Acrocentric chromosome

A chromosome with the centromere near one end such that it has one long arm plus a stalk and a satellite.

Activators

The major class of transcription regulatory proteins in eukaryotes. Binding of these proteins to regulatory DNA sequences associated with specific genes determines the efficiency of transcription initiation. Some bacterial genes are controlled by activators.

Adenine (A)

A purine base found in DNA and RNA. In double stranded DNA, adenine pairs with thymine, a pyrimidine, by hydrogen bonding. In double-stranded RNA, adenine pairs with uracil, a pyrimidine, by hydrogen bonding.

Agarose gel electrophoresis

An experimental procedure in which an electric field is used to move DNA or RNA molecules, which are negatively charged, through a gel matrix of agarose from the negative pole to the positive pole.

Allele

One of two or more alternative forms of a single gene that can exist at the same locus in the genome. All the alle les of a gene determine the same hereditary trait (e.g., seed colour), but each has a unique nucleotide sequence, which may result in different phenotypes (e.g., yellow or green seeds).

Allele frequency

Proportion of a particular allele at a locus within a gene pool. The sum of the allele frequencies at a given locus is 1.
Allele-specific oligonucleotide (ASO) hybridization: A procedure, using PCR primers, to distinguish alleles that differ by one base pair.

Allelomorph

One of two or more alternative forms of a single gene that can exist at the same locus in the genome.

Allopolyploidy

Condition in which a cell or organism has two or more genetically distinct sets of chromosomes that originate in different, though usually related, species.

Alternation of generations

Type of life cycle characteristic of green plants in which haploid cells (gametophytes) alternate with diploid cells (sporophytes).

Alternative polyadenylation

Process for generating different functional mRNAs from a single gene by cleavage and polyadenylation of the primary transcript at different poly(A) sites.

Alternative splicing

In eukaryotes, a process for generating different functional mRNAs from a single precursor mRNA (pre-mRNA) by incorporating different exons in the mature mRNA.

Ames test

An assay that measures the ability of chemicals to cause mutations in certain bacteria. It can identify potential carcinogens.

Amino acid

Any of the small molecules, containing a carboxyl group and amino group, that are joined together to form polypeptides and proteins.

Aminoacyl–tRNA

A tRNA molecule covalently bound to an amino acid; also called charged tRNA. This complex brings the amino acid to the ribosome so that it can be used in polypeptide synthesis.

Aminoacyl–tRNA synthetase

An enzyme that catalyzes the addition of a specific amino acid to the tRNA for that amino acid.

Amniocentesis

A procedure in which a sample of amniotic sac fluid is withdrawn from the amniotic sac of a developing fetus and cells are cultured and examined for chromosomal abnormalities.

Analysis of variance (ANOVA)

A series of statistical procedures for determining whether differences in the means of a variable in two samples are significant and for partitioning the variance into components.

Anaphase

The stage in mitosis when the sister chromatids separate and migrate toward the opposite poles of the cell.

Anaphase I

The stage in meiosis I when the chromosomes in each bivalent separate and begin moving toward opposite poles of the cell.

Anaphase II

The stage in meiosis II when the sister chromatids are pulled to the opposite poles of the cell.

Aneuploid

Referring to an organism or cell that has a chromo some number that is not an exact multiple of the haploid set of chromosomes.

Aneuploidy

Any condition in which the number of chromosomes differs from an exact multiple of the normal haploid number in a cell or organism. It commonly results from the gain or loss of individual chromosomes but also can result from the duplication or deletion of part(s) of a chromo some or chromosomes.

Antibody

A protein molecule that recognizes and binds to a foreign substance introduced into the organism.

Anticodon

A group of three adjacent nucleotides in a tRNA molecule that pairs with a codon in mRNA by complementary base pairing.

Antigen

Any large molecule that stimulates the production of specific antibodies or binds specifically to an antibody.

Antiparallel

In the case of double-stranded DNA, referring to the opposite orientations of the strands, with the 5′ end of one strand paired with the 3′ end of the other strand.

Antisense mRNA

An mRNA transcribed from a cloned gene that is complementary to the mRNA produced by the nor mal gene.

Apoptosis

Controlled process leading to cell death that is triggered by intracellular damage (e.g., DNA lesions) or by external signals from neighbouring cells. Also called programmed cell death.

Aporepressor protein

An inactive repressor that is activated when bound to an effector molecule.

Applied research

Research done with the objective of developing products or processes that can be commercialized or at least made available to humankind for practical benefit.

Archaea Prokaryotes

Organisms that constitute one of the three main evolutionary domains of organisms. Members of this domain are called archaeans.

Artificial selection

Process for deliberating changing the phenotypic traits of a population by determining which individuals will survive and reproduce.

Attenuation

A regulatory mechanism in certain bacterial biosynthetic operons that controls gene expression by causing RNA polymerase to terminate transcription.

Autonomously replicating sequence (ARS)

A specific sequence in yeast chromosomes that, when included as part of an extrachromosomal, circular DNA molecule, confers on that molecule the ability to replicate autonomously; one type of eukaryotic replicator.

Autopolyploidy

Condition in which a cell or organism has two or more genetically distinct sets of chromosomes of the same species.

Autosome

A chromosome other than a sex chromosome.

Auxotroph

A mutant strain of an organism that cannot synthesize a molecule required for growth and therefore must have the molecule supplied in the growth medium for it to grow. Also called auxotrophic mutant or nutritional mutant.

Auxotrophic mutant

A mutant strain of an organism that cannot synthesize a molecule required for growth and therefore must have the molecule supplied in the growth medium for it to grow. Also called auxotrophic mutant or nutritional mutant.

Auxotrophic mutation

A mutation that affects an organism’s ability to make a particular molecule essential for growth. Also called nutritional mutation.

Back mutation

A point mutation in a mutant allele that changes it back to a wild-type allele. Also called reversion.

Bacteria Prokaryotes

Micro-organisms that constitute one of the three main evolutionary domains of organisms. Members of this domain are called bacteria.

Bacterial artificial chromosome (BAC)

A vector for cloning DNA fragments up to about 200 kb long in E. coli. A BAC contains the origin of replication of the F factor, a multiple cloning site, and a selectable marker.

Bacteriophages

Viruses that attack bacteria. Also called phages.

Barr body

A highly condensed and transcriptionally inactive X chromosome found in the nuclei of somatic cells of female mammals.

Base

Also called nitrogenous base. Purine or pyrimidine component of a nucleotide.

Base analog

A chemical whose molecular structure is very similar to that of one of the bases normally found in DNA. Some chemical mutagens, such as 5-bromouracil (5BU), are base analogs.

Base excision repair

An enzyme-catalysed process for repairing damaged DNA by removal of the altered base, followed by excision of the baseless nucleotide. The correct nucleotide then is inserted in the gap.

Base-modifying agent

A chemical mutagen that modifies the chemical structure of one or more bases normally found in DNA. Nitrous oxide, hydroxylamine, and methylmethane sulfonate are common base-modifying agents.

Base-pair substitution mutation

A change in the genetic material such that one base pair is replaced by another base pair; for instance, an A-T is replaced by a G-C pair.

Basic research

Research done to further knowledge for knowledge’s sake.

Bidirectional replication

Synthesis of DNA in both directions away from an origin of replication.

Bioinformatics

Application of mathematics and computer science to store, retrieve, and analyse biological data, particularly nucleic acid and protein sequence data.

Bivalent

A pair of homologous, synapsed chromosomes, consisting of four chromatids, during the first meiotic division.

Bootstrap procedure

A method for determining confidence levels attached to the branching patterns of a phylogenetic tree chosen by the parsimony approach.

Bottleneck effect

A form of genetic drift that occurs when a population is drastically reduced in size and some genes are lost from the gene pool as a result of chance.

Branch-point sequence

Specific sequence within introns of precursor mRNAs (pre-mRNAs) of eukaryotes containing an adenylate (A) nucleotide to which the free 5¿ end of an intron binds during mRNA splicing.

Broad-sense heritability

The proportion of the phenotypic variance within a population that results from genetic differences among individuals.

cAMP (cyclic AMP)

Adenosine 3’, 5’ monophosphate; an intracellular regulatory molecule involved in controlling gene expression and some other processes in both prokaryotes and eukaryotes.

Cancer

Disease characterized by the uncontrolled and abnormal division of cells and by the spread of malignant tumor cells (metastasis) to disparate sites in the organism.

5’ capping

The addition of a methylated guanine nucleotide (a “cap”) to the 5’ end of a precursor mRNA (pre-mRNA) molecule in eukaryotes; the cap is retained on the mature mRNA molecule.

Carcinogen

Any physical or chemical agent that increases the frequency with which cells become cancerous.

Carrier

An individual who is heterozygous for a recessive mutation. A carrier usually does not exhibit the mutant phenotype.

Catabolite activator protein (CAP)

A regulatory protein that binds with cyclic AMP (cAMP) at low glucose concentrations, forming a complex that stimulates transcription of the lac operon and some other bacterial operons.

Catabolite repression

The inactivation of some inducible operons in the presence of glucose even though the operon’s inducer is present. Also called glucose effect.

cDNA

DNA copies made from RNA templates in a reaction catalyzed by the enzyme reverse transcriptase.

cDNA library

Collection of cloned complementary DNAs (cDNAs) produced from the entire mRNA population of a cell.

Cell cycle

The cyclical process of growth and cellular reproduction in unicellular and multicellular eukaryotes. The cycle includes nuclear division, or mitosis, and cell (cytoplasmic) division, or cytokinesis.

Cell division

A process whereby one cell divides to produce two cells.

CEN sequence

Nucleotide sequence of DNA in the centromere region of yeast chromosomes. Centromere sequences differ among species and between chromosomes in the same species.

Centimorgan (cM)

The unit of distance on a genetic map. Equivalent to map unit.

Centromere

The region of a chromosome containing DNA sequences to which mitotic and meiotic spindle fibres attach. Under the microscope a centromere is seen as a constriction in the chromosome. The centromere region of each chromosome is responsible for the accurate segregation of replicated chromosomes to the daughter cells during mitosis and meiosis.

Chain-terminating codon

One of three codons in mRNA for which no normal tRNA molecule exists and that signals the termination of polypeptide synthesis. .

Character

A characteristic that results from gene action and is transmitted from one generation to another. .

Charged tRNA

An enzyme that catalyzes the addition of a specific amino acid to the tRNA for that amino acid.

Charging

Addition of an amino acid to a tRNA that contains an anticodon for that amino acid. Also called aminoacylation. checkpoints, cell-cycle Stages in the cell cycle at which progression of a cell through the cycle is blocked if there is damage to the genome or the mitotic machinery.

Chiasma

(plural, chiasmata) A cross-shaped structure formed during crossing-over and visible during the diplonema stage of meiosis.

Chiasma interference

Phenomenon in which the presence of one crossover interferes with the formation of another crossover nearby. Mathematically defined as 1 minus the coefficient of coincidence.

Chi-square test

A statistical procedure that determines what constitutes a significant difference between observed results and results expected on the basis of a particular hypothesis; a goodness-of-fit test.

Chloroplasts

Triple-membraned, chlorophyll-containing organelles found in green plants in which photosynthesis occurs.

Chorionic villus sampling

A procedure in which a sample of chorionic villus tissue of a developing fetus is examined for chromosomal abnormalities.

Chromatid

One of the two visibly distinct replicated copies of each chromosome that becomes visible between early prophase and metaphase of mitosis and is joined to its sister chromatid at their centromeres.

Chromatin

The DNA–protein complex that constitutes eukaryotic chromosomes and can exist in various degrees of folding or compaction.

Chromatin remodelling

Alteration of the structure of chromatin in the vicinity of a core promoter in a way that stimulates or represses transcription initiation. Remodelling is carried out by enzymes catalyzing histone acetylation or deacetylation and by nucleosome remodelling complexes.

Chromosomal aberration

The variation from the wild-type condition in chromosome number or structure.

Chromosomal mutation

The variation from the wild-type condition in chromosome number or structure.

Chromosome

In eukaryotic cells, a linear structure composed of a single DNA molecule complexed with protein. Each eukaryotic species has a characteristic number of chromosomes in the nucleus of its cells. Most prokaryotic cells contain a single, usually circular chromosome.

Chromosome library

Collection of cloned DNA fragments produced from a particular chromosome (e.g., the human X chromosome).

Chromosome theory of inheritance

The theory that genes are located on chromosomes and that the transmission of chromosomes from one generation to the next accounts for the inheritance of hereditary traits.

Cis-dominant

Referring to a gene or DNA sequence that can control genes on the same DNA molecule but not on other DNA molecules.

Cis-trans test

See A test used to determine whether two independently isolated mutations that confer the same phenotype are located within the same gene or in two different genes.

Classical model

An early model for genetic variation that was based on the assumption that most natural populations had a wild-type allele with very few mutant alleles present.

Cline

A systematic change in allele frequencies within a continuous population distributed over a geographic region.

Clonal selection

A process whereby cells that express cellsurface antibodies specific for a particular antigen are stimulated to proliferate and secrete that antibody by exposure to that antigen.

Cloning

The production of many identical copies of a DNA molecule by replication in a suitable host; also called DNA cloning, gene cloning, and molecular cloning. (b) The generation of cells (or individuals) genetically identical to themselves and to their parent.

Cloning vector

A double-stranded DNA molecule that is able to replicate autonomously in a host cell and into which a DNA fragment (or fragments) can be inserted to form a recombinant DNA molecule for cloning.

Coactivator

In eukaryotes, a large multiprotein complex that interacts with activators bound at enhancers, general transcription factors bound near the promoter, and RNA polymerase II. These interactions help stimulate transcription of regulated genes.

Coding sequence

The part of an mRNA molecule that specifies the amino acid sequence of a polypeptide during translation.

Codominance

The condition in which an individual heterozygous for a gene exhibits the phenotypes of both homozygotes.

Codon

A group of three adjacent nucleotides in an mRNA molecule that specifies either one amino acid in a polypeptide chain or the termination of polypeptide synthesis.

Codon usage bias

A disproportionate use of one or a few synonymous codons within a codon family for a particular gene or across a genome.

Coefficient of coincidence

A measure of the extent of chiasma interference throughout a genetic map; ratio of the observed to the expected frequency of double crossovers.

Combinatorial gene regulation

In eukaryotes, control of transcription by the combined action of several activators and repressors, which bind to particular gene regulatory sequences.

Comparative genomics

Comparison of the nucleotide sequences of entire genomes of different species, with the Glossary 3 goal of understanding the functions and evolution of genes. Such comparisons can identify which genome regions are evolutionarily conserved and likely to represent functional genes.

Complementary base pairs

The specific A-T and G-C base pairs in double-stranded DNA. The bases are held together by hydrogen bonds between the purine and pyrimidine base in each pair.

Complementary DNA

DNA copies made from RNA templates in a reaction catalyzed by the enzyme reverse transcriptase.

Complementation test

A test used to determine whether two independently isolated mutations that confer the same phenotype are located within the same gene or in two different genes. Also called cis-trans test.

Complete dominance

The condition in which an allele is phenotypically expressed when one or both copies are present, so that the phenotype of the heterozygote is essentially indistinguishable from that of the homozygote.

Complete medium

For a microorganism, a medium that supplies all the ingredients required for growth and reproduction, including those normally produced by the wild-type organism.

Complete recessiveness

The condition in which an allele is phenotypically expressed only when two copies are present.

Conditional mutation

A mutation that results in a wild-type phenotype under one set of conditions but a mutant phenotype under other conditions. Temperature-sensitive mutations are a common type of conditional mutation.

Conjugation

In bacteria, process of unidirectional transfer of genetic material through direct cellular contact between a donor (“male”) cell and a recipient (“female”) cell.

Consensus sequence

The series of nucleotides found most frequently at each position in a particular DNA sequence among different species.

Conservative model

A model for DNA replication in which the two parental strands of DNA remain together and serve as a template for the synthesis of a new daughter double helix. The results of the Meselson–Stahl experiment did not support this model.

Constitutive gene

A gene whose expression is unregulated. The products of constitutive genes are essential to the normal functioning of the cell and are always produced in growing cells regardless of the environmental conditions.

Constitutive heterochromatin

Condensed chromatin that is always transcriptionally inactive and is found at homologous sites on chromosome pairs.

Contributing allele

An allele that affects the phenotype of a quantitative trait.

Coordinate induction

The simultaneous transcription and translation of two or more genes brought about by the action of an inducer.

Core enzyme

The portion of E. coli RNA polymerase that is the active enzyme; it is bound to the sigma factor, which directs the enzyme to the promoter region of genes.

Corepressor

In eukaryotes, a large multiprotein complex that interacts with repressors bound at enhancers, general transcription factors bound near the promoter, and RNA polymerase II. These interactions help inhibit transcription of regulated genes.

Core promoter

In eukaryotic genomes, the gene regulatory elements closest to the transcription start site that are required for RNA synthesis to begin at the correct nucleotide.

Correlation coefficient

A statistical measure of the strength of the association between two variables. See also regression.

Cotransduction

The simultaneous transduction of two or more bacterial genes, a good indication that the bacterial genes are closely linked.

Coupling

In individuals heterozygous at two genetic loci, the arrangement in which the wild-type alleles of both genes are on one homologous chromosome and the recessive mutant alleles are on the other; also called cis configuration.

Covariance

A statistical measure of the tendency for two variables to vary together; used to calculate the correlation coefficient between the two variables.

CpG island

DNA region containing many copies of the dinucleotide CpG. Many genes in eukaryotic DNA have CpG islands in or near the promoter. Methylation of the cytosines (C) in these islands represses transcription.

Crisscross inheritance

Transmission of a gene from a male parent to a female child to a male grandchild.

Cross

The fusion of male gametes from one individual and female gametes from another.

Cross-fertilization

The fusion of male gametes from one individual and female gametes from another.

Crossing-over

The process of reciprocal chromosomal interchange that occurs frequently during meiosis and gives rise to recombinant chromosomes.

C-value

The amount of DNA found in the haploid set of chromosomes.

Cyclin

Any of a group of proteins whose concentrations increase and decrease in a regular pattern through the cell cycle. The cyclins act in conjunction with cyclindependent kinases to regulate cell-cycle progression.

Cyclin-dependent kinase (Cdk)

Any of a group of protein kinases, activated by binding of specific cyclins, that regulate cell-cycle progression.

Cytokinesis

Division of the cytoplasm following mitosis or meiosis I and II during which the two new nuclei compartmentalize into separate daughter cells.

Cytosine (C) A pyrimidine

found in RNA and DNA. In doublestranded DNA, cytosine pairs with guanine, a purine, by hydrogen bonding.

Dark repair

An enzyme-catalyzed process for removal of thymine dimers from DNA and synthesis of a new DNA segment complementary to the undamaged strand.

Darwinian fitness (w)

The relative reproductive ability of individuals with a particular genotype.

Daughter chromosomes

Detached sister chromatids after they separate at the beginning of mitotic anaphase or meiotic anaphase II.

Deamination

Removal of an amino group from a nucleotide in DNA.

Degeneracy

In the genetic code, the existence of more than one codon corresponding to each amino acid.

Degradation control

The regulation of the rate of breakdown (turnover) of RNA molecules in the cell.

Deletion

A chromosomal mutation resulting in the loss of a segment of a chromosome and the gene sequences it contains.

Deoxyribonuclease (DNase)

An enzyme that catalyzes the degradation of DNA to nucleotides.

Deoxyribonucleic acid (DNA)

A polymeric molecule consisting of deoxyribonucleotide building blocks that in a double-stranded, double-helical form is the genetic material of all living organisms.

Deoxyribonucleotide

Any of the nucleotides that make up 4 DNA, consisting of a sugar (deoxyribose), a base, and a phosphate group.

Deoxyribose

The pentose (five-carbon) sugar found in DNA.

Depurination

Loss of a purine base (adenine or guanine) from a nucleotide in DNA.

Determination

Process early in development that establishes the fate of a cell, that is, the differentiated cell type it will become.

Development

Overall process of growth, differentiation, and morphogenesis by which a zygote gives rise to an adult organism. It involves a programmed sequence of phenotypic events that are typically irreversible.

Diakinesis

The final stage in prophase I of meiosis, during which the replicated chromosomes (bivalents) are most condensed, the nuclear envelope breaks down, and the spindle begins to form.

Dicentric bridge

A homologous chromosome pair in meiosis I in which one chromatid has two centromeres as the result of crossing-over within a paracentric inversion. As the two centromeres begin to migrate to opposite poles, a dicentric bridge stretching across the cell forms and eventually breaks.

Dicentric chromosome

A homologous chromosome pair in meiosis I in which one chromatid has two centromeres as the result of crossing-over within a paracentric inversion. As the two centromeres begin to migrate to opposite poles, a dicentric bridge stretching across the cell forms and eventually breaks.

Dideoxynucleotide (ddNTP)

A modified nucleotide that has a 3¿-H on the deoxyribose sugar rather than a 3¿-OH. A ddNTP can be incorporated into a growing DNA chain, but no further DNA synthesis can occur because no phosphodiester bond can be formed with an incoming nucleotide.

Dideoxy sequencing

A method for rapidly sequencing DNA molecules in which the DNA to be sequenced is used as the template for in vitro DNA synthesis in the presence of dideoxynucleotides (ddNTPs). When a dideoxynucleotide is incorporated into a growing DNA chain, no further DNA synthesis occurs, generating a truncated chain in which the terminal dideoxynucleotide corresponds to the normal nucleotide that occurs at that point in the sequence.

Differentiation

Series of cell-specific changes in gene expression by which determined cells give rise to cell types with characteristic structures and functions.

Dihybrid cross

A cross between two individuals of the same genotype that are heterozygous for two pairs of alleles at two different loci (e.g.,,).

Dioecious

Referring to plant species in which individual plants possess either male or female sex organs.

Diploid (2N)

A cell or an individual with two copies of each chromosome.

Diplonema

The stage in prophase I of meiosis during which the synaptonemal complex disassembles and homologous chromosomes begin to move apart.

Discontinuous trait

A heritable characteristic that exhibits a small number of distinct phenotypes, which commonly are determined by variant alleles at a single locus.

Dispersed repeated DNA

Repetitive DNA sequences that are distributed at irregular intervals in the genome.

Dispersive model

A model for DNA replication in which the parental double helix is cleaved into double-stranded DNA segments that act as templates for the synthesis of new double-stranded DNA segments, which are reassembled into complete DNA double helices, with parental and Ss Yy!Ss Yy progeny DNA segments interspersed. The results of the Meselson–Stahl experiment did not support this model.

DNA

A polymeric molecule consisting of deoxyribonucleotide building blocks that in a double-stranded, double-helical form is the genetic material of all living organisms.

DNA chip

An ordered grid of DNA molecules of known sequence—probes—fixed at known positions on a solid substrate, either a silicon chip, glass, or less commonly, a nylon membrane. Labelled free DNA molecules—targets— are added to the unlabelled fixed probes to analyze identities or quantities of target molecules. DNA microarrays allow for the simultaneous analysis of thousands of DNA target molecules.

DNA-dependent RNA polymerase

The more complete name for RNA polymerase, the enzyme responsible for transcription, the process of RNA synthesis using a DNA template.

DNA fingerprinting

Molecular analysis of DNA polymorphisms to identify individuals based on the unique characteristics of their DNA.

DNA helicase

An enzyme that catalyzes unwinding of the DNA double helix at a replication fork during DNA replication.

DNA ladder

Also known as DNA size markers, a set of DNA molecules of known size used in agarose gel electrophoresis experiments.

DNA ligase

An enzyme that catalyzes the formation of a phosphodiester bond between the 5’ end of one DNA chain and 3’ end of another DNA chain during DNA replication and DNA repair.

DNA markers

Sequence variations among individuals in a specific region of DNA that are detected by molecular analysis of the DNA and can be used in genetic analysis.

DNA microarray

An ordered grid of DNA molecules of known sequence—probes—fixed at known positions on a solid substrate, either a silicon chip, glass, or less commonly, a nylon membrane. Labelled free DNA molecules—targets— are added to the unlabelled fixed probes to analyze identities or quantities of target molecules. DNA microarrays allow for the simultaneous analysis of thousands of DNA target molecules.

DNA molecular testing

A type of genetic testing that focuses on the molecular nature of mutations associated with a particular disease.

DNA polymerase

Any enzyme that catalyzes the polymerization of deoxyribonucleotides into a DNA chain. All DNA polymerases synthesize DNA in the 5’ to 3’ direction.

DNA polymerase I (DNA Pol I)

One of several E. coli enzymes that catalyze DNA synthesis; originally called the Kornberg enzyme.

DNA polymorphism

Variation in the nucleotide sequence or number of tandem repeat units at a particular locus in the genome. Most commonly, this term is used for DNA markers, variations that are located outside of genes and that are detected by molecular analysis.

DNA primase

An enzyme that catalyzes formation of a short RNA primer in DNA replication.

DNA profiling

Molecular analysis of DNA polymorphisms to identify individuals based on the unique characteristics of their DNA. Also called DNA fingerprinting.

DNA typing

Molecular analysis of DNA polymorphisms to identify individuals based on the unique characteristics of their DNA. Also called DNA fingerprinting.

Domain shuffling

Proposed mechanism for evolution of genes with new functions by the duplication and rearrangement of exons encoding protein domains in different combinations. Also called exon shuffling.

Dominant

Describing an allele or phenotype that is expressed in either the homozygous or the heterozygous state.

Dominant lethal allele

An allele that results in the death of an organism that is homozygous or heterozygous for the allele.

Dosage compensation

Any mechanism in organisms with genotypic sex determination for equalizing expression of genes on the sex chromosomes in males and females.

Down syndrome

The presence of an extra copy of chromosome 21, which causes Down syndrome in humans. 2N+1.

Duplication

A chromosomal mutation that results in the doubling of a segment of a chromosome and the gene sequences it contains.

Effective population size

The effective number of adults contributing gametes to the next generation.

Effector

A small molecule involved in controlling expression of a regulated gene or the activity of a protein.

Elongation factor (EF)

Accessory proteins required for the elongation phase of translation in prokaryotes and eukaryotes.

Embryonic stem (ES) cell

A cell derived from a very early embryo that retains the ability to differentiate into a cell type characteristic of any part of the organism.

Enhancer

A set of gene regulatory elements in eukaryotic genomes that can act over distances up to thousands of base pairs upstream or downstream from a gene. Most enhancers bind activators and act to stimulate transcription.

Environmental genomics

A branch of comparative genomics involving the analysis of genomes in entire communities of microbes isolated from the environment.

Environmental variance

Component of the phenotypic variance for a trait that is due to any nongenetic source of variation among individuals in a population. includes variation arising from general environmental effects, which permanently influence phenotype; special environmental effects, which temporarily influence phenotype; and family environmental effects, which are shared by family members.

Epigenetic

Referring to a heritable change in gene expression that does not result from a change in the nucleotide sequence of the genome.

Episome

In bacteria, a plasmid that is capable of integrating into the host cell’s chromosome.

Epistasis

Interaction between two or more genes that controls a single phenotype. For instance, the expression of a gene at one locus can mask or suppress the expression of a second gene at another locus.

Epitope

The specific short region of a protein (or other molecule recognized by an antibody) that is bound specifically by the antibody.

Essential gene

A gene that when mutated can result in the death of the organism.

Euchromatin

Chromatin that is condensed during mitosis but becomes uncoiled during interphase, when it can be transcribed.

Eukarya

One of the three major evolutionary domains. Organisms in this domain have genetic material in a membranebound nucleus as well as a number of membrane-bounded organelles such as mitochondria.

Eukaryote

Any organism whose cells have a membrane-bound nucleus in which the genetic material is located and membrane-bound organelles (e.g., mitochondria). Eukaryotes can be unicellular or multicellular and constitute one of the three major evolutionary domains of organisms.

Euploid

Referring to an organism or cell that has one complete set of chromosomes or an exact multiple of complete sets.

Evolution

Genetic change that takes place over time within a group of organisms.

Evolutionary domains

The three major lineages of organisms—Bacteria, Archaea, and Eukarya—thought to have evolved from a common, single-celled ancestor.

Excision repair

An enzyme-catalyzed process for removal of thymine dimers from DNA and synthesis of a new DNA segment complementary to the undamaged strand.

Exon

A segment of a protein-coding gene and its precursor (pre-mRNA) that specifies an amino acid sequence and is retained in the functional mRNA.

Exon shuffling

Proposed mechanism for evolution of genes with new functions by the duplication and rearrangement of exons encoding protein domains in different combinations. Also called exon shuffling.

Expected heterozygosity (He)

The number of heterozygotes expected if the population is in Hardy–Weinberg equilibrium.

Expression

Vector A cloning vector carrying a promoter and other sequences required for expression of a cloned gene in a host cell.

Expressivity

The degree to which a particular gene is expressed in the phenotype. A gene with variable expressivity can cause a range of phenotypes.

Extranuclear inheritance

The inheritance of characters determined by genes located on mitochondrial or chloroplast chromosomes. Such extra nuclear genes show inheritance patterns distinctly different from those of genes on chromosomes in the nucleus. Also called non-Mendelian inheritance.

Facultative heterochromatin

Chromatin that may become condensed and therefore transcriptionally inactive in certain cell types, at different developmental stages, or in one member of a homologous chromosome pair.

Familial trait

A characteristic shared by members of a family as the result of shared genes and/or environmental factors.

Fate map

A diagram of an early embryo showing the cell types and tissues that different embryonic cells subsequently develop into.

F-duction

Transfer of host genes carried on an f factor in conjugation between an f and an f^–cell. If the genes are different between the two cell types, the recipient becomes partially diploid for the genes on the f.

F factor

In e. Coli, a plasmid—a self-replicating circular DNA molecule—that confers the ability to act as a donor cell in conjugation. Excision of an f factor from the bacterial chromosome may generate an f¿ factor, which may carry host cell genes.

F₁ generation

The offspring that result from the first experimental crossing of two parental strains of animals or plants; the first filial generation.

F₂ generation

The offspring that result from crossing f₁ individuals; the second filial generation. Fine-structure mapping procedures for generating a high resolution map of allele sites within a gene.

Formylmethionine (fmet)

A modified form of the amino acid methionine that has a formyl group attached to the amino group. It is the first amino acid incorporated into a polypeptide chain in prokaryotes and in eukaryotic organelles.

Forward mutation

A point mutation in a wild-type allele that changes it to a mutant allele.

Founder effect

A form of genetic drift that occurs when a population is formed by migration of a small number of individuals from a large population.

F-pili (singular, f-pilus)

Hairlike cell surface components produced by cells containing the f factor, which allow the physical union of f⁺ and f^–cells or hfr and f^–cells to take place. Also called sex pili.

Frameshift mutation

A mutational addition or deletion of a base pair in a gene that disrupts the normal reading frame of the corresponding mRNA.

Frequency distribution

In genetics, a graphical representation of the numbers of individuals within a population who fall within the same range of phenotypic values for a continuous quantitative trait. Typically, the phenotypic classes are plotted on the horizontal axis and the number of individuals in each class are plotted on the vertical axis.

Functional genomics

The comprehensive analysis of the functions of genes and of non gene sequences in entire genomes, including patterns of gene expression and its control.