my-ms logo my-ms title
Genetics Definitions


Additive genetic effects
When the combined effects of alleles at different loci are equal to the sum of their individual effects.

Adenine (A)
A nitrogenous base, one member of the base pair AT (adenine-thymine).

Affected relative pair
Individuals related by blood, each of whom is affected with the same trait. Examples are affected sibling, cousin, and avuncular pairs.

Aggregation technique
A technique used in model organism studies in which embryos at the 8-cell stage of development are pushed together to yield a single embryo (used as an alternative to microinjection).

Alternative form of a genetic locus; a single allele for each locus is inherited from each parent (e.g., at a locus for eye color the allele might result in blue or brown eyes).

Variation in alleles among members of the same species.

Alternative splicing
Different ways of combining a gene's exons to make variants of the complete protein.

Amino acid
Any of a class of 20 molecules that are combined to form proteins in living things. The sequence of amino acids in a protein and hence protein function are determined by the genetic code.

An increase in the number of copies of a specific DNA fragment; can be in vivo or in vitro.

Adding pertinent information such as gene coded for, amino acid sequence, or other commentary to the database entry of raw sequence of DNA bases.

Each generation of offspring has increased severity of a genetic disorder, (e.g., a grandchild may have earlier onset and more severe symptoms than the parent, who had earlier onset than the grandparent).

Nucleic acid that has a sequence exactly opposite to an mRNA molecule made by the body; binds to the mRNA molecule to prevent a protein from being made.

Programmed cell death, the body's normal method of disposing of damaged, unwanted, or unneeded cells.

Arrayed library
Individual primary recombinant clones (hosted in phage, cosmid, YAC, or other vector) that are placed in two-dimensional arrays in microtiter dishes. Each primary clone can be identified by the identity of the plate and the clone location (row and column) on that plate. Arrayed libraries of clones can be used for many applications, including screening for a specific gene or genomic region of interest.

Putting sequenced fragments of DNA into their correct chromosomal positions.

A technique that uses X-ray film to visualize radioactively labeled molecules or fragments of molecules; used in analyzing length and number of DNA fragments after they are separated by gel electrophoresis.

Autosomal dominant
A gene on one of the non-sex chromosomes that is always expressed, even if only one copy is present. A person only needs one copy of the changed gene in order to have the disorder. Usually, the changed gene is inherited from a parent who also has the disorder and every generation in the family may have members with the disorder. There are some instances in which a person has the gene that causes the disorder and does not show symptoms of the disorder, but can still pass the gene to his or her children. A person who carriers a gene for an autosomal dominant disorder has a 50% chance of passing the gene to each child.

Autosomal recessive
Autosomal recessive means that it is necessary to have two copies of the changed gene to have the disorder. Each parent contributes one changed copy of the gene to the child who has the disorder. The parents are called carriers of the disorder because they have one normal copy of the gene and one changed copy of the gene, but they do not show symptoms of the disorder. When both parents are carriers of the changed gene, each of their children has a 25% chance of having the disorder, a 50% chance of being a carrier of the disorder (like their parents), and a 25% chance of neither being a carrier nor having the disorder. These risks are the same for each pregnancy. When there is more than one person in a family who has the disease, these people are often in the same generation.

A chromosome not involved in sex determination. The diploid human genome consists of a total of 46 chromosomes: 22 pairs of autosomes, and 1 pair of sex chromosomes (the X and Y chromosomes).

Avuncular relationship
The genetic relationship between nieces and nephews and their aunts and uncles.

^ Top of Page


A cross between an animal that is heterozygous for alleles obtained from two parental strains and a second animal from one of those parental strains. Also used to describe the breeding protocol of an outcross followed by a backcross.

Bacterial artificial chromosome (BAC)
A vector used to clone DNA fragments (100- to 300-kb insert size; average, 150 kb) in Escherichia coli cells. Based on naturally occurring F-factor plasmid found in the bacterium E. coli.

One of the molecules that form DNA and RNA molecules.

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; determines structure of proteins encoded by that DNA.

Base sequence analysis
A method, sometimes automated, for determining the base sequence.

Behavioral genetics
The study of genes that may influence behavior.

The science of managing and analyzing biological data using advanced computing techniques. Especially important in analyzing genomic research data.

The use of biological organisms such as plants or microbes to aid in removing hazardous substances from an area.

A set of biological techniques developed through basic research and now applied to research and product development. In particular, biotechnology refers to the use by industry of recombinant DNA, cell fusion, and new bioprocessing techniques.

Birth defect
Any harmful trait, physical or biochemical, present at birth, whether a result of a genetic mutation or some other nongenetic factor.

A computer program that identifies homologous (similar) genes in different organisms, such as human, fruit fly, or nematode.

^ Top of Page


Diseases in which abnormal cells divide and grow unchecked. Cancer can spread from its original site to other parts of the body and can be fatal.

Candidate gene
A gene located in a chromosome region suspected of being involved in a disease.

Capillary array
Gel-filled silica capillaries used to separate fragments for DNA sequencing. The small diameter of the capillaries permit the application of higher electric fields, providing high speed, high throughput separations that are significantly faster than traditional slab gels.

Something which causes cancer to occur by causing changes in a cell's DNA.

An individual who possesses an unexpressed, recessive trait.

Causative SNPs
Causative SNPs affect the way a protein functions, correlating with a disease or influencing a person's response to medication. Causative SNPs come in two forms: 1) Coding SNPs, located within the coding region of a gene, change the amino acid sequence of the gene's protein product. 2) Non-coding SNPs, located within the gene's regulatory sequences, change the level of gene expression and, therefore, how much RNA and protein is produced.

cDNA library
A collection of DNA sequences that code for genes. The sequences are generated in the laboratory from mRNA sequences.

The basic unit of any living organism that carries on the biochemical processes of life.

Centimorgan (cM)
A unit of measure of recombination frequency. One centimorgan is equal to a 1% chance that a marker at one genetic locus will be separated from a marker at a second locus due to crossing over in a single generation. In human beings, one centimorgan is equivalent, on average, to one million base pairs.

A specialized chromosome region to which spindle fibers attach during cell division.

Chimera (plural: chimaera)
An organism that contains cells or tissues with a different genotype. These can be mutated cells of the host organism or cells from a different organism or species.

An experimental targeted repair process in which a desirable sequence of DNA is combined with RNA to form a chimeraplast. These molecules bind selectively to the target DNA. Once bound, the chimeraplast activates a naturally occurring gene-correcting mechanism. Does not use viral or other conventional gene-delivery vectors.

Chloroplast chromosome
Circular DNA found in the photosynthesizing organelle (chloroplast) of plants instead of the cell nucleus where most genetic material is located.

One of the serially aligned beads or granules of a eukaryotic chromosome, resulting from local coiling of a continuous DNA thread.

Chromosomal deletion
The loss of part of a chromosome's DNA.

Chromosomal inversion
Chromosome segments that have been turned 180 degrees. The gene sequence for the segment is reversed with respect to the rest of the chromosome.

A chromosome is an organized structure of DNA and protein that is found in cells. A chromosome is a single piece of DNA that contains many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions. The self-replicating genetic structure of cells containing the cellular DNA that bears in its nucleotide sequence the linear array of genes. In prokaryotes, chromosomal DNA is circular, and the entire genome is carried on one chromosome. Eukaryotic genomes consist of a number of chromosomes whose DNA is associated with different kinds of proteins.

Chromosome painting
Attachment of certain fluorescent dyes to targeted parts of the chromosome. Used as a diagnostic for particular diseases, (e.g., types of leukemia).

Chromosome region p
A designation for the short arm of a chromosome.

Chromosome region q
A designation for the long arm of a chromosome.

An exact copy made of biological material such as a DNA segment (e.g., a gene or other region), a whole cell, or a complete organism.

Using specialized DNA technology to produce multiple, exact copies of a single gene or other segment of DNA to obtain enough material for further study. This process, used by researchers in the Human Genome Project, is referred to as cloning DNA. The resulting cloned (copied) collections of DNA molecules are called clone libraries. A second type of cloning exploits the natural process of cell division to make many copies of an entire cell. The genetic makeup of these cloned cells, called a cell line, is identical to the original cell. A third type of cloning produces complete, genetically identical animals such as the famous Scottish sheep, Dolly.

Cloning vector
DNA molecule originating from a virus, a plasmid, or the cell of a higher organism into which another DNA fragment of appropriate size can be integrated without loss of the vector's capacity for self-replication; vectors introduce foreign DNA into host cells, where the DNA can be reproduced in large quantities. Examples are plasmids, cosmids, and yeast artificial chromosomes; vectors are often recombinant molecules containing DNA sequences from several sources.

Situation in which two different alleles for a genetic trait are both expressed.

Coisogenic or congenic
Nearly identical strains of an organism; they vary at only a single locus.

Comparative genomics
The study of human genetics by comparisons with model organisms such as mice, the fruit fly, and the bacterium E. coli.

Complementary DNA (cDNA)
DNA that is synthesized in the laboratory from a messenger RNA template.

Complementary sequence
Nucleic acid base sequence that can form a double-stranded structure with another DNA fragment by following base-pairing rules (A pairs with T and C with G). The complementary sequence to GTAC for example, is CATG.

Complex trait
Trait that has a genetic component that does not follow strict Mendelian inheritance. May involve the interaction of two or more genes or gene-environment interactions.

In genetics, the expectation that genetic material and the information gained from testing that material will not be available without the donor's consent.

Any trait present at birth, whether the result of a genetic or nongenetic factor.

Conserved sequence
A base sequence in a DNA molecule (or an amino acid sequence in a protein) that has remained essentially unchanged throughout evolution.

Constitutive ablation
Gene expression that results in cell death.

Group of cloned (copied) pieces of DNA representing overlapping regions of a particular chromosome.

Contig map
A map depicting the relative order of a linked library of overlapping clones representing a complete chromosomal segment.

Artificially constructed cloning vector containing the cos gene of phage lambda. Cosmids can be packaged in lambda phage particles for infection into E. coli; this permits cloning of larger DNA fragments (up to 45kb) than can be introduced into bacterial hosts in plasmid vectors.

Crossing over
The breaking during meiosis of one maternal and one paternal chromosome, the exchange of corresponding sections of DNA, and the rejoining of the chromosomes. This process can result in an exchange of alleles between chromosomes.

The study of the physical appearance of chromosomes.

Cytological band
An area of the chromosome that stains differently from areas around it.

Cytological map
A type of chromosome map whereby genes are located on the basis of cytological findings obtained with the aid of chromosome mutations.

Cytoplasmic trait
A genetic characteristic in which the genes are found outside the nucleus, in chloroplasts or mitochondria. Results in offspring inheriting genetic material from only one parent.

Cytosine (C)
A nitrogenous base, one member of the base pair GC (guanine and cytosine) in DNA.

^ Top of Page


Data warehouse
A collection of databases, data tables, and mechanisms to access the data on a single subject.

A loss of part of the DNA from a chromosome which can lead to a disease or abnormality.

Deletion map
A description of a specific chromosome that uses defined mutations (specific deleted areas in the genome) as "biochemical signposts" or markers for specific areas.

A type of sugar that is one component of DNA (deoxyribonucleic acid).

A full set of genetic material consisting of paired chromosomes, one from each parental set. Most animal cells except the gametes have a diploid set of chromosomes. The diploid human genome has 46 chromosomes.

Directed evolution
A laboratory process used on isolated molecules or microbes to cause mutations and identify subsequent adaptations to novel environments.

Directed mutagenesis
Alteration of DNA at a specific site and its reinsertion into an organism to study any effects of the change.

Directed sequencing
Successively sequencing DNA from adjacent stretches of chromosome.

Disease-associated genes
Alleles carrying particular DNA sequences associated with the presence of disease.

DNA (deoxyribonucleic acid)
The molecule that encodes genetic information. DNA constitutes the chemical basis for genes, the basic units of heredity. DNA is a long polymer made from repeating units called nucleotides. DNA is a double-stranded molecule held together by weak bonds between base pairs of nucleotides. The four nucleotides in DNA contain the bases adenine (A), guanine (G), cytosine (C), and thymine (T). In nature, base pairs form only between A and T and between G and C; thus the base sequence of each single strand can be deduced from that of its partner. The DNA chain is 22 to 26 Ångströms (Å) wide (2.2 to 2.6 nanometres (nm)), and one nucleotide unit is 3.3 Å (0.33 nm) long. Although each individual repeating unit is very small, DNA polymers can be very large molecules containing millions of nucleotides.

DNA bank
A service that stores DNA extracted from blood samples or other human tissue.

DNA repair genes
Genes encoding proteins that correct errors in DNA sequencing.

DNA replication
The use of existing DNA as a template for the synthesis of new DNA strands. In humans and other eukaryotes, replication occurs in the cell nucleus.

DNA sequence
The relative order of base pairs, whether in a DNA fragment, gene, chromosome, or an entire genome.

A discrete portion of a protein with its own function. The combination of domains in a single protein determines its overall function.

An allele that is almost always expressed, even if only one copy is present.

Double helix
The twisted-ladder shape that two linear strands of DNA assume when complementary nucleotides on opposing strands bond together.

Draft sequence
The sequence generated by the HGP as of June 2000 that, while incomplete, offers a virtual road map to an estimated 95% of all human genes. Draft sequence data are mostly in the form of 10,000 base pair-sized fragments whose approximate chromosomal locations are known.

^ Top of Page


A method of separating large molecules (such as DNA fragments or proteins) from a mixture of similar molecules. An electric current is passed through a medium containing the mixture, and each kind of molecule travels through the medium at a different rate, depending on its electrical charge and size. Agarose and acrylamide gels are the media commonly used for electrophoresis of proteins and nucleic acids.

A process using high-voltage current to make cell membranes permeable to allow the introduction of new DNA; commonly used in recombinant DNA technology.

Embryonic stem cells (ES)
An embryonic cell that can replicate indefinitely, transform into other types of cells, and serve as a continuous source of new cells.

A protein that acts as a catalyst, speeding the rate at which a biochemical reaction proceeds but not altering the direction or nature of the reaction.

One gene interfers with or prevents the expression of another gene located at a different locus.

Escherichia coli
Common bacterium that has been studied intensively by geneticists because of its small genome size, normal lack of pathogenicity, and ease of growth in the laboratory.

The study of improving a species by artificial selection; usually refers to the selective breeding of humans.

Cell or organism with membrane-bound, structurally discrete nucleus and other well-developed subcellular compartments. Eukaryotes include all organisms except viruses, bacteria, and bluegreen algae.

Exogenous DNA
DNA originating outside an organism that has been introduced into the organism.

The protein-coding DNA sequence of a gene.

An enzyme that cleaves nucleotides sequentially from free ends of a linear nucleic acid substrate.

Expressed sequence tag (EST)
A short strand of DNA that is a part of a cDNA molecule and can act as identifier of a gene. Used in locating and mapping genes.

^ Top of Page


Filial generation (F1, F2)
Each generation of offspring in a breeding program, designated F1, F2, etc.

In genetics, the identification of multiple specific alleles on a person's DNA to produce a unique identifier for that person.

Finished DNA Sequence
High-quality, low error, gap-free DNA sequence of the human genome. Achieving this ultimate 2003 HGP goal requires additional sequencing to close gaps, reduce ambiguities, and allow for only a single error every 10,000 bases, the agreed-upon standard for HGP finished sequence.

Flow cytometry
Analysis of biological material by detection of the light-absorbing or fluorescing properties of cells or subcellular fractions (i.e., chromosomes) passing in a narrow stream through a laser beam. An absorbance or fluorescence profile of the sample is produced. Automated sorting devices, used to fractionate samples, sort successive droplets of the analyzed stream into different fractions depending on the fluorescence emitted by each droplet.

Flow karyotyping
Use of flow cytometry to analyze and separate chromosomes according to their DNA content.

Fluorescence in situ hybridization (FISH)
A physical mapping approach that uses fluorescein tags to detect hybridization of probes with metaphase chromosomes and with the less-condensed somatic interphase chromatin.

The use of DNA for identification. Some examples of DNA use are to establish paternity in child support cases; establish the presence of a suspect at a crime scene, and identify accident victims.

Fraternal twin
Siblings born at the same time as the result of fertilization of two ova by two sperm. They share the same genetic relationship to each other as any other siblings.

Full gene sequence
The complete order of bases in a gene. This order determines which protein a gene will produce.

Functional genomics
The study of genes, their resulting proteins, and the role played by the proteins the body's biochemical processes.

^ Top of Page


Mature male or female reproductive cell (sperm or ovum) with a haploid set of chromosomes (23 for humans).

GC-rich area
Many DNA sequences carry long stretches of repeated G and C which often indicate a gene-rich region.

The fundamental physical and functional unit of heredity containing coded instructions for manufacturing a protein. Genes are subunits of chromosomes, which are strands of DNA contained within most cells. A gene is an ordered sequence of nucleotides located in a particular position on a particular chromosome that encodes a specific functional product (i.e., a protein or RNA molecule).

Gene amplification
Repeated copying of a piece of DNA; a characteristic of tumor cells.

Gene chip technology
Development of cDNA microarrays from a large number of genes. Used to monitor and measure changes in gene expression for each gene represented on the chip.

Gene expression
The process by which a gene's coded information is converted into the structures present and operating in the cell. Expressed genes include those that are transcribed into mRNA and then translated into protein and those that are transcribed into RNA but not translated into protein (e.g., transfer and ribosomal RNAs).

Gene family
Group of closely related genes that make similar products.

Gene mapping
Determination of the relative positions of genes on a DNA molecule (chromosome or plasmid) and of the distance, in linkage units or physical units, between them.

Gene pool
All the variations of genes in a species.

Gene prediction
Predictions of possible genes made by a computer program based on how well a stretch of DNA sequence matches known gene sequences.

Gene product
The biochemical material, either RNA or protein, resulting from expression of a gene. The amount of gene product is used to measure how active a gene is; abnormal amounts can be correlated with disease-causing alleles.

Gene therapy
An experimental procedure aimed at replacing, manipulating, or supplementing nonfunctional or misfunctioning genes with healthy genes.

Gene transfer
Incorporation of new DNA into and organism's cells, usually by a vector such as a modified virus and used in gene therapy.

Genetic code
This is the set of rules by which information encoded in genetic material, DNA or RNA sequences, is translated into proteins or amino acid sequences by living cells. The sequence of nucleotides, coded in triplets (codons) along the mRNA, that determines the sequence of amino acids in protein synthesis. A gene's DNA sequence can be used to predict the mRNA sequence, and the genetic code can in turn be used to predict the amino acid sequence.

Genetic counseling
Provides patients and their families with education and information about genetic-related conditions and helps them make informed decisions.

Genetic discrimination
Prejudice against those who have or are likely to develop an inherited disorder.

Genetic disorders
A genetic disorder is a disease that is caused by an abnormality in an individual's DNA. Abnormalities can range from a small mutation in a single gene to the addition or subtraction of an entire chromosome or set of chromosomes.

Genetic engineering
Altering the genetic material of cells or organisms to enable them to make new substances or perform new functions.

Genetic illness
Sickness, physical disability, or other disorder resulting from the inheritance of one or more deleterious alleles.

Genetic marker
A gene or other identifiable portion of DNA whose inheritance can be followed.

Genetic mosaic
An organism in which different cells contain different genetic sequence. This can be the result of a mutation during development or fusion of embryos at an early developmental stage.

Genetic polymorphism
Difference in DNA sequence among individuals, groups, or populations (e.g., genes for blue eyes versus brown eyes).

Genetic predisposition
A genetic predisposition to a condition, disorder, or disease means that a person’s inherited genetic composition makes it more likely that he or she will develop that condition, disorder, or disease. It is the susceptibility to a genetic disease. May or may not result in actual development of the disease.

Genetic screening
Testing a group of people to identify individuals at high risk of having or passing on a specific genetic disorder.

Genetic testing
Analyzing an individual's genetic material to determine predisposition to a particular health condition or to confirm a diagnosis of genetic disease.

The study of inheritance patterns of specific traits.

All the genetic material in the chromosomes of a particular organism; its size is generally given as its total number of base pairs.

Genome project
Research and technology-development effort aimed at mapping and sequencing the genome of human beings and certain model organisms.

Genomic library
A collection of clones made from a set of randomly generated overlapping DNA fragments that represent the entire genome of an organism.

The study of genes and their function.

The genotype is the genetic constitution of a cell, an organism, or an individual usually with reference to a specific character under consideration. The genetic constitution of an organism, as distinguished from its physical appearance (its phenotype).

Germ cell
Sperm and egg cells and their precursors. Germ cells are haploid and have only one set of chromosomes (23 in all), while all other cells have two copies (46 in all).

Germ line
The continuation of a set of genetic information from one generation to the next.

Germ line gene therapy
An experimental process of inserting genes into germ cells or fertilized eggs to cause a genetic change that can be passed on to offspring. May be used to alleviate effects associated with a genetic disease.

Guanine (G)
A nitrogenous base, one member of the base pair GC (guanine and cytosine) in DNA.

Organisms that have both male and female cells and therefore express both male and female characteristics.

^ Top of Page


A single set of chromosomes (half the full set of genetic material) present in the egg and sperm cells of animals and in the egg and pollen cells of plants. Human beings have 23 chromosomes in their reproductive cells.

A way of denoting the collective genotype of a number of closely linked loci on a chromosome.

Having only one copy of a particular gene. For example, in humans, males are hemizygous for genes found on the Y chromosome.

Hereditary cancer
Cancer that occurs due to the inheritance of an altered gene within a family.

The presence of different alleles at one or more loci on homologous chromosomes.

Highly conserved sequence
DNA sequence that is very similar across several different types of organisms.

High-throughput sequencing
A fast method of determining the order of bases in DNA.

A short stretch of nucleotides whose base sequence is virtually identical in all the genes that contain it. Homeoboxes have been found in many organisms from fruit flies to human beings. In the fruit fly, a homeobox appears to determine when particular groups of genes are expressed during development.

A member of a chromosome pair in diploid organisms or a gene that has the same origin and functions in two or more species.

Homologous chromosome
Chromosome containing the same linear gene sequences as another, each derived from one parent.

Homologous recombination
Swapping of DNA fragments between paired chromosomes.

Similarity in DNA or protein sequences between individuals of the same species or among different species.

An organism that has two identical alleles of a gene.

Human artificial chromosome (HAC)
A vector used to hold large DNA fragments.

Human genome
The human genome is the complete set of genetic information for humans. This information is encoded as DNA sequences within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual mitochondria. Human genomes include both protein-coding DNA genes and noncoding DNA. Haploid human genomes (contained in egg and sperm cells) consist of three billion DNA base pairs, while diploid genomes (found in somatic cells) have twice the DNA content.

Human Genome Initiative
This is the early name of the Human Genome Project.
(See: Human Genome Project)

Human Genome Project (HGP)
Collective name for several projects begun in 1986 by DOE to create an ordered set of DNA segments from known chromosomal locations, develop new computational methods for analyzing genetic map and DNA sequence data, and develop new techniques and instruments for detecting and analyzing DNA. The joint national effort, led by DOE and NIH, previously known as the Human Genome Initiative.

The offspring of genetically different parents.

The process of joining two complementary strands of DNA or one each of DNA and RNA to form a double-stranded molecule.

^ Top of Page


Using the immune system to treat disease, for example, in the development of vaccines. May also refer to the therapy of diseases caused by the immune system.

A phenomenon in which the disease phenotype depends on which parent passed on the disease gene. For instance, both Prader-Willi and Angelman syndromes are inherited when the same part of chromosome 15 is missing. When the father's complement of 15 is missing, the child has Prader-Willi, but when the mother's complement of 15 is missing, the child has Angelman syndrome.

In situ hybridization
Use of a DNA or RNA probe to detect the presence of the complementary DNA sequence in cloned bacterial or cultured eukaryotic cells.

In vitro
Studies performed outside a living organism such as in a laboratory.

In vivo
Studies carried out in living organisms.

Independent assortment
During meiosis each of the two copies of a gene is distributed to the germ cells independently of the distribution of other genes.

Informed consent
An individual willingly agrees to participate in an activity after first being advised of the risks and benefits.

In genetics, to receive genetic material from parents through biological processes.

A chromosome abnormality in which a piece of DNA is incorporated into a gene and thereby disrupts the gene's normal function.

Intellectual property rights
Patents, copyrights, and trademarks.

One crossover event inhibits the chances of another crossover event. Also known as positive interference. Negative interference increases the chance of a second crossover.

The period in the cell cycle when DNA is replicated in the nucleus; followed by mitosis.

DNA sequence that interrupts the protein-coding sequence of a gene; an intron is transcribed into RNA but is cut out of the message before it is translated into protein.

An enzyme performing the same function as another enzyme but having a different set of amino acids. The two enzymes may function at different speeds.

^ Top of Page


Junk DNA
Stretches of DNA that do not code for genes; most of the genome consists of so-called junk DNA which may have regulatory and other functions. Also called non-coding DNA.

^ Top of Page


A photomicrograph of an individual's chromosomes arranged in a standard format showing the number, size, and shape of each chromosome type; used in low-resolution physical mapping to correlate gross chromosomal abnormalities with the characteristics of specific diseases.

Kilobase (kb)
Unit of length for DNA fragments equal to 1000 nucleotides.

Deactivation of specific genes; used in laboratory organisms to study gene function.

^ Top of Page


An unordered collection of clones (e.g., cloned DNA from a particular organism) whose relationship to each other can be established by physical mapping.

The proximity of two or more markers (e.g., genes, RFLP markers) on a chromosome; the closer the markers, the lower the probability that they will be separated during DNA repair or replication processes (binary fission in prokaryotes, mitosis or meiosis in eukaryotes), and hence the greater the probability that they will be inherited together.

Linkage disequilibrium
Where alleles occur together more often than can be accounted for by chance. Indicates that the two alleles are physically close on the DNA strand.

Linkage map
A map of the relative positions of genetic loci on a chromosome, determined on the basis of how often the loci are inherited together. Distance is measured in centimorgans (cM).

Linked SNPs (Indicative SNPs)
These do not reside within genes and do not affect protein function, but they do correspond to a particular drug response or to the risk for getting a certain disease.

Determination of the original position (locus) of a gene or other marker on a chromosome.

Locus (plural: loci)
The position on a chromosome of a gene or other chromosome marker; also, the DNA at that position. The use of locus is sometimes restricted to mean expressed DNA regions.

Long-Range Restriction Mapping
Restriction enzymes are proteins that cut DNA at precise locations. Restriction maps depict the chromosomal positions of restriction-enzyme cutting sites. These are used as biochemical "signposts," or markers of specific areas along the chromosomes. The map will detail the positions where the DNA molecule is cut by particular restriction enzymes.

^ Top of Page


Macrorestriction map
Map depicting the order of and distance between sites at which restriction enzymes cleave chromosomes.

Major histocompatibility complex (MHC)
A large cluster of genes found on the short arm of chromosome 6. The complex spans four million base pairs of DNA and contains 128 genes as well as 96 pseudogenes (non-functional gene remnants). Many of the genes in this complex play important roles in the immune system.

Mapping population
The group of related organisms used in constructing a genetic map.

Mass spectrometry
An instrument used to identify chemicals in a substance by their mass and charge.

Megabase (Mb)
Unit of length for DNA fragments equal to 1 million nucleotides and roughly equal to 1 cM.

The process of two consecutive cell divisions in the diploid progenitors of sex cells. Meiosis results in four rather than two daughter cells, each with a haploid set of chromosomes.

Mendelian inheritance
One method in which genetic traits are passed from parents to offspring. Named for Gregor Mendel, who first studied and recognized the existence of genes and this method of inheritance.

Messenger RNA (mRNA)
RNA that serves as a template for protein synthesis.

A stage in mitosis or meiosis during which the chromosomes are aligned along the equatorial plane of the cell.

Sets of miniaturized chemical reaction areas that may also be used to test DNA fragments, antibodies, or proteins.

Microbial genetics
The study of genes and gene function in bacteria, archaea, and other microorganisms. Often used in research in the fields of bioremediation, alternative energy, and disease prevention.

A technique for introducing a solution of DNA into a cell using a fine microcapillary pipet.

Chromosome fragments that are not incorporated into the nucleus at cell division.

Mitochondrial DNA
The genetic material found in mitochondria, the organelles that generate energy for the cell. Not inherited in the same fashion as nucleic DNA.

The process of nuclear division in cells that produces daughter cells that are genetically identical to each other and to the parent cell.

Model organisms
A laboratory animal or other organism useful for research.

The use of statistical analysis, computer analysis, or model organisms to predict outcomes of research.

Molecular biology
The study of the structure, function, and makeup of biologically important molecules.

Molecular farming
The development of transgenic animals to produce human proteins for medical use.

Molecular genetics
The study of macromolecules important in biological inheritance.

Molecular medicine
The treatment of injury or disease at the molecular level. Examples include the use of DNA-based diagnostic tests or medicine derived from DNA sequence information.

Monogenic disorder
A disorder caused by mutation of a single gene.

Possessing only one copy of a particular chromosome instead of the normal two copies.

Morbid map
A diagram showing the chromosomal location of genes associated with disease.

A laboratory approach that performs multiple sets of reactions in parallel (simultaneously); greatly increasing speed and throughput.

Organism in the genus Mus. A rat or mouse.

A physical or chemical agent that changes the genetic material or information of an organism thus causing mutations.

The capacity of a chemical or physical agent to cause permanent genetic alterations.

A mutation is a permanent change in the DNA sequence of a gene. Mutations in a gene's DNA sequence can alter the amino acid sequence of the protein encoded by the gene.

^ Top of Page


Nitrogenous base
A nitrogen-containing molecule having the chemical properties of a base. DNA contains the nitrogenous bases adenine (A), guanine (G), cytosine (C), and thymine (T).

Northern blot
A gel-based laboratory procedure that locates mRNA sequences on a gel that are complementary to a piece of DNA used as a probe.

Nuclear transfer
A laboratory procedure in which a cell's nucleus is removed and placed into an oocyte with its own nucleus removed so the genetic information from the donor nucleus controls the resulting cell. Such cells can be induced to form embryos. This process was used to create the cloned sheep "Dolly".

Nucleic acid
A large molecule composed of nucleotide subunits. This is a macromolecule composed of chains of monomeric nucleotides and carry genetic information or form structures within cells. The most common nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

Nucleolar organizing region
A part of the chromosome containing rRNA genes.

A subunit of DNA or RNA consisting of a nitrogenous base (adenine, guanine, thymine, or cytosine in DNA; adenine, guanine, uracil, or cytosine in RNA), a phosphate molecule, and a sugar molecule (deoxyribose in DNA and ribose in RNA). Thousands of nucleotides are linked to form a DNA or RNA molecule.

Nucleotide base
The heterocyclic pyrimidine and purine compounds which are constituents of all nucleic acids. Adenine (A), guanine (G) and cytosine (C) are found in both DNA and RNA, thymine (T) is found (primarily) in DNA and uracil (U) only in RNA and they are considered the main nucleotide basis.

The cellular organelle in eukaryotes that contains most of the genetic material.

^ Top of Page


A phenotypic trait produced by two or more genes working together.

A molecule usually composed of 25 or fewer nucleotides; used as a DNA synthesis primer.

A gene, one or more forms of which is associated with cancer. Many oncogenes are involved, directly or indirectly, in controlling the rate of cell growth.

Open reading frame (ORF)
The sequence of DNA or RNA located between the start-code sequence (initiation codon) and the stop-code sequence (termination codon).

A set of genes transcribed under the control of an operator gene.

^ Top of Page


P1-derived artificial chromosome (PAC)
One type of vector used to clone DNA fragments (100- to 300-kb insert size; average, 150 kb) in Escherichia coli cells. Based on bacteriophage (a virus) P1 genome.

In genetics, conferring the right or title to genes, gene variations, or identifiable portions of sequenced genetic material to an individual or organization.

A family tree diagram that shows how a particular genetic trait or disease has been inherited.

The probability of a gene or genetic trait being expressed. "Complete" penetrance means the gene or genes for a trait are expressed in all the population who have the genes. "Incomplete" penetrance means the genetic trait is expressed in only part of the population. The percent penetrance also may change with the age range of the population.

Two or more amino acids joined by a bond called a "peptide bond."

A virus for which the natural host is a bacterial cell.

The study of the interaction of an individual's genetic makeup and response to a drug.

A trait not caused by inheritance of a gene but appears to be identical to a genetic trait.

The physical characteristics of an organism or the presence of a disease that may or may not be genetic.

Physical map
A map of the locations of identifiable landmarks on DNA (e.g., restriction-enzyme cutting sites, genes), regardless of inheritance. Distance is measured in base pairs. For the human genome, the lowest-resolution physical map is the banding patterns on the 24 different chromosomes; the highest-resolution map is the complete nucleotide sequence of the chromosomes.

Autonomously replicating extra-chromosomal circular DNA molecules, distinct from the normal bacterial genome and nonessential for cell survival under nonselective conditions. Some plasmids are capable of integrating into the host genome. A number of artificially constructed plasmids are used as cloning vectors.

One gene that causes many different physical traits such as multiple disease symptoms.

The potential of a cell to develop into more than one type of mature cell, depending on environment.

Polygenic disorder
Genetic disorder resulting from the combined action of alleles of more than one gene (e.g., heart disease, diabetes, and some cancers). Although such disorders are inherited, they depend on the simultaneous presence of several alleles; thus the hereditary patterns usually are more complex than those of single-gene disorders.

Polymerase chain reaction (PCR)
A method for amplifying a DNA base sequence using a heat-stable polymerase and two 20-base primers, one complementary to the (+) strand at one end of the sequence to be amplified and one complementary to the (-) strand at the other end. Because the newly synthesized DNA strands can subsequently serve as additional templates for the same primer sequences, successive rounds of primer annealing, strand elongation, and dissociation produce rapid and highly specific amplification of the desired sequence. PCR also can be used to detect the existence of the defined sequence in a DNA sample.

Polymerase, DNA or RNA
Enzyme that catalyzes the synthesis of nucleic acids on preexisting nucleic acid templates, assembling RNA from ribonucleotides or DNA from deoxyribonucleotides.

Difference in DNA sequence among individuals that may underlie differences in health. Genetic variations occurring in more than 1% of a population would be considered useful polymorphisms for genetic linkage analysis.

A protein or part of a protein made of a chain of amino acids joined by a peptide bond.

Population genetics
The study of variation in genes among a group of individuals.

Positional cloning
A technique used to identify genes, usually those that are associated with diseases, based on their location on a chromosome.

Premature chromosome condensation (PCC)
A method of studying chromosomes in the interphase stage of the cell cycle.

Short preexisting polynucleotide chain to which new deoxyribonucleotides can be added by DNA polymerase.

In genetics, the right of people to restrict access to their genetic information.

Single-stranded DNA or RNA molecules of specific base sequence, labeled either radioactively or immunologically, that are used to detect the complementary base sequence by hybridization.

Cell or organism lacking a membrane-bound, structurally discrete nucleus and other subcellular compartments. Bacteria are examples of prokaryotes.

A DNA site to which RNA polymerase will bind and initiate transcription.

The nucleus of a sperm or egg prior to fertilization.

A large molecule composed of one or more chains of amino acids in a specific order; the order is determined by the base sequence of nucleotides in the gene that codes for the protein. Proteins are required for the structure, function, and regulation of the body's cells, tissues, and organs; and each protein has unique functions. Examples are hormones, enzymes, and antibodies.

Proteins expressed by a cell or organ at a particular time and under specific conditions.

The study of the full set of proteins encoded by a genome.

A sequence of DNA similar to a gene but nonfunctional; probably the remnant of a once-functional gene that accumulated mutations.

A nitrogen-containing, double-ring, basic compound that occurs in nucleic acids. The purines in DNA and RNA are adenine and guanine.

A nitrogen-containing, single-ring, basic compound that occurs in nucleic acids. The pyrimidines in DNA are cytosine and thymine; in RNA, cytosine and uracil.

^ Top of Page


^ Top of Page


Radiation hybrid
A hybrid cell containing small fragments of irradiated human chromosomes. Maps of irradiation sites on chromosomes for the human, rat, mouse, and other genomes provide important markers, allowing the construction of very precise STS maps indispensable to studying multifactorial diseases.

Recessive gene
A gene which will be expressed only if there are 2 identical copies or, for a male, if one copy is present on the X chromosome.

Reciprocal translocation
When a pair of chromosomes exchange exactly the same length and area of DNA. Results in a shuffling of genes.

Produced by a biotechnological process in the laboratory.

Recombinant clone
Clone containing recombinant DNA molecules.

Recombinant DNA molecules
A combination of DNA molecules of different origin that are joined using recombinant DNA technologies.

Recombinant DNA technology
Procedure used to join together DNA segments in a cell-free system (an environment outside a cell or organism). Under appropriate conditions, a recombinant DNA molecule can enter a cell and replicate there, either autonomously or after it has become integrated into a cellular chromosome.

The process by which progeny derive a combination of genes different from that of either parent. In higher organisms, this can occur by crossing over.

Regulatory region or sequence
A DNA base sequence that controls gene expression.

Repetitive DNA
Sequences of varying lengths that occur in multiple copies in the genome; it represents much of the human genome.

Degree of molecular detail on a physical map of DNA, ranging from low to high.

Restriction enzyme, endonuclease
A protein that recognizes specific, short nucleotide sequences and cuts DNA at those sites. Bacteria contain over 400 such enzymes that recognize and cut more than 100 different DNA sequences.

Restriction fragment length polymorphism (RFLP)
Variation between individuals in DNA fragment sizes cut by specific restriction enzymes; polymorphic sequences that result in RFLPs are used as markers on both physical maps and genetic linkage maps. RFLPs usually are caused by mutation at a cutting site.

Restriction-enzyme cutting site
A specific nucleotide sequence of DNA at which a particular restriction enzyme cuts the DNA. Some sites occur frequently in DNA (e.g., every several hundred base pairs); others much less frequently (rare-cutter; e.g., every 10,000 base pairs).

Retroviral infection
The presence of retroviral vectors, such as some viruses, which use their recombinant DNA to insert their genetic material into the chromosomes of the host's cells. The virus is then propogated by the host cell.

Reverse transcriptase
An enzyme used by retroviruses to form a complementary DNA sequence (cDNA) from their RNA. The resulting DNA is then inserted into the chromosome of the host cell.

The five-carbon sugar that serves as a component of RNA.

Ribosomal RNA (rRNA)
A class of RNA found in the ribosomes of cells.

Small cellular components composed of specialized ribosomal RNA and protein; site of protein synthesis.

Risk communication
In genetics, a process in which a genetic counselor or other medical professional interprets genetic test results and advises patients of the consequences for them and their offspring.

RNA (Ribonucleic acid)
A chemical found in the nucleus and cytoplasm of cells; it plays an important role in protein synthesis and other chemical activities of the cell. It consists of a long chain of nucleotide units central to the synthesis of proteins. RNA is very similar to DNA, but differs in a few important structural details: in the cell, RNA is usually single-stranded, while DNA is usually double-stranded; RNA nucleotides contain ribose while DNA contains deoxyribose; and RNA has the base uracil rather than thymine that is present in DNA. RNA is transcribed from DNA by enzymes called RNA polymerases and is generally further processed by other enzymes. There are several classes of RNA molecules, including messenger RNA, transfer RNA, ribosomal RNA, and other small RNAs, each serving a different purpose.

^ Top of Page


Sanger sequencing
A widely used method of determining the order of bases in DNA.

A chromosomal segment that branches off from the rest of the chromosome but is still connected by a thin filament or stalk.

In genomic mapping, a series of contigs that are in the right order but not necessarily connected in one continuous stretch of sequence.

The normal biological process whereby the two pieces of a chromosome pair are separated during meiosis and randomly distributed to the germ cells.

Sequence assembly
A process whereby the order of multiple sequenced DNA fragments is determined.

Sequence tagged site (STS)
Short (200 to 500 base pairs) DNA sequence that has a single occurrence in the human genome and whose location and base sequence are known. Detectable by polymerase chain reaction, STSs are useful for localizing and orienting the mapping and sequence data reported from many different laboratories and serve as landmarks on the developing physical map of the human genome. Expressed sequence tags (ESTs) are STSs derived from cDNAs.

Determination of the order of nucleotides (base sequences) in a DNA or RNA molecule or the order of amino acids in a protein.

Sequencing technology
The instrumentation and procedures used to determine the order of nucleotides in DNA.

It's one of the 20 naturally occurring proteinogenic (coded for in the standard genetic code) amino acids.

Serine endopeptidases or serine proteases
These are proteases (enzymes that cut peptide bonds in proteins) in which one of the amino acids at the active site is serine.

Sex chromosome
The X or Y chromosome in human beings that determines the sex of an individual. Females have two X chromosomes in diploid cells; males have an X and a Y chromosome. The sex chromosomes comprise the 23rd chromosome pair in a karyotype.

Traits or diseases associated with the X or Y chromosome; generally seen in males.

Shotgun method
Sequencing method that involves randomly sequenced cloned pieces of the genome, with no foreknowledge of where the piece originally came from. This can be contrasted with "directed" strategies, in which pieces of DNA from known chromosomal locations are sequenced. Because there are advantages to both strategies, researchers use both random (or shotgun) and directed strategies in combination to sequence the human genome.

Single nucleotide polymorphism (SNP)
DNA sequence variations that occur when a single nucleotide (A, T, C, or G) in the genome sequence is altered. SNPs are single nucleotide substitutions of one base for another that occur in more than one percent of the general population. Each SNP location in the genome can have up to four versions: one for each nucleotide, A, C, G and T.

Single-gene disorder
Hereditary disorder caused by a mutant allele of a single gene (e.g., Duchenne muscular dystrophy, retinoblastoma, sickle cell disease).

SNP (pronounced "snip")
(See: Single nucleotide polymorphism)

Somatic cell
Any cell in the body except gametes and their precursors.

Somatic cell gene therapy
Incorporating new genetic material into cells for therapeutic purposes. The new genetic material cannot be passed to offspring.

Somatic cell genetic mutation
A change in the genetic structure that is neither inherited nor passed to offspring. Also called acquired mutations.

Southern blotting
Transfer by absorption of DNA fragments separated in electrophoretic gels to membrane filters for detection of specific base sequences by radio-labeled complementary probes.

Spectral karyotype (SKY)
A graphic of all an organism's chromosomes, each labeled with a different color. Useful for identifying chromosomal abnormalities.

Splice site
Location in the DNA sequence where RNA removes the noncoding areas to form a continuous gene transcript for translation into a protein.

Spontaneous mutations
Changes to the genetic information due to errors in replication, repair, or recombination of DNA sequences.

Sporadic cancer
Cancer that occurs randomly and is not inherited from parents. Caused by DNA changes in one cell that grows and divides, spreading throughout the body.

Stem cell
Undifferentiated, primitive cells in the bone marrow that have the ability both to multiply and to differentiate into specific blood cells.

Structural genomics
The effort to determine the 3D structures of large numbers of proteins using both experimental techniques and computer simulation.

In genetics, a type of mutation due to replacement of one nucleotide in a DNA sequence by another nucleotide or replacement of one amino acid in a protein by another amino acid.

Suppressor gene
A gene that can suppress the action of another gene.

The group or recognizable pattern of symptoms or abnormalities that indicate a particular trait or disease.

Genetically identical members of the same species.

Genes occurring in the same order on chromosomes of different species.

^ Top of Page


Tandem repeat sequences
Multiple copies of the same base sequence on a chromosome; used as markers in physical mapping.

Targeted mutagenesis
Deliberate change in the genetic structure directed at a specific site on the chromosome. Used in research to determine the targeted region's function.

Technology transfer
The process of transferring scientific findings from research laboratories to the commercial sector.

The enzyme that directs the replication of telomeres.

The end of a chromosome. This specialized structure is involved in the replication and stability of linear DNA molecules.

Substances such as chemicals or radiation that cause abnormal development of a embryo.

Thymine (T)
A nitrogenous base, one member of the base pair AT (adenine-thymine).

The study of how genomes respond to environmental stressors or toxicants. Combines genome-wide mRNA expression profiling with protein expression patterns using bioinformatics to understand the role of gene-environment interactions in disease and dysfunction.

The synthesis of an RNA copy from a sequence of DNA (a gene); the first step in gene expression.

Transcription factor
A protein that binds to regulatory regions and helps control gene expression.

The full complement of activated genes, mRNAs, or transcripts in a particular tissue at a particular time.

The introduction of foreign DNA into a host cell.

Transfer RNA (tRNA)
A class of RNA having structures with triplet nucleotide sequences that are complementary to the triplet nucleotide coding sequences of mRNA. The role of tRNAs in protein synthesis is to bond with amino acids and transfer them to the ribosomes, where proteins are assembled according to the genetic code carried by mRNA.

A process by which the genetic material carried by an individual cell is altered by incorporation of exogenous DNA into its genome.

An experimentally produced organism in which DNA has been artificially introduced and incorporated into the organism's germ line.

The process in which the genetic code carried by mRNA directs the synthesis of proteins from amino acids.

A mutation in which a large segment of one chromosome breaks off and attaches to another chromosome.

Transposable element
A class of DNA sequences that can move from one chromosomal site to another.

Possessing three copies of a particular chromosome instead of the normal two copies.

Twins - Dizygotic
Also known as fraternal twins, two babies that come from separate, simultaneously-fertilized eggs. If one dizygotic twin develops MS, the other has the same genetic risk.

Twins - Identical
Twins produced by the division of a single zygote; both have identical genotypes.

Twins - Monozygotic
Also known as identical twins, two babies that come from single fertilized egg and share identical genetic makeup. If one monozygotic twin develops MS, the other has a 30/100 risk of developing the disease, indicating that factors other than genetic makeup contribute to the etiology of MS.

^ Top of Page


A nitrogenous base normally found in RNA but not DNA; uracil is capable of forming a base pair with adenine.

^ Top of Page


A noncellular biological entity that can reproduce only within a host cell. Viruses consist of nucleic acid covered by protein; some animal viruses are also surrounded by membrane. Inside the infected cell, the virus uses the synthetic capability of the host to produce progeny virus.

^ Top of Page


Western blot
A technique used to identify and locate proteins based on their ability to bind to specific antibodies.

Wild type
The form of an organism that occurs most frequently in nature.

^ Top of Page


X chromosome
One of the two sex chromosomes, X and Y.

X-linked Dominant Inheritance
X-linked dominant inheritance follows a pattern similar to autosomal dominant inheritance except that more females are affected than males. However, X-linked dominant disorders are very rare.

X-linked Recessive Disorders
X-linked recessive disorders are usually only seen in males and they are much more common than X-linked dominant disorders. People with an X-linked recessive disorder do not have any normal copies of the gene. Males only have one X chromosome, so if a male inherits a changed gene on his X chromosome (which is always inherited from his mother), then he does not have another copy of the working gene to compensate. Females with one copy of a changed gene on one X chromosome are called carriers of X-linked recessive disorder. It's rare for a female to have the changed gene on both her X chromosomes. In most cases, females who are carriers do not show symptoms because the working copy of the gene compensates for the non-working copy of the gene. Carrier females have a 25% of having a son with the disorder, a 25% chance of having a son without the disorder, a 25% chance of having a carrier daughter and a 25% chance of having a daughter who is not a carrier. Males with an X-linked recessive disorder cannot pass the disorder to their sons, but 100% of their daughters will be carriers.

Tissue or organs from an individual of one species transplanted into or grafted onto an organism of another species, genus, or family. A common example is the use of pig heart valves in humans.

^ Top of Page


Y chromosome
One of the two sex chromosomes, X and Y.

Yeast artificial chromosome (YAC)
Constructed from yeast DNA, it is a vector used to clone large DNA fragments.

^ Top of Page


Zinc-finger protein
A secondary feature of some proteins containing a zinc atom; a DNA-binding protein.

^ Top of Page