck, axillary, groin, and inframammary regions. Also called acrochordon.

DNA sequence variations that occur when a single nucleotide (adenine, thymine, cytosine, or guanine) in the genome sequence is altered; usually present in at least 1% of the population. Also called single nucleotide polymorphism.

An alteration in DNA that occurs after conception and is not present within the germline. Somatic variants can occur in any of the cells of the body except the germ cells (sperm and egg) and therefore are not passed on to children. Somatic variants can (but do not always) cause cancer or other diseases.

Electrophoresis-based technique used in genetic testing to detect large deletions in DNA that can be missed by PCR-based genetic testing methods.

The frequency with which a test yields a true negative result among individuals who do not have the disease or the gene mutation in question. A test with high specificity has a low false-positive rate and thus does a good job of correctly classifying unaffected individuals.

A genetic alteration in the DNA sequence that occurs at the boundary of an exon and an intron (splice site). This change can disrupt RNA splicing resulting in the loss of exons or the inclusion of introns and an altered protein-coding sequence. Also called splice-site variant.

A genetic alteration in the DNA sequence that occurs at the boundary of an exon and an intron (splice site). This change can disrupt RNA splicing resulting in the loss of exons or the inclusion of introns and an altered protein-coding sequence. Also called splice-site mutation.

The process by which introns, the noncoding regions of genes, are excised out of the primary messenger RNA transcript, and the exons (i.e., coding regions) are joined together to generate mature messenger RNA. The latter serves as the template for synthesis of a specific protein.

This term has two meanings. It is sometimes used to differentiate cancers occurring in people who do not have a germline mutation that confers increased susceptibility to cancer from cancers occurring in people who are known to carry a mutation. Cancer developing in people who do not carry a high-risk mutation is referred to as sporadic cancer. The distinction is not absolute, because genetic background may influence the likelihood of cancer even in the absence of a specific predisposing mutation. Alternatively, sporadic is also sometimes used to describe cancer occurring in individuals without a family history of cancer.

A type of mutation scanning; the identification of abnormally-migrating single-stranded DNA segments on gel electrophoresis. Also called single-stranded conformational polymorphism.

A laboratory test used to separate single-stranded nucleic acids based on subtle differences in their DNA sequence, often a single base pair, which results in a different secondary structure and a measurable difference in mobiity through a gel. Also called single-stranded conformation polymorphism analysis.

The summation of all positive pedigree LOD scores (statistical estimates of whether two genetic loci are physically near enough to each other on a particular chromosome that they are likely to be inherited together) at each point in the genome. Also called summary logarithm of the odds score.

The summation of all positive pedigree LOD scores (statistical estimates of whether two genetic loci are physically near enough to each other on a particular chromosome that they are likely to be inherited together) at each point in the genome. Also called sumLOD score.

Periodic clinical evaluation of an individual who is at increased risk of developing a condition (compared with the general population) aimed at detecting new or recurrent disease. In public health, surveillance may also refer to the systematic collection of information regarding the incidence, prevalence, and mortality related to various medical conditions or health-related events.

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, pathogenic variant, and predisposing mutation.

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A single nucleotide polymorphism, or SNP, that is used to “tag” a particular haplotype in a region of the genome. As a subset of all of the SNPs in the genome, tagging SNPs can be extremely useful for testing the association of a marker locus with a qualitative or quantitative trait locus in that it may not be necessary to genotype all of the SNPs. Also called tagSNP.

A single nucleotide polymorphism, or SNP, that is used to “tag” a particular haplotype in a region of the genome. As a subset of all of the SNPs in the genome, tagSNPs can be extremely useful for testing the association of a marker locus with a qualitative or quantitative trait locus in that it may not be necessary to genotype all of the SNPs. Also called tagging SNP.

The end of a chromosome. Telomeres are made of repetitive sequences of non-coding DNA that protect the chromosome from damage. Telomeres become shorter each time the cell divides.

The quantity of information, people, or materials that is put through a process in a specific period of time. In medicine, it can be used to describe the efficiency of laboratory procedures, such as genetic sequencing, or the number of patients seen in a clinic in a certain period of time.

The process of synthesizing messenger RNA (mRNA) from DNA.

The process of synthesizing an amino acid sequence (protein product) from the messenger RNA code.

A type of chromosomal abnormality in which a chromosome breaks and a portion of it reattaches to a different chromosomal location.

A benign tumor arising from the outer cells of the hair follicle.

Sequences of 3 nucleotides repeated in tandem on the same contiguous section of chromosome. A certain amount of normal (polymorphic) variation in repeat number with no clinical significance commonly occurs between individuals; however, repeat numbers over a certain threshold can, in some cases, lead to adverse effects on the function of the gene, resulting in genetic disease.

Triple-negative breast cancer is defined by a lack of expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2/neu). Also called ER-negative PR-negative HER2/neu-negative breast cancer.

The presence of an extra chromosome, resulting in a total of three copies of that chromosome instead of the normal 2 copies (e.g., trisomy 21, or Down syndrome).

Sequencing of somatic tissue, such as tumors, refers to looking for variants in DNA that typically occur after conception. Somatic mutations can occur in any of the cells of the body except the germ cells (sperm and egg) and therefore are not passed on to children. These variants can (but do not always) cause cancer or other diseases.

A type of gene that regulates cell growth. When a tumor suppressor gene is mutated, uncontrolled cell growth may occur. This may contribute to the development of cancer. Also called antioncogene.

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An individual who does not manifest symptoms of a condition or disease occurring in his or her family.

A variation in a genetic sequence for which the association with disease risk is unclear. Also called variant of uncertain significance, variant of unknown significance, and VUS.

A negative test result in an individual where a clearly deleterious mutation has not been found in any family members. The genetic risk status of such an individual must be interpreted in the context of his or her personal and family history. Also called inconclusive result and indeterminate result.

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Variation in the manner in which a trait is manifested. When there is variable expressivity, the trait may vary in clinical expression from mild to severe. For example, the condition neurofibromatosis type 1 may be mild, presenting with café-au-lait spots only, or may be severe, presenting with neurofibromas and brain tumors.

A variation in a genetic sequence for which the association with disease risk is unclear. Also called unclassified variant, variant of unknown significance, and VUS.

A variation in a genetic sequence for which the association with disease risk is unclear. Also called unclassified variant, variant of uncertain significance, and VUS.

A variation in a genetic sequence for which the association with disease risk is unclear. Also called unclassified variant, variant of uncertain significance, and variant of unknown significance.

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A laboratory process that is used to determine the nucleotide sequence primarily of the exonic (or protein-coding) regions of an individual’s genome and related sequences, representing approximately 1% of the complete DNA sequence.

A laboratory process that is used to determine nearly all of the approximately 3 billion nucleotides of an individual’s complete DNA sequence, including non-coding sequence.

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X-linked dominant inheritance refers to genetic conditions associated with mutations in genes on the X chromosome. A single copy of the mutation is enough to cause the disease in both males (who have one X chromosome) and females (who have two X chromosomes). In some conditions, the absence of a functional gene results in the death of affected males.

X-linked recessive inheritance refers to genetic conditions associated with mutations in genes on the X chromosome. A male carrying such a mutation will be affected, because he carries only one X chromosome. A female carrying a mutation in one gene, with a normal gene on the other X chromosome, is generally unaffected. A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

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A score that indicates how many standard deviations a value is above or below the mean.


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