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Term Glossary

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Glossary of Terms:


Allele: One version of a gene at a given location (locus) along a chromosome.

Atrophy: Wasting away or diminution.

Autosomal: Refers to any of the chromosomes other than the sex-determining chromosomes (i.e., the X and Y) or the genes on these chromosomes.

Autosomal Dominant: Describes a trait or disorder in which the phenotype is expressed in those who have inherited only one copy of a particular gene mutation (heterozygotes); specifically refers to a gene on one of the 22 pairs of autosomes (non-sex chromosomes).

Autosomal Recessive: Describes a trait or disorder requiring the presence of two copies of a gene mutation at a particular locus in order to express observable phenotype; specifically refers to genes on one of the 22 pairs of autosomes (non-sex chromosomes).


Base Pair (bp): Two nitrogenous bases (adenine and thymine or guanine and cytosine) held together by weak bonds. Two strands of DNA are held together in the shape of a double helix by the bonds between base pairs.

Base Sequence: The order of nucleotide bases in a DNA molecule.

Base Sequence Analysis: A method, sometimes automated, for determining the base sequence.


Chromosome: Physical structure consisting of a large DNA molecule organized into genes and supported by proteins called chromatin.

Compound Heterozygote: An individual who has two different abnormal alleles at a particular locus, one on each chromosome of a pair; usually refers to individuals affected with an autosomal recessive disorder.

Congenita or Congenital: Occurring at or near birth.


Deletion: Absence of a segment of DNA; may be as small as a single base or as large as one or more genes.

De Novo Mutation: (synonyms: de novo gene mutation new gene mutation, 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 in the fertilized egg itself.

Diplopia: The condition in which a single object appears as two objects. Also called "double vision."

Diploid: The normal number of chromosomes in a somatic cell; in humans, 46 chromosomes (22 pairs of autosomes and two sex chromosomes).

Direct DNA Analysis: (synonym: direct DNA) The use of any test method, such as sequence analysis, mutation scanning, or mutation analysis to detect a mutation in a gene

Disease-Causing Mutation: A gene alteration that causes or predisposes an individual to a specific disease.

DNA: (synonym: deoxyribonucleic acid) the molecule, which encodes the genes responsible for the structure and function of an organism and allows for transmission of genetic information from one generation to the next.

Dysarthria: Speech that is characteristically slurred, slow, and difficult to produce (difficult to understand). The person with dysarthria may also have problems controlling the pitch, loudness, rhythm, and voice qualities of their speech.


Exacerbation: The abrupt worsening.


Gene: the basic unit of heredity, consisting of a segment of DNA arranged in a linear manner along a chromosome. A gene codes for a specific protein or segment of protein leading to a particular characteristic or function.

Gene Product: Genes are transcribed into segments of RNA (ribonucleic acid), which are translated into proteins. Both RNA and proteins are products of the expression of the gene.

Gene Symbol: A unique abbreviation of a gene name consisting of italicized uppercase Latin letters and Arabic numbers formally assigned by the by HUGO Gene Nomenclature Committee after a gene has been identified (Note: a putative gene may be referred to by its locus name prior to its identification).

Genome: The complete DNA sequence, containing all genetic information and supporting proteins, in the chromosomes of an individual or species.

Genotype: The genetic constitution of an organism or cell; also refers to the specific set of alleles inherited at a locus.

Genotyping: Testing that reveals the specific alleles inherited by an individual; particularly useful for situations in which more than one genotypic combination can produce the same clinical presentation, as in the ABO blood group, where both the AO and AA genotypes yield type A blood.

Genotype-Phenotype Correlation: The association between the presence of a certain mutation or mutations (genotype) and the resulting pattern of abnormalities (phenotype).

Genetic Counseling: A process, involving an individual or family, comprising: evaluation to confirm, diagnose, or exclude a genetic condition, malformation syndrome, or isolated birth defect; discussion of natural history and the role of heredity; identification of medical management issues; calculation and communication of genetic risks; provision of or referral for psychosocial support.


Heterogeneous: A combination of different kinds.

Heterozygote: An individual who has two different alleles at a particular locus one on each chromosome of a pair; one allele is usually normal and the other abnormal.

Homozygote: An individual who has two identical alleles at a particular locus, one on each chromosome of a pair.

Hyperkalemic: High in potassium.

Hypokalemic: Low in potassium.


Ion Channels: Ion channels are present in the membranes that surround all biological cells. By conducting and controlling the flow of ions, these pore-forming enzymes help establish the small negative voltage that all cells possess at rest (see cell potential). An ion channel is an integral membrane protein or more typically an assembly of several proteins. Such "multi-subunit" assemblies usually involve a circular arrangement of identical or related proteins closely packed around a water-filled pore. Ions move through the pore single file--nearly as fast as the ions move through free fluid. Access to the pore is governed by "gates," which may be opened or closed by chemical or electrical signals, or mechanical force, depending on the variety of channel.

In-Frame Mutation: A mutation that does not cause a shift in the triplet reading frame; such mutations can, however, lead to the synthesis of an abnormal protein product.

Insertion: A chromosome abnormality in which material from one chromosome is inserted into another nonhomologous chromosome; a mutation in which a segment of DNA is inserted into a gene or other segment of DNA, potentially disrupting the coding sequence.

Interictal: Referring to the period between seizures.


Kalemic: Having to do with potassium.


Linkage Analysis: (synonym: indirect DNA analysis) Testing DNA sequence polymorphisms (normal variants) that are near or within a gene of interest to track within a family the inheritance of a disease-causing mutation in a given gene.

Locus Name: An informally assigned abbreviation used in the process of mapping to designate a putative gene prior to gene identification; once the gene is identified. The locus name is generally replaced by a formally assigned gene symbol (which often differs from the locus name).


Missense Mutation: A missense mutation is a change in one DNA base pair that changes a codon into a codon corresponding to a different amino acid.

Molecular Genetic Testing: (synonyms: DNA testing, DNA-based testing, molecular testing) Testing that involves the analysis of DNA, either through linkage analysis, sequencing, or one of several methods of mutation detection

Mutation: (synonyms: sequence alteration, splicing mutation) any alteration in a gene from its natural state; may be disease-causing or a benign, normal variant.

Mutation Analysis: (synonym: targeted mutation analysis) Testing for the presence of a specific mutation (e.g., Glu6Val for sickle cell anemia), a specific type of mutation (e.g., the trinucleotide repeat expansion associated with spinocerebellar ataxia type 1, deletions associated with Duchenne muscular dystropy), or set of mutations (e.g., a panel of mutations for cystic fibrosis), as opposed to complete gene sequencing or mutation scanning, which detect most mutations in the tested region.

Mutation Scanning: A process by which a segment of DNA is screened via one of a variety of methods to identify variant gene region(s). Variant regions are further analyzed (by sequence analysis or mutation analysis) to identify the sequence alteration.

Myokymia: Continuous involuntary muscle twitchings that give the appearance of worm like rippling of muscle. The muscle contractions are involuntary (spontaneous) and brief.

Myotonia: Difficulty relaxing muscles after contraction.


Nonsenses Mutation: A nonsense mutation is a change in one DNA base pair. Instead of substituting one amino acid for another, however, the altered DNA sequence prematurely signals the cell to stop building a protein (i.e. a stop codon). This type of mutation results in a shortened protein that may function improperly or not at all.

Nucleotide: A molecule consisting of a nitrogenous 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 of many nucleotides.

Nystagmus: Rapid rhythmic repetitious involuntary (unwilled) eye movements. Nystagmus can be horizontal, vertical or rotary.


Oscillopsia: Oscillating vision. Swinging vision. In oscillopsia, objects seem to swing, move back and forth, jerk, or wiggle. A classical language hybrid derived from the Latin "oscillo", meaning to swing, and the Greek "opsis" meaning vision.


Paroxysm: In medicine, a paroxysm is a violent attack. It may be due to the sudden occurrence of symptoms or the acute exacerbation of preexisting symptoms.

Penetrance: The proportion of individuals with a mutation causing a particular disorder who exhibit clinical symptoms of that disorder; a condition (most commonly inherited in an autosomal dominant manner) is said to have complete penetrance if clinical symptoms are present in all individuals who have the disease-causing mutation, and to have reduced or incomplete penetrance if clinical symptoms are not always present in individuals who have the disease-causing mutation.

Phenotype: The observable physical and/or biochemical characteristics of the expression of a gene; the clinical presentation of an individual with a particular genotype.

Point Mutations: nucleotide substitutions that result in missense mutations, nonsense mutations or deletions and insertions.


Reading Frame: (synonym: exon) A sequence of messenger RNA that is translated into an amino acid chain, three bases at a time, each triplet sequence coding for a single amino acid.

Recessive: see autosomal recessive.

RNA: (synonym: ribonucleic acid) The molecule synthesized from the DNA template; contains the sugar ribose instead of deoxyribose, which is present in DNA; three types of RNA exist, messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).


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

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


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