Glossary of Genetic Terms

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Acquired aberration: Chromosomal abnormality that develops throughout a person’s lifetime. These chromosomal changes are observed in the neoplastic disease process.
Acrocentric: Acrocentric chromosomes are those with the centromere very close to one end, giving the chromosome a “V” shaped appearance. The short (p) arms are very short and usually have small dot-like appendages on stalks, known as “satellites.” The acrocentric chromosomes are 13, 14, 15, 21, 22, and Y.
Amniocentesis: A prenatal diagnosis procedure which involves the removal of a small amount of amniotic fluid. The amniotic fluid contains fetal cells which can be examined to look for chromosomal, biochemical, or gene alterations in the baby.
Aneuploidy: The gain or loss of one chromosome compared to the typical chromosome complement of 46.
Aneuvysion: A set of FISH DNA probes from VYSIS, Inc. that can be used to detect the numerical status of chromosomes 13, 18, 21, X, and Y in interphase cells. Used primarily for rapid aneuploid screening of amniotic fluid cells.
Autosomes: The 22 pairs of chromosomes; not including the two sex chromosomes.
Band: The series of darkly and lightly stained stripes across a chromosome produced by a variety of chemical treatments. Each chromosome has its own characteristic banding pattern that allows specific identification of chromosomes or parts of chromosomes.
Banding level: The number of bands seen in a haploid (23 chromosomes) karyotype.
Biparental: Involving or inherited from both parents.
C-banding: Banding that specifically stains constitutive heterochromatin (eg. centromeres and Y-chromosome heterochromatin).
Cell division: The mechanism by which cells multiply. The type of cell division involved in the growth of the body is known as mitosis. The cell division that produces sperm or ova in the testis or ovary is known as meiosis.
Centromere: The primary constriction of the chromosome separating the short (p) arm from the long (q) arm. It is the attachment point for the mitotic spindle during cell division.
Centromeric fission: splitting of the chromosome horizontally through the centromere, separating the short (p) and long (q) arms.
Chimerism: A special kind of mosaicism in which more than one genetic cell line in an individual represents separate origins of the lineages. Tissue transplant carriers artificially represent a form of chimerism.
Chorionic Villus Sampling (CVS): A prenatal diagnosis procedure performed at 10-12 weeks gestation, which involves obtaining a sample of the placenta. The cells of the placenta generally represent the cells of the fetus and can be examined to look for chromosomal, biochemical, or gene alterations in the fetus.
Chromatid: A chromosome, at the metaphase stage, is made up of two identical sister chromatids joined at the centromere.
Chromosome: Thread-like structures consisting of DNA which “package” genetic information within the nucleus of most cells of the body. Human cells contain 46 chromosomes in 23 pairs. The 46 chromosomes contain all the genetic information inherited from both parents, thus one of each pair is inherited from the mother, and the other is from the father. The ability to see chromosomes through a microscope led to the field of science known as cytogenetics.
Chromosome number: In humans, the chromosome number is 23 pairs giving a total of 46. Half of the chromosomes (23) are inherited from the mother and half from the father.
Clones: A group of cells derived from a single ancestor cell.
Constitutional study: A study for a chromosome abnormality that has been present since conception and affects cells distributed throughout the affected individual. Such abnormalities may comprise an alteration in the structure of a chromosome or an entire extra, (or missing), chromosome. If the net result of a constitutional abnormality is duplication or deficiency of chromosome material (termed ‘unbalanced’), then adverse clinical effects (mental / physical handicap) are likely. If the constitutional structural abnormality has simply resulted in re-arranged chromosome material, (i.e. no duplication or deficiency, termed a ‘balanced’ rearrangement), no adverse effects are likely, although the individual carrying such a rearrangement may be predisposed to having a handicapped child with a derivative ‘unbalanced’ chromosome constitution.
Culture: To grow cells in a special medium. Also used to differentiate between culture flasks. Two or more cultures may be initiated from a single sample.
Cytogenetics: The study of the number, structure, behavior, and effects of chromosomes.
Deletion: Loss of a whole or part of a chromosome. The term usually refers to loss of part of a chromosome (partial deletion). A terminal deletion refers to breakage and loss of the end of a chromosome; an interstitial deletion is the loss of material from within the chromosome, between the ends.
De novo: An alteration in a chromosome or 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. A new abnormality or variation, not inherited from parents.
Diploidy: A cell with a normal chromosome number including two sets of parental chromosomes (23 from each parent). See triploidy and tetraploidy.
Disomy: Meaning two bodies. Refers to a normal chromosome pair.
DNA (deoxyribonucleic acid): The long double-stranded chemical molecule found in the nucleus of cells and packaged into chromosomes. DNA contains the “genetic code” (genes), which we inherit from our parents.

Duplication: A double copy of part of a chromosome resulting in an extra (abnormal) dose of the duplicated material.

Euchromatin: The part of the chromosomal DNA which is genetically active and pale staining with G- and C- banding. Euchromatin is relatively rich in GC base pairs. During interphase it is uncoiled and transcriptionally active whereas heterochromatin is condensed and inactive.
Familial: Something which has a tendency to occur within a particular family. For instance, Down syndrome that has occurred more than once within a family because of an inherited chromosome translocation can be said to be familial in that family.
F.I.S.H.: Fluorescence In-Situ Hybridization is a technique whereby small lengths of DNA (probes), which “recognize” complementary DNA on particular chromosomes or parts of chromosomes, are labeled with a colored fluorescent dye and then used to highlight “target” DNA in chromosomes.
Fragment: A piece of chromosome without a centromere.
Gamete: The germ cell (sperm or ovum) containing a single set of (23) chromosomes from one parent.
G-band: see also band. This common banding technique is produced by treatment with trypsin and subsequent Giemsa/Wright’s staining and used to routinely identify chromosomes.
Gene: A sequence of DNA within a chromosome, containing genetic information (genetic code) that is responsible for a particular bodily function or characteristic. Genes are not visible through a microscope.
Genetics: The scientific study of characteristics that are passed on (inherited) through successive generations in humans and other organisms due to the transmission of genetic material (DNA).
Genome: Refers to the sum total of genetic information contained in the chromosomes from a cell representing an individual person. A haploid genome would be the information representing the chromosomes from one gamete.

Genotype: The actual genetic makeup of an individual.

Haploid: The chromosome number (23) found in the parental gamete. i.e. half the diploid number.
Heterochromatin: Chromosomal material which is condensed and genetically inactive during interphase. It consists of repetitive DNA sequences that are relatively rich in AT base pairs and is late replicating in the cell cycle. In metaphase chromosomes, it is dark staining with G- and C- banding. (see euchromatin)
Homologous chromosomes: A pair of like chromosomes, one inherited from each parent.
Ideogram: An idealized drawing of a banded chromosome, with associated numerical locators to describe the bands according to accepted nomenclature.
Imprinting: The “memory” held by a chromosome as to which parent it was inherited from. The memory is chemically “stamped” into the DNA and can result in chromosomes behaving differently, depending on the parent of origin.
Insertion: The addition of a piece of chromosomal material into a place on a chromosome where it is normally not found.
Interphase: That phase of the cell cycle when the nuclear membranes have formed, the chromosomes have decondensed, and the cell is going about its normal activities. A non-dividing cell that is appropriate for interphase FISH analysis.
Interstitial: Any part of a chromosome between the ends (telomeres). see deletion.

Inversion: The breakage of a chromosome in two places followed by a 180-degree switch of the segment between the breaks. There is no loss or gain of material but the sequence of the inverted segment is reversed.

ISCN: The International System of Cytogenetic Nomenclature provides the terminology used to describe chromosome constitutions

Isochromosome: A metacentric chromosome in which the arms are of equal length and the information in each of the two arms is genetically identical. (iso- = equal)

Karyotype: Usually refers to the cut out and arranged pairs of chromosomes from a photograph of a cell representing an individual. The term can also refer the chromosomal constitution of a person, i.e. “she has a normal female karyotype”
Locus: Positional location of a gene on a chromosome.
Marker: A small chromosome containing a centromere occasionally seen in tissue culture, often in a mosaic state (present in some cells but not in others). A marker chromosome may be of little clinical significance or, if it contains material from one or both arms of another chromosome, may create an imbalance for whatever genes are present; assessment to establish clinical significance, particularly if found in a fetal karyotype, is often difficult.
Maternal: Originating from the mother.
Meiosis: see cell division.
Metacentric: A metacentric chromosome has its centromere in the middle and the p and q arms, which are of similar length.
Metaphase: A cell at the stage in cell replication where the duplicated DNA is highly condensed as chromosomes and is about to divide.

Microdeletion: An absence of DNA, undetectable by classical cytogenetics. A microdeletion may be detected by FISH.

Mitosis: see cell division.

Modal number: The chromosome count that is characteristic of cells from a particular individual or cell line (usually 46 in a normal human).

Monosomy: A single copy of a chromosome rather than a pair (disomy).

Mosaicism: The co-existence of genetically different cells within the same individual or tissue. e.g. a mixture of both normal cells and cells with a chromosome abnormality.

Nondisjunction: The failure of homologous chromosomes to segregate into each daughter cell during mitotic or meiotic anaphase. i.e. both members of a pair move to the same daughter cell, resulting in deficiency in one cell and excess in the other.
NOR region: Nucleolar organizer regions that are located on the short arms of the human acrocentric chromosome regions.
NOR staining: A method of staining that identifies the nucleolar organizer regions on the short arms of the acrocentric chromosomes. This is also called silver staining.
p-arm: Each chromosome is divided into two parts joined by the centromere. The ‘p’ arm is the shorter of the two segments and is at the top of the chromosome. The longer segment is called the ‘q’ arm.
Paternal: Originating from the father.
Phenotype: External appearance of an individual and their characteristics produced by that individual’s genotype interacting with their environments.
Polymorphism: Literally, many shapes. Variations of chromosome appearance that are considered to be normal. These “normal variants” are passed on within families without causing any problems in development.
Proband: Individual seen in genetic consultation, usually the identified patient.

Probe: A small segment of DNA of known origin, designed to recognize complementary DNA on specific parts of chromosomes. The probe can be labeled with a colored stain and used to confirm the presence or absence of the target material or to identify unknown chromosomal material.

Pseudo-mosaicism: In prenatal diagnosis, a single cell or cell line(s), confined to a single independent culture. Believed to arise in culture.

q-arm: Each chromosome is divided into two parts, joined by the centromere. The ‘q’ arm is the longer of the two segments and is at the bottom of the chromosome. The shorter segment is called the ‘p’ arm.
Q-banding: A fluorescent banding technique that permits identification of chromosomes and can specifically stain the Y chromosome heterochromatin.
Reciprocal translocation: The breakage and exchange of material between two chromosomes without any gain or loss of genetic material. Such a reciprocal translocation is said to be balanced. When translocated chromosomes are passed on to offspring in a way that results in gain or loss of material, the translocation is said to be unbalanced.
Recurrence risk: The chances of a genetic problem occurring or recurring in the family members
Ring chromosome: A chromosome which has broken at each end, lost the terminal segments, curled around, and rejoined to form a ring shape. Formation of a ring may result in the loss or deletion of the terminal segments.
Robertsonian translocation: A type of translocation exclusive to the acrocentric chromosomes (13,14,15,21,22), in which two chromosomes join at or near their centromeres. This is effectively a fusion between two whole chromosomes rather than an exchange of material as in reciprocal translocation.
Satellites: Small round appendages attached by fine stalks to the ends of the short (p) arms of the acrocentric chromosomes (13,14,15,21,22). They do not always stain darkly and may be difficult to see. Satellites on different chromosomes are often attracted to one another, causing the acrocentric chromosomes to be in satellite association.
Sex Chromosome: The X and Y chromosomes that are responsible for sexual development. In humans, females have two X chromosomes while males have an X and a Y. Genes on the Y chromosome are responsible for male sexual development.
Silver staining: A method of staining that identifies the nucleolar organizer regions on the short arms of the acrocentric chromosomes. This is also called NOR staining.
Sub-metacentric: Chromosomes with centromeres that are not centrally placed. The chromosome arms are not of equal length. This term does not apply to the acrocentric chromosomes that have an almost terminal centromere.
Sub-telomeric region: The chromosomal region just proximal to the telomere (end of the chromosome) composed of highly polymorphic repetitive DNA sequences that are typically situated adjacent to gene-rich areas. Microdeletions and subtle rearrangements that disrupt genes in the sub-telomeric regions can cause mental retardation; use of Fluorescent in situ Hybridization (FISH) to evaluate sub-telomeric regions is usually required for detection of these abnormalities.

Syndrome: The particular collection of physical features or characteristics in an individual that help to identify or characterize a genetic abnormality. A group of symptoms, occurring together, which characterize a disease.

Telomere: The terminal segment of each chromosome arm.
Tetraploidy: The presence in a cell of four haploid sets (23) of parental chromosomes, giving a total chromosome count of 92.
Tetrasomy: The presence of four copies of a particular chromosome.
Tissue Culture: The growth of cells in a special medium.
Translocation: The fusion or exchange of material between chromosomes. When there is no gain or loss of material, the translocation is said to be balanced; when there is gain or loss resulting in trisomy or monosomy for a particular chromosome segment, it is said to be unbalanced.

Triploidy: The presence in a cell of three haploid sets (23) of parental chromosomes, giving a total chromosome count of 69.

Trisomy: The presence of three copies of a particular chromosome, i.e. trisomy 21. Three copies of part of a chromosome is partial trisomy.

Uniparental disomy: both members of a chromosome pair are contributed by one parent rather than one from each parent.
X-chromosome: see sex chromosome.
Y-chromosome: see sex chromosome.
Zygote: The single cell with 46 chromosomes resulting from the fertilization of an ovum (23) by a sperm (23). Through cell division (mitosis), the zygote develops into a multicellular embryo and then into a fetus.