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Welcome to the NCI Dictionary of Genetics Terms, which contains definitions for more than 100 terms related to genetics. These definitions were developed by the PDQ® Cancer Genetics Editorial Board to support the comprehensive, evidence-based, peer-reviewed PDQ genetics cancer information summaries.
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affected
Individuals in a pedigree who exhibit the specific phenotype under study.
allele
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. (NCI Thesaurus)
allelic heterogeneity
Different mutations in the same gene that cause different phenotypic manifestations or severity of disease.
aneuploidy
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 46). (NCI Thesaurus)
Ashkenazi Jews
One of two major ancestral groups of Jewish individuals, comprised of those whose ancestors lived in Eastern Europe (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. Also called Eastern European Jews. (NCI Thesaurus)
assisted reproductive technology
ART. 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).
attributable risk
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.
autosomal
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
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
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).
band
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. (NCI Thesaurus)
base pair
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.
cancer screening
Clinical testing designed to identify the presence of a specific cancer in an asymptomatic individual or population thought to be at risk of that specific cancer. The intent is to find cancers at the earliest possible stage in their development, in order to improve the chances for disease cure. (NCI Thesaurus)
carrier
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.
carrier frequency
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.
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.
chromosome
Discrete physical structures inside a cell nucleus that consist of proteins and DNA organized into genes. (NCI Thesaurus)
clone
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. (NCI Thesaurus)
clone
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. (NCI Thesaurus)
CNV
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 copy number variant.
conformation-sensitive gel electrophoresis
CSGE. 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.
congenital
A condition or trait present at birth, whether the result of a genetic or non-genetic factors.
consanguinity
Genetic relatedness between individuals who are descendants of at least one common ancestor.
consultand
An individual who presents for genetic counseling. Also called counselee.
copy number variant
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.
cosegregation
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).
counselee
An individual who presents for genetic counseling. Also called consultand.
CSGE
Conformation-sensitive gel electrophoresis. 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.
cytogenetics
The study of the structure, function, and abnormalities of human chromosomes. (NCI Thesaurus)
de novo 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 new mutation. (NCI Thesaurus)
deleterious mutation
A mutation that is documented to be associated with risk of disease.
deletion
Absence of a segment of DNA; may be as small as a single base or as large as a whole chromosome. (NCI Thesaurus)
deoxyribonucleic acid
DNA. 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. (NCI Thesaurus)
dirty necrosis
Presence of necrotic cellular debris within the lumen of the neoplastic glands in the colorectal mucosa.
disease-causing mutation
A gene alteration that causes or predisposes an individual to a specific disease.
DNA
Deoxyribonucleic acid. 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. (NCI Thesaurus)
domain
A specific physical region or amino acid sequence in a protein which is associated with a particular function or corresponding segment of DNA.
Eastern European Jews
One of two major ancenstral groups of Jewish individuals, comprised of those whose ancestors lived in Eastern Europe (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 Eastern European descent. Also called Ashkenazi Jews. (NCI Thesaurus)
exon
Coding sequence of DNA present in mature messenger RNA. Most genes have multiple exons. (NCI Thesaurus)
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