Glossary of Genetic Terms

Compiled by the Genetics Education Center, University of Kansas Medical Center

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Achondroplasia -- the most common and well known form of short limbed dwarfism characterized by a normal trunk size with disproportionally short arms and legs, and a disproportionally large head; autosomal dominant condition.
 
Advanced maternal age -- women over age 34 (age 35 at delivery) at increased risk for nondisjunction trisomy in fetus.
 
Alcoholism -- a chronic and progressive condition characterized by the inability to control the consumption of alcohol.
 
Allele -- an alternative form of a gene; any one of several mutational forms of a gene.
 
Alpha-fetoprotein (AFP) -- a protein excreted by the fetus into the amniotic fluid and from there into the mother's bloodstream through the placenta.
 
Alu repetitive sequence -- the most common dispersed repeated DNA sequence in the human genome accounting for 5% of human DNA. The name is derived from the fact that these sequences are cleaved by the restriction endonuclease Alu.
 
Amino acid sequence -- the linear order of the amino acids in a protein or peptide.
 
Amniocentesis -- prenatal diagnosis method using cells in the amniotic fluid to determine the number and kind of chromosomes of the fetus and, when indicated, perform biochemical studies.
 
Amniocyte -- cells obtained by amniocentesis.
 
Amplification -- any process by which specific DNA sequences are replicated disproportionately greater than their representation in the parent molecules.
 
Aneuploidy -- state of having variant chromosome number (too many or too few). (i.e. Down syndrome, Turner syndrome).
 
Angelman syndrome -- a condition characterized by severe mental deficiency, developmental delay and growth deficiency, puppet-like gait and frequent laughter unconnected to emotions of happiness.
 
Apert syndrome -- a condition caused by the premature closure of the sutures of the skull bones, resulting in an altered head shape, with webbed fingers and toes. Autosomal dominant.
 
Artificial insemination -- the placement of sperm into a female reproductive tract or the mixing of male and female gametes by other than natural means.
 
Autosome -- a nuclear chromosome other than the X- and Y-chromosomes.
 
Autoradiograph -- a photographic picture showing the position of radioactive substances in tissues.
 
Bacteriophage -- a virus whose host is a bacterium; commonly called phage.
 
Barr body -- the condensed single X-chromosome seen in the nuclei of somatic cells of female mammals. base pair a pair of hydrogen-bonded nitrogenous bases (one purine and one pyrimidine) that join the component strands of the DNA double helix.
 
Base sequence -- a partnership of organic bases found in DNA and RNA; adenine forms a base pair with thymine (or uracil) and guanine with cytosine in a double-stranded nucleic acid molecule.
 
Baysian analysis -- a mathematical method to further refine recurrence risk taking into account other known factors.
 
Becker muscular dystrophy -- X-linked condition characterized by progressive muscle weakness and wasting; manifests later in life with progression less severe than Duchenne muscular dystrophy.
 
Carrier -- an individual heterozygous for a single recessive gene.
 
cDNA -- complementary DNA produced from a RNA template by the action of RNA- dependent DNA polymerase.
 
Centromere -- a region of a chromosome to which spindle traction fibers attach during mitosis and meiosis; the position of the centromere determines whether the chromosome is considered an acrocentric, metacentric or telomeric chromosome.
 
Charcot-Marie Tooth disease -- a condition characterized by degeneration of the motor and sensory nerves that control movement and feeling in the arm below the elbow and the leg below the knee; transmitted in autosomal dominant, autosomal recessive and X-linked forms.
 
Chorionic villus sampling -- an invasive prenatal diagnostic procedure involving removal of villi from the human chorion to obtain chromosomes and cell products for diagnosis of disorders in the human embryo.
 
Chromosome -- in the eukaryotic nucleus, one of the threadlike structures consisting of chromatin and carry genetic information arranged in a linear sequence.
 
Chromosome banding -- a technique for staining chromosomes so that bands appear in a unique pattern particular to the chromosome.
 
Cleft lip/palate -- congenital condition with cleft lip alone, or with cleft palate; cause is thought to be multifactorial.
 
Clone -- genetically engineered replicas of DNA sequences.
 
Cloned DNA -- any DNA fragment that passively replicates in the host organism after it has been joined to a cloning vector.
 
Codon -- a sequence of three nucleotides in mRNA that specifies an amino acid.
 
Consanguinity -- genetic relationship. Consanguineous individuals have at least one common ancestor in the preceding few generations.
 
Conservative change -- an amino acid change that does not affect significantly the function of the protein.
 
Contiguous genes -- genes physically close on a chromosome that when acting together express a phenotype.
 
Cosmids -- plasmid vectors designed for cloning large fragments of eukaryotic DNA; the vector is a plasmid into which phage lambda cohesive end sites have been inserted.
 
CpG islands -- areas of multiple CG repeats in DNA.
 
Cri-du-chat syndrome -- a chromosomal condition (monosomy 5p). Name comes from the distinctive mewing cry of affected infants; characterized by significant mental deficiency, low birthweight, failure to thrive and short stature; deletion of a small section of the short arm of chromosome 5.
 
Crossovers -- the exchange of genetic material between two paired chromosome during meiosis.
 
Cornelia de Lange syndrome -- condition involving growth deficiency, significant developmental delay, anomalies of the extremities and a characteristic facial appearance.
 
Cytogenetics -- the study of chromosomes.
 
Cystic fibrosis -- an autosomal recessive genetic condition of the exocrine glands, which causes the body to produce excessively thick, sticky mucus that clogs the lungs and pancreas, interfering with breathing and digestion.
 
Degenerate codon -- a codon that specifies the same amino acid as another codon.
 
Deletion -- the loss of a segment of the genetic material from a chromosome.
 
Deletion mapping -- the use of overlapping deletions to localize the position of an unknown gene on a chromosome or linkage map.
 
Disease -- any deviation from the normal structure or function of any part, organ, or system of the body that is manifested by a characteristic set of symptoms and signs whose pathology and prognosis may be known or unknown.
 
DMD -- Duchenne muscular dystrophy.
 
DNA fingerprint technique -- a method employed to determine differences in amino acid sequences between related proteins; relies upon the presence of a simple tandem-repetitive sequences that are scattered throughout the human genome.
 
DNA hybridization -- a technique for selectively binding specific segments of single-stranded (ss) DNA or RNA by base pairing to complementary sequences on ssDNA molecules that are trapped on a nitrocellulose filter.
 
DNA probe -- any biochemical used to identify or isolate a gene, a gene product, or a protein.
 
DNA sequencing -- "plus and minus" or "primed synthesis" method, developed by Sanger, DNA is synthesized in vitro in such a way that it is radioactively labeled and the reaction terminates specifically at the position corresponding to a given base; the "chemical" method, ssDNA is subjected to several chemical cleavage protocols that selectively make breaks on one side of a particular base.
 
DOE -- Department of Energy.
 
Dominant -- alleles that determine the phenotype displayed in a heterozygote with another (recessive) allele.
 
Down syndrome -- a type of mental deficiency due to trisomy (three copies) of autosome 21, a translocation of 21 or mosaicism.
 
Duchenne/Becker muscular dystrophy -- the most common and severe form of muscular dystrophy; transmitted as an X-linked trait. X-linked recessive. Symptoms include onset at 2-5 years with difficulty with gait and stairs, enlarged calf muscles, progression to wheelchair by adolescence, shortened life span.
 
Dystonia -- neurologic condition involving repeated twisting and movement. Involves a variety of muscle groups. Intelligence not effected. Three forms: childhood - autosomal dominant, autosomal recessive, adult-acquired.
 
Dwarfism -- conditions of short stature with adult height under 4'10" as adult, usually with normal intelligence and lifespan. Ehlers Danlos Syndrome connective tissue condition including problems with tendons, ligaments, skin, bones, cartilage, and membranes surrounding blood vessels and nerves. Symptoms include joint laxity, elastic skin, dislocations. Many forms: autosomal dominant, autosomal recessive, X-linked forms.
 
ELSI -- ethical, legal and social implications (of HGP).
 
Endonuclease -- an enzyme that breaks the internal phosphodiester bonds in a DNA molecule.
 
Ethics -- the study of fundamental principles which defines values and determines moral duty and obligation.
 
Erythrocytes -- the hemoglobin-containing cell found in the blood of vertebrates.
 
Euchromatin -- the chromatin that shows the staining behavior characteristic of the majority of the chromosomal complement.
 
Eugenics -- the improvement of humanity by altering its genetic composition by encouraging breeding of those presumed to have desirable genes.
 
Exons -- portion of a gene included in the transcript of a gene and survives processing of the RNA in the cell nucleus to become part of a spliced messenger of a structural RNA in the cell cytoplasm; an exon specifies the amino acid sequence of a portion of the complete polypeptide.
 
Fetal alcohol syndrome -- a link between excessive alcohol consumption during pregnancy and birth defects; characteristics include small head and eyes, folds of the skin that obscure the inner juncture of the eyelids, short, upturned nose, and thin lips.
 
FISH -- florescent in situ hybridization: a technique for uniquely identifying whole chromosomes or parts of chromosomes using florescent tagged DNA.
 
5' - end -- the end of a polynucleotide with a free (or phosphorylated or capped) 5' - hydroxyl group; transcription/translation begins at this end.
 
Fragile sites -- a non-staining gap of variable width that usually involves both chromatids and is always at exactly the same point on a specific chromosome derived from an individual or kindred.
 
Fragile-X syndrome -- X-linked trait; the second most common identifiable cause of genetic mental deficiency.
 
Gamete -- an haploid cell.gel electrophoresis the process by which nucleic acids (DNA or RNA) or proteins are separated by size according to movement of the charged molecules in an electrical field.
 
Gene -- a hereditary unit that occupies a certain position on a chromosome; a unit that has one or more specific effects on the phenotype, and can mutate to various allelic forms.
 
Gene amplification -- any process by which specific DNA sequences are replicated disproportionately greater than their representation in the parent molecules; during development, some genes become amplified in specific tissues.
 
Gene map -- the linear arrangement of mutable sites on a chromosome as deduced from genetic recombination experiments.
 
Gene therapy -- addition of a functional gene or group of genes to a cell by gene insertion to correct an hereditary disease.
 
Genetic counseling -- the educational process that helps individuals, couples, or families to understand genetic information and issues that may have an impact on them.
 
Genetic linkage map -- a chromosome map showing the relative positions of the known genes on the chromosomes of a given species.
 
Genetic screening -- testing groups of individuals to identify defective genes capable of causing hereditary conditions.
 
Genetic variation -- a phenotypic variance of a trait in a population attributed to genetic heterogeneity.
 
Genome -- all of the genes carried by a single gamete; the DNA content of an individual, which includes all 44 autosomes, 2 sex chromosomes, and the mitochondrial DNA.
 
Genotype -- genetic constitution of an organism.
 
Germ cell -- a sex cell or gamete (egg or spermatozoan).Haldane equation Haldane's law: the generalization that if first generation hybrids are produced between two species, but one sex is absent, rare, or sterile, that sex is the heterogamic sex.
 
Hardy-Weinberg Law -- the concept that both gene frequencies and genotype frequencies will remain constant from generation to generation in an infinitely large, interbreeding population in which mating is at random and there is no selection, migration or mutation.
 
Heterozygote -- having two alleles that are different for a given gene.
 
Hemophilia -- a sex-linked disease in humans in which the blood-clotting process is defective.
 
Heterogeneity -- the production of identical or similar phenotypes by different genetic mechanisms.
 
HGP -- Human Genome Project.
 
HHMI -- Howard Hughes Medical Institute.
 
Homologous chromosomes -- chromosomes that pair during meiosis; each homologue is a duplicate of one chromosome from each parent.
 
Homozygote -- having identical alleles at one or more loci in homologous chromosome segments.
 
Housekeeping genes -- those genes expressed in all cells because they provide functions needed for sustenance of all cell types.
 
HUGO -- Human Genome Organization.
 
Huntington disease -- a disease characterized by irregular, spasmodic involuntary movements of the limbs and facial muscles, mental deterioration and death, usually within 20 years of the onset of symptoms.
 
Hybridization -- the pairing of a single-stranded, labeled probe (usually DNA) to its complementary sequence.
 
Ichthyosis -- any of several hereditary or congenital skin conditions; skin of affected individuals has a dry, scaly appearance.
 
Imprinting -- a chemical modification of a gene allele which can be used to identify maternal or paternal origin of chromosome.
 
Incomplete penetrance -- the gene for a condition is present, but not obviously expressed in all individuals in a family with the gene.
 
In situ hybridization -- hybridization of a labeled probe to its complementary sequence within intact, banded chromosomes.
 
Introns -- a segment of DNA (between exons) that is transcribed into nuclear RNA, but are removed in the subsequent processing into mRNA.
 
Isochromosome -- a metacentric chromosome produced during mitosis or meiosis when the centromere splits transversely instead of longitudinally; the arms of such chromosome are equal in length and genetically identical, however, the loci are positioned in reverse sequence in the two arms.
 
Klinefelter syndrome -- an endocrine condition caused by a an extra X-chromosome (47,XXY); characterized by the lack of normal sexual development and testosterone, leading to infertility and adjustment problems if not detected and treated early.
 
Karyotype -- a set of photographed, banded chromosomes arranged in order from largest to smallest.
 
Lligase -- an enzyme that functions in DNA repair.
 
Linkage -- the greater association in inheritance of two or more nonallelic genes than is to be expected from independent assortment; genes are linked because they reside on the same chromosome.
 
Linkage -- analysis of pedigree the tracking of a gene through a family by following the inheritance of a (closely associated) gene or trait and a DNA marker.
 
Lod score -- logarithm of the odd score; a measure of the likelihood of two loci being within a measurable distance of each other.
 
Marfan syndrome -- autosomal dominant condition of connective tissue; affects the skeletal, ocular and cardiovascular systems.
 
Marker -- a gene with a known location on a chromosome and a clear-cut phenotype, used as a point of reference when mapping a new mutant.
 
Meiosis -- the doubling of gametic chromosome number.
 
Methylation -- addition of a methyl group (-CH3) to DNA or RNA.
 
Methylmalonic acidemia -- a group of conditions characterized by the inability to metabolize methylmalonic acid or by a defect in the metabolism of Vitamin B12.
 
Missense mutation -- a change in the base sequence of a gene that alters or eliminates a protein.
 
Mitochondrial DNA -- the mitochondrial genome consists of a circular DNA duplex, with 5 to 10 copies per organelle.
 
Mitosis -- nuclear division.
 
mRNA -- messenger RNA; an RNA molecular that functions during translation to specify the sequence of amino acids in a nascent polypeptide.
 
Multifactorial -- a characteristic influenced in its expression by many factors, both genetic and environmental.
 
Mutation -- process by which genes undergo a structural change.
 
Myotonic dystrophy -- a combination of progressive weakening of the muscles and muscle spasms or rigidity, with difficulty relaxing a contracted muscle; inherited as an autosomal dominant trait.
 
Neurofibromatosis -- one of the most common single gene conditions affecting the human nervous system; in most cases, "cafe au lait" spots, are the only symptom; inherited as an autosomal dominant trait, with 50% being new mutations.
 
NIH -- National Institutes of Health.
 
Nonsense mutation -- a mutation in which a codon is changed to a stop codon, resulting in a truncated protein product.
 
Noonan syndrome -- a condition characterized by short stature and ovarian or testicular dysfunction, mental deficiency, and lesions of the heart.
 
Northern analysis -- a technique for transferring electrophoretically resolved RNA segments from an agarose gel to a nitrocellulose filter paper sheet via capillary action.
 
Nucleotide -- one of the monomeric units from which DNA or RNA polymers are constructed; consists of a purine or pyrimidine base, a pentose sugar and a phosphoric acid group.
 
Oncogenes -- genes involved in cell cycle control (growth factors, growth factor regulator genes, etc), a mutation can lead to tumor growth.
 
Osteogenesis imperfecta -- a condition also known as brittle bone disease; characterized by a triangular shaped face with yellowish brown teeth, short stature and stunted growth, scoliosis, high pitched voice, excessive sweating and loose joints.
 
Parthenogenesis -- the development of an individual from an egg without fertilization.
 
PCR -- polymerase chain reaction; a technique for copying the complementary strands of a target DNA molecule simultaneously for a series of cycles until the desired amount is obtained.
 
Pedigree -- a diagram of the heredity of a particular trait through many generations of a family.
 
Phenotype -- observable characteristics of an organism produced by the organism's genotype interacting with the environment.
 
Physical map -- map where the distance between markers is the actual distance, such as the number of base pairs.
 
PKU -- phenylketonuria, an enzyme deficiency condition characterized by the inability to convert one amino acid, phenylalanine, to another, tyrosine, resulting in mental deficiency. plasmid double-stranded, circular, bacterial DNA into which a fragment of DNA from another organism can be inserted.
 
Pleiotropy -- the phenomenon of variable phenotypes for a number of distinct and seemingly unrelated phenotypic effects.
 
Polycystic kidney disease (PKD) -- a group of conditions characterized by fluid filled sacs that slowly develop in both kidneys, eventually resulting in kidney malfunction.
 
Polymerase -- any enzyme that catalyzes the formation of DNA or RNA from deoxyribonucleotides or ribonucleotides.
 
Prader-Willi syndrome -- a condition characterized by obesity and insatiable appetite, mental deficiency, small genitals, and short stature. May be deletion of #15 chromosome.
 
Predisposition -- to have a tendency or inclination towards something in advance.
 
Presymptomatic diagnosis -- diagnosis of a genetic condition before the appearance of symptoms.
 
Primer -- nucleotides used in the polymerase chain reaction to initiate DNA synthesis at a particular location.
 
Probability -- the long term frequency of an event relative to all alternative events, and usually expressed as decimal fraction.
 
Proband -- individual in a family who brought the family to medical attention.
 
Probe -- single-stranded DNA labeled with radioactive isotopes or tagged in other ways for ease in identification.
 
Prognosis -- prediction of the course and probable outcome of a disease.
 
Proteus syndrome -- a condition characterized by distorted asymmetric growth of the body and enlarged head, enlarged feet, multiple nevi on the skin; mode of inheritance is unknown.
 
Public policy -- a set of action guidelines or rules that result from the actions or lack of actions of governmental entities.
 
Recessive -- a gene that is phenotypically manifest in the homozygous state but is masked in the presence of a dominant allele.
 
Recombination -- the natural process of breaking and rejoining DNA strands to produce new combinations of genes and, thus, generate genetic variation. Gene crossover during meiosis.
 
Repeat sequences -- the length of a nucleotide sequence that is repeated in a tandem cluster.
 
Retinitis pigmentosa -- group of hereditary ocular disorders with progressive retinal degeneration. Autosomal dominant, autosomal recessive, and x-linked forms.
 
Retinoblastoma -- a childhood malignant cancer of the retina of the eye. reverse transcriptase viral enzyme used to make cDNA.
 
RFLP -- restriction fragment length polymorphism; variations occurring within a species in the length of DNA fragments generated by a species endonuclease.
 
Ribosomal protein -- one of the ribonucleoprotein particles that are the sites of translation.
 
Rubinstein-Taybi syndrome -- condition with multiple congenital anomalies including: mental deficiency, broad thumbs, small head, broad nasal bridge and beaked nose.
 
Sanger sequence -- "plus and minus" or "primed synthesis" method; DNA is synthesized so it is radioactively labeled and the reaction terminates specifically at the position corresponding to a given base.
 
Selection -- the process of determining the relative share allotted individuals of different genotypes in the propagation of a population; the selective effect of a gene can be defined by the probability that carriers of the gene will reproduce.
 
Sex determination -- the mechanism in a given species by which sex is determined; in many species sex is determined at fertilization by the nature of the sperm that fertilizes the egg.
 
Sickle cell anemia -- an hereditary, chronic form of hemolytic anemia characterized by breakdown of the red blood cells; red blood cells undergo a reversible alteration in shape when the oxygen tension of the plasma falls slightly and a sickle-like shape forms.
 
Somatic cell hybrid -- hybrid cell line derived from two different species; contains a complete chromosomal complement of one species and a partial chromosomal complement of the other; human/hamster hybrids grow and divide, losing human chromosomes with each generation until they finally stabilize, the hybrid cell line established is then utilized to detect the presence of genes on the remaining human chromosome.
 
Somatic mutation -- a mutation occurring in any cell that is not destined to become a germ cell; if the mutant cell continues to divide, the individual will come to contain a patch of tissue of genotype different from the cells of the rest of the body.
 
Southern blotting -- a technique for transferring electrophoretically resolved DNA segments from an agarose gel to a nitrocellulose filter paper sheet via capillary action; the DNA segment of interest is probed with a radioactive, complementary nucleic acid, and its position is determined by autoradiography.
 
Spina bifida -- a congenital condition that results from altered fetal development of the spinal cord, part of the neural plate fails to join together and bone and muscle are unable to grow over this open section.
 
Syndrome -- a recognizable pattern or group of multiple signs, symptoms or malformations that characterize a particular condition; syndromes are thought to arise from a common origin and result from more than one developmental error during fetal growth.
 
Tay-Sachs disease -- a fatal degenerative disease of the nervous system due to a deficiency of hexosamidase A, causing mental deficiency, paralysis, mental deterioration, and blindness; found primarily but not exclusively among Ashkenazi Jews. Autosomal recessive.
 
Teratogens -- any agent that raises the incidence of congenital malformations.
 
3' - end -- the end of a polynucleotide with a free (or phosphorylated) 3' - hydroxyl group.
 
Trait -- any detectable phenotypic property of an organism.
 
Transduction -- the transfer of bacterial genetic material from one bacterium to another using a phage as a vector.
 
Transferase -- enzymes that catalyze the transfer of functional groups between donor and acceptor molecules.
 
Transcription -- the formation of an RNA molecule upon a DNA template by complementary base pairing.
 
Translation -- the formation of a polypeptide chain in the specific amino acid sequence directed by the genetic information carried by mRNA.
 
Translocation -- a chromosome aberration which results in a change in position of a chromosomal segment within the genome, but does not change the total number of genes present.
 
Triplet code -- a code in which a given amino acid is specified by a set of three nucleotides.
 
Tumor suppressor gene -- genes that normally function to restrain the growth of tumors; the best understood case is for hereditary retinoblastoma.
 
Transgenic organism -- one into which a cloned genetic material has been experimentally transferred, a subset of these foreign gene express themselves in their offspring.Turner syndrome a chromosomal condition in females (usually 45,XO) due to monosomy of the X- chromosome; characterized by short stature, failure to develop secondary sex characteristics, and infertility.
 
UNESCO -- United National Educational, Scientific, and Cultural Organization.
 
VNTR -- variable number tandem repeats; any gene whose alleles contain different numbers of tandemly repeated oligonucleotide sequences.
 
Vector -- a self-replicating DNA molecule that transfers a DNA segment between host cells.
 
Von Hippel-Lindau syndrome -- an autosomal dominant condition characterized by the anomalous growth and proliferation of blood vessels on the retina of the eye and the cerebellum of the brain; cysts and cancers in the kidneys, pancreas, and adrenal glands.
 
Western blotting analysis -- a technique used to identify a specific protein; the probe is a radioactively labeled antibody raised against the protein in question.
 
X-inactivation -- the repression of one of the two X-chromosomes in the somatic cells of females as a method of dosage compensation; at an early embryonic stage in the normal female, one of the two X-chromosomes undergoes inactivation, apparently at random, from this point on all descendent cells will have the same X-chromosome inactivated as the cell from which they arose, thus a female is a mosaic composed of two types of cells, one which expresses only the paternal X-chromosome, and another which expresses only the maternal X-chromosome.
 
XYY syndrome -- genetic condition in males with extra Y chromosome (in 1 in 1000 male births). Symptoms: tall stature (over 6'), may including sterility, developmental delay, learning problems.
 
YAC -- yeast artificial chromosome; a linear vector into which a large fragment of DNA can be inserted; the development of YAC's in 1987 has increased the number of nucleotides which can be cloned.
 
Zoo blot -- northern analysis of mRNA from different organisms.
 

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