NCBI Bookshelf. A service of the National Library of Medicine, National Institutes of Health.

Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000.

  • By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.
Cover of An Introduction to Genetic Analysis

An Introduction to Genetic Analysis. 7th edition.

Show details




Adenine or adenosine.

abortive transduction

The failure of a transducing DNA segment to be incorporated into the recipient chromosome.

acentric chromosome

A chromosome having no centromere.


A type of dwarfism in humans, inherited as an autosomal dominant phenotype.

acrocentric chromosome

A chromosome having the centromere located slightly nearer one end than the other.

active site

The part of a protein that must be maintained in a specific shape if the protein is to be functional—for example, in an enzyme, the part to which the substrate binds.


In the evolutionary sense, some heritable feature of an individual’s phenotype that improves its chances of survival and reproduction in the existing environment.

adaptive landscape

The surface plotted in a three-dimensional graph, with all possible combinations of allele frequencies for different loci plotted in the plane and with mean fitness for each combination plotted in the third dimension.

adaptive peak

A high point (perhaps one of several) on an adaptive landscape; selection tends to drive the genotype composition of the population toward a combination corresponding to an adaptive peak.

adaptive surface

See adaptive landscape.

adaptor protein

A protein that binds to certain specific phosphorylated amino acid sequences on a second protein, often a transmembrane receptor, and associates with still other proteins, thereby allowing a complex of proteins to “dock” to the receptor. This docking brings proteins of this complex in proximity to one another and, by doing so, permits propagation of an intracellular signal in a signal transduction pathway.

additive genetic variance

Genetic variance associated with the average effects of substituting one allele for another.


A purine base that pairs with thymine in the DNA double helix.


The nucleoside containing adenine as its base.

adenosine triphosphate

See ATP.

adjacent segregation

In a reciprocal translocation, the passage of a translocated and a normal chromosome to each of the poles.


Adenosine diphosphate.


Alanine (an amino acid).


A pigmentless “white” phenotype, determined by a mutation in a gene coding for a pigment-synthesizing enzyme.

alkylating agent

A chemical agent that can add alkyl groups (for example, ethyl or methyl groups) to another molecule; many mutagens act through alkylation.


One of the different forms of a gene that can exist at a single locus.

allele frequency

A measure of the commonness of an allele in a population; the proportion of all alleles of that gene in the population that are of this specific type.

allopatric speciation

The process of the splitting of a species into two or more new species that occurs between populations that are geographically isolated from each other.


See amphidiploid.

allosteric transition

A change from one conformation of a protein into another.

alternate segregation

In a reciprocal translocation, the passage of both normal chromosomes to one pole and both translocated chromosomes to the other pole.

alternation of generations

The alternation of gametophyte and sporophyte stages in the life cycle of a plant.

alternative splicing

The process by which different mRNAs are produced from the same primary transcript, through variations in the splicing pattern of the transcript. Multiple mRNA “isoforms” can be produced in a single cell or the different isoforms can display different tissue-specific patterns of expression. If the alternative exons fall within the open reading frames of the mRNA isoforms, different proteins will be produced by the alternative mRNAs.

amber codon

The codon UAG, a nonsense codon.

amber suppressor

A mutant allele coding for a tRNA whose anticodon is altered in such a way that the tRNA inserts an amino acid at an amber codon in translation.

Ames test

A widely used test to detect possible chemical carcinogens; based on mutagenicity in the bacterium Salmonella.

amino acid

A peptide; the basic building block of proteins (or polypeptides).


A technique for testing the genotype of an embryo or fetus in utero with minimal risk to the mother or the child.


Adenosine monophosphate.


An allopolyploid; a polyploid formed from the union of two separate chromosome sets and their subsequent doubling.


The production of many DNA copies from one master region of DNA.


An intermediate stage of nuclear division during which chromosomes are pulled to the poles of the cell.

aneuploid cell

A cell having a chromosome number that differs from the normal chromosome number for the species by a small number of chromosomes.

animal breeding

The practical application of genetic analysis for development of lines of domestic animals suited to human purposes.


Spontaneous alignment of two single DNA strands to form a double helix.


A protein (immunoglobulin) molecule, produced by the immune system, that recognizes a particular substance (antigen) and binds to it.


A nucleotide triplet in a tRNA molecule that aligns with a particular codon in mRNA under the influence of the ribosome so that the amino acid carried by the tRNA is inserted in a growing protein chain.


A molecule that is recognized by antibody (immunoglobulin) molecules. Generally, multiple antibody molecules can recognize a given antigen.


A term used to describe the opposite orientations of the two strands of a DNA double helix; the 5′ end of one strand aligns with the 3′ end of the other strand.

AP site

Apurinic or apyrimidinic site resulting from the loss of a purine or pyrimidine residue from the DNA.


Arginine (an amino acid).


A sexual spore from certain fungus species in which spores are found in a sac called an ascus.


In fungi, a sac that encloses a tetrad or an octad of ascospores.

asexual spore

See spore.


Asparagine (an amino acid).


Aspartate (an amino acid).

ATP (adenosine triphosphate)

The “energy molecule” of cells, synthesized mainly in mitochondria and chloroplasts; energy from the breakdown of ATP drives many important cell reactions.

attached X

A pair of Drosophila X chromosomes joined at one end and inherited as a single unit.


A region adjacent to the structural genes of the trp operon; this region acts in the presence of tryptophan to reduce the rate of transcription from the structural genes.

autonomous controlling element

A controlling element that seems to have both regulator and receptor functions combined in a single unit that enters a gene and causes an unstable mutation.

autonomous phenotype

A genetic trait in multicellular organisms in which only genotypically mutant cells exhibit the mutant phenotype. Conversely, a nonautonomous trait is one in which genotypically mutant cells cause other cells (regardless of their genotype) to exhibit a mutant phenotype.

autonomous replication sequence (ARS)

A segment of a DNA molecule needed for the initiation of its replication; generally a site recognized and bound by the proteins of the replication system.


The process by which a protein kinase phosphorylates specific amino acid residues on itself.


A polyploid formed from the doubling of a single genome.


A pattern of dark spots in a developed photographic film or emulsion, in the technique of autoradiography.


A process in which radioactive materials are incorporated into cell structures, which are then placed next to a film or photographic emulsion, thus forming a pattern on the film corresponding to the location of the radioactive compounds within the cell.

autoregulatory loop

The process by which the expression of a gene is controlled by its own gene product.


Any chromosome that is not a sex chromosome.


A strain of microorganisms that will proliferate only when the medium is supplemented with some specific substance not required by wild-type organisms.



Bacterial artificial chromosome; an F plasmid engineered to act as a cloning vector that can carry large inserts.

back mutation

See reversion.

bacteriophage (phage)

A virus that infects bacteria.

balanced polymorphism

Stable genetic polymorphism maintained by natural selection.


A chromosome with multiple inversions, used to retain favorable allele combinations in the uninverted homolog.

Balbiani ring

A large chromosome puff.

Barr body

A densely staining mass that represents an inactivated X chromosome.

base analog

A chemical whose molecular structure mimics that of a DNA base; because of the mimicry, the analog may act as a mutagen.

bead theory

The disproved hypothesis that genes are arranged on the chromosome like beads on a necklace, indivisible into smaller units of mutation and recombination.

bimodal distribution

A statistical distribution having two modes.

binary fission

The process in which a parent cell splits into two daughter cells of approximately equal size.

biparental zygote

A Chlamydomonas zygote that contains cpDNA from both parents; such cells are generally rare.

blast cell

A cell that divides, generally asymmetrically, to give rise to two different progeny cells. One is a blast cell just like the parental cell and the other is a cell that enters a differentiation pathway. In this manner, a continuously propagating cell population can maintain itself and spin off differentiating cells.


In an insect embryo, the layer of cells that completely surrounds an internal mass of yolk.


An early developmental stage of lower vertebrate embryos, in which the embryo consists of a single layer of cells surrounding the central yolk.

blending inheritance

A discredited model of inheritance suggesting that the characteristics of an individual result from the smooth blending of fluidlike influences from its parents.


A human phenotype of unusually short digits, generally inherited as an autosomal dominant.

branch migration

The process by which a single “invading” DNA strand extends its partial pairing with its complementary strand as it displaces the resident strand.

bridging cross

A cross made to transfer alleles between two sexually isolated species by first transferring the alleles to an intermediate species that is sexually compatible with both.

broad heritability (H2)

The proportion of total phenotypic variance at the population level that is contributed by genetic variance.


A daughter cell formed by mitosis in yeast; one daughter cell retains the cell wall of the parent, and the other (the bud) forms a new cell wall.

buoyant density

A measure of the tendency of a substance to float in some other substance; large molecules are distinguished by their differing buoyant densities in some standard fluid. Measured by density-gradient ultracentrifugation.

Burkitt lymphoma

A cancer of the lymphatic system manifested by tumors in the jaw, associated with a chromosomal translocation bringing a specific oncogene next to regulatory elements of one of the immunoglobulin genes.



Cytosine or cytidine.


An undifferentiated clone of plant cells.

cAMP (cyclic adenosine monophosphate)

A molecule that plays a key role in the regulation of various processes within the cell.


The class of disease characterized by rapid and uncontrolled proliferation of cells within a tissue of a multitissued eukaryote. Cancers are generally thought to be genetic diseases of somatic cells, arising through sequential mutations that create oncogenes and inactivate tumor-suppressor genes.

candidate gene

A sequenced gene of previously unknown function that, because of its chromosomal position or some other property, becomes a candidate for a particular function such as disease determination.

CAP (catabolite activator protein)

A protein whose presence is necessary for the activation of the lac operon.


A 7-methylguanosine molecule added to the 5′ end of the pre-mRNA.

carbon source

A nutrient (such as sugar) that provides carbon “skeletons” needed in an organism’s synthesis of organic molecules.


A substance that causes cancer.

cardinal gene

Those pattern-formation genes in Drosophila that are the zygotically acting genes directly responding to the gradients of anterior–posterior and dorsal–ventral positional information created by the maternally expressed pattern-formation genes.


An individual who possesses a mutant allele but does not express it in the phenotype, because of a dominant allelic partner; thus, an individual of genotype A a is a carrier of a if there is complete dominance of A over a.

cassette model

A model to explain mating-type interconversion in yeast. Information for both a and α mating types is assumed to be present as silent “cassettes”; a copy of either type of cassette may be transposed to the mating-type locus, where it is “played” (transcribed).

catabolite activator protein

See CAP.

catabolite repression

The inactivation of an operon caused by the presence of large amounts of the metabolic end product of the operon.


A positively charged ion (such as K+).


See cyclin-dependent protein kinase.


See complementary DNA.

cDNA library

A library composed of cDNAs, not necessarily representing all mRNAs.

cell autonomous

Refers to a genetic trait in multicellular organisms in which only genotypically mutant cells exhibit the mutant phenotype. Conversely, a nonautonomous trait is one in which genotypically mutant cells cause other cells (regardless of their genotype) to exhibit a mutant phenotype.

cell cycle

The set of events that take place in the divisions of mitotic cells. The cell cycle oscillates between mitosis (M phase) and interphase. Interphase can be subdivided in order into G1, S phase, and G2. DNA synthesis takes place during S phase. The length of the cell cycle is regulated through a special option in G1, in which G1 cells can enter a resting phase called G0.

cell division

The process by which two cells are formed from one.

cell fate

The ultimate differentiated state to which a cell has become committed.

cell lineage

A pedigree of cells related through asexual division.

cellular blastoderm

The stage of blastoderm in insects after the nuclei have each been packaged in an individual cellular membrane.

centimorgan (cM)

See map unit.

central dogma

The hypothesis that information flows only from DNA to RNA to protein; although some exceptions are now known, the rule is generally valid.


A specialized region of DNA on each eukaryotic chromosome; it acts as a site for the binding of the kinetochore proteins.


Some attribute of individuals within a species for which various heritable differences can be defined.

character difference

Alternative forms of the same attribute within a species.


See pulse-chase experiment.


Stage of the cell cycle at which the completion of a certain event of the cell cycle, such as chromosome replication, must have been successfully completed in order for the cell cycle to progress to the next stage.

chiasma (chiasmata)

A cross-shaped structure commonly observed between nonsister chromatids in meiosis; the site of crossing-over.


See mosaic.

chimeric DNA

See recombinant DNA.

chi-square(χ2) test

A statistical test used to determine the probability of obtaining the observed results by chance under a specific hypothesis.


A chlorophyll-containing organelle in plants that is the site of photosynthesis.


One of the two side-by-side replicas produced by chromosome division.

chromatid conversion

A type of gene conversion that is inferred from the existence of identical sister-spore pairs in a fungal octad that shows a non-Mendelian allele ratio.

chromatid inference

A situation in which the occurrence of a crossover between any two nonsister chromatids can be shown to affect the probability of those chromatids taking part in other crossovers in the same meiosis.


The substance of chromosomes; now known to include DNA, chromosomal proteins, and chromosomal RNA.


The point at which the polytene chromosomes appear to be attached together.


A small beadlike structure visible on a chromosome during prophase of meiosis and mitosis.


A linear end-to-end arrangement of genes and other DNA, sometimes with associated protein and RNA.

chromosome aberration

Any type of change in the chromosome structure or number.

chromosome loss

Failure of a chromosome to become incorporated into a daughter nucleus at cell division.

chromosome map

See linkage map.

chromosome mutation

Any type of change in the chromosome structure or number.

chromosome puff

A swelling at a site along the length of a polytene chromosome; the site of active transcription.

chromosome rearrangement

A chromosome mutation in which chromosome parts are newly juxtaposed.

chromosome set

The group of different chromosomes that carries the basic set of genetic information of a particular species.

chromosome theory of inheritance

The unifying theory stating that inheritance patterns may be generally explained by assuming that genes are located in specific sites on chromosomes.

chromosome walking

A method for the dissection of large segments of DNA, in which a cloned segment of DNA, usually eukaryotic, is used to screen recombinant DNA clones from the same genome bank for other clones containing neighboring sequences.

cis conformation

In a heterozygote involving two mutant sites within a gene or within a gene cluster, the arrangement a1a2/+ +.

cis dominance

The ability of a gene to affect genes next to it on the same chromosome.

cis-trans test

A test to determine whether two mutant sites of a gene are in the same functional unit or gene.


Originally defined as a functional genetic unit within which two mutations cannot complement. Now equated with the term gene as the region of DNA that encodes a single polypeptide (or functional RNA molecule such as tRNA or rRNA).


A group of genetically identical cells or individuals derived by asexual division from a common ancestor.

(colloquial) An individual formed by some asexual process so that it is genetically identical with its “parent.”

See DNA clone.

cM (centimorgan)

See map unit.

code dictionary

A listing of the 64 possible codons and their translational meanings (the corresponding amino acids).


The situation in which a heterozygote shows the phenotypic effects of both alleles equally.


A section of DNA (three nucleotide pairs in length) that encodes a single amino acid.

coefficient of coincidence

The ratio of the observed number of double recombinants to the expected number.

cohesive end

End of DNA that is cut in a staggered pattern and then can hydrogen bond with a complementary base sequence from other similarly formed end.


The product of the fusion of two circular elements to form a single, larger circle.


The correspondence between the location of a mutant site within a gene and the location of an amino acid substitution within the polypeptide translated from that gene.


A visible clone of cells.


The existence of boundaries within an organism beyond which a specific clone of cells will never extend during development.


Able to take up exogenous DNA and thereby be transformed.

complementary DNA (cDNA)

Synthetic DNA transcribed from a specific RNA through the action of the enzyme reverse transcriptase.

complementary RNA (cRNA)

Synthetic RNA produced by transcription from a specific DNA single-stranded template.


The production of a wild-type phenotype when two different mutations are combined in a diploid or a heterokaryon.

complementation test

See cis-trans test.

complex inheritance

The transmission pattern of discontinuous variants that can be explained only by the interaction of several genes plus the environment.

conditional mutation

A mutation that has the wild-type phenotype under certain (permissive) environmental conditions and a mutant phenotype under other (restrictive) conditions.


The union of two bacterial cells, during which chromosomal material is transferred from the donor to the recipient cell.

conjugation tube

See pilus.

conservative replication

A disproved model of DNA synthesis suggesting that one-half of the daughter DNA molecules should have both strands composed of newly polymerized nucleotides.

constant region

A region of an antibody molecule that is nearly identical with the corresponding regions of antibodies of different specificities.


Always expressed in an unregulated fashion (in reference to gene control).

constitutive heterochromatin

Specific regions of heterochromatin always present and in both homologs of a chromosome.


A set of ordered overlapping clones that constitute a chromosomal region or a genome.

continuous variation

Variation showing an unbroken range of phenotypic values.

controlling element

A mobile genetic element capable of producing an unstable mutant target gene; two types exist, the regulator and the receptor elements.

copy-choice model

A model of the mechanism for crossing-over, suggesting that crossing-over occurs in the course of chromosome division and can occur only between two supposedly “new” nonsister chromatids; the experimental evidence does not support this model.


The production (possibly by excision and repair) of a properly paired nucleotide pair from a sequence of hybrid DNA that contains an illegitimate pair.

correlation coefficient

A statistical measure of the extent to which variations in one variable are related to variations in another.


In Chlamydomonas, parallel behavior of different chloroplast markers in a cross, due to their close linkage on cpDNA.


A cloning vector that, like a plasmid, can replicate autonomously and be packaged into phage.


The simultaneous transduction of two bacterial marker genes.


The simultaneous transformation of two bacterial marker genes.

coupling conformation

Linked heterozygous gene pairs in the arrangement A B/a b.


A statistical measure used in computing the correlation coefficient between two variables; the covariance is the mean of (xImage xbar.jpg)(yImage ybar.jpg) overall pairs of values for the variables x and y, where Image xbar.jpg is the mean of the x values and Image ybar.jpg is the mean of the y values.


Chloroplast DNA.

cri du chat syndrome

A lethal human condition in infants caused by deletion of part of one homolog of chromosome 5.

crisscross inheritance

Transmission of a gene from male parent to female child to male grandchild—for example, X-linked inheritance.


See complementary RNA.


The deliberate mating of two parental types of organisms in genetic analysis.


The exchange of corresponding chromosome parts between homologs by breakage and reunion.

crossover suppressor

An inversion (usually complex) that makes pairing and crossing-over impossible.

cruciform configuration

A region of DNA with palindromic sequences in both strands; each strand pairs with itself to form a helix extending sideways from the main helix.


Tissue or cells multiplying by asexual division, grown for experimentation.


Chorionic villus sampling, a placental sampling procedure for obtaining fetal tissue for chromosome and DNA analysis to assist in prenatal diagnosis of genetic disorders.

cyclic adenosine monophosphate

See cAMP.


A family of labile proteins that are synthesized and degraded at specific times within each cell cycle and that regulate cell cycle progression through their interactions with specific cyclin-dependent protein kinases.

cyclin-dependent protein kinase

A family of protein kinases that, on activation by cyclins and an elaborate set of positive and negative regulatory proteins, phosphorylate certain transcription factors whose activity is necessary for a particular stage of the cell cycle.


Cysteine (an amino acid).

cystic fibrosis

A potentially lethal human disease of secretory glands; the most prominent symptom is excess secretion of lung mucus; inherited as an autosomal recessive.


The nucleoside containing cytosine as its base.


A class of proteins, found in mitochondrial membranes, whose main function is oxidative phosphorylation of ADP to form ATP.


The cytological approach to genetics, mainly entailing microscopic studies of chromosomes.


A cell containing two genetically distinct types of a specific organelle.


The material between the nuclear and cell membranes; includes fluid (cytosol), organelles, and various membranes.

cytoplasmic inheritance

Inheritance through genes found in cytoplasmic organelles.


A pyrimidine base that pairs with guanine.


The protein cable systems and associated proteins that together form the architecture of a eukaryotic cell.


The fluid part of the cytoplasm, outside the organelles.


Darwinian fitness

The relative probability of survival and reproduction for a genotype.

daughter cells

Two identical cells formed by the asexual division of a cell.

daughter chromatids

Two identical chromatids formed by the replication of one chromosome.

default state

The developmental state of a cell (or group of cells) in the absence of the activation of a developmental regulatory switch.

degenerate code

A genetic code in which some amino acids may be encoded by more than one codon each.


Removal of a chromosomal segment from a chromosome set.


The separation of the two strands of a DNA double helix or the severe disruption of the structure of any complex molecule without breaking the major bonds of its chains.

denaturation map

A map of a stretch of DNA showing the locations of local denaturation loops, which correspond to regions of high AT content.


See DNase.

deoxyribonucleic acid

See DNA.


A spatially localized molecule that causes cells to adopt a particular fate or set of related fates.


The process of commitment of cells to particular fates.


The process whereby a single cell becomes a differentiated organism.

developmental field

A set of cells that together interact to form a developing structure (for example, an embryo, a tissue, an organ, a limb, and so forth).

developmental pathway

The chain of molecular events that take a set of equivalent cells and produce the assignment of different fates among those cells.

dicentric chromosome

A chromosome with two centromeres.


The changes in cell shape and physiology associated with the production of the final cell types of a particular organ or tissue.

dihybrid cross

A cross between two individuals identically heterozygous at two loci—for example, A/a · B/b × A/a · B/b.


Apolymorphism” including only two forms.

dioecious plant

A plant species in which male and female organs appear on separate individuals.


A cell having two chromosome sets or an individual having two chromosome sets in each of its cells.

directed mutagenesis

Altering some specific part of a cloned gene and reintroducing the modified gene back into the organism.

directional selection

Selection that changes the frequency of an allele in a constant direction, either toward or away from fixation for that allele.

discontinuous variation

Variation in which there are distinct classes of phenotypes for a particular character.

dispersive replication

Disproved model of DNA synthesis suggesting more or less random interspersion of parental and new segments in daughter DNA molecules.


See statistical distribution.

distribution function

A graph of some precise quantitative measure of a character against its frequency of occurrence.


Differentiation between different lines of descent in the course of evolution.

DNA (deoxyribonucleic acid)

A double chain of linked nucleotides (having deoxyribose as their sugars); the fundamental substance of which genes are composed.

DNA clone

A section of DNA that has been inserted into a vector molecule, such as a plasmid or a phage chromosome, and then replicated to form many copies.

DNA fingerprint

The largely individual-specific autoradiographic banding pattern produced when DNA is digested with a restriction enzyme that cuts outside a family of VNTRs and a Southern blot of the electrophoretic gel is probed with a VNTR-specific probe.

DNA polymerase

An enzyme that can synthesize new DNA strands from a DNA template; several such enzymes exist.

DNase (deoxyribonuclease)

An enzyme that degrades DNA to nucleotides.

dominance variance

Genetic variance at a single locus attributable to dominance of one allele over another.

dominant allele

An allele that expresses its phenotypic effect even when heterozygous with a recessive allele; thus if A is a dominant over a, then A/A and A/a have the same phenotype.

dominant phenotype

The phenotype of a genotype containing the dominant allele; the parental phenotype that is expressed in a heterozygote.

donor DNA

Any DNA to be used in cloning.

dosage compensation

The process in organisms using a chromosomal sex determination mechanism (such as XX versus XY) that allows standard structural genes on the sex chromosome to be expressed at the same levels in females and males, regardless of the number of sex chromosomes. In mammals, dosage compensation operates by maintaining only a single active X chromosome in each cell; in Drosophila, it operates by hyperactivating the male X chromosome.


See gene dose.

double crossover

Two crossovers occurring in a chromosomal region under study.

double helix

The structure of DNA first proposed by Watson and Crick, with two interlocking helices joined by hydrogen bonds between paired bases.

double infection

Infection of a bacterium with two genetically different phages.

Down syndrome

An abnormal human phenotype, including mental retardation, due to a trisomy of chromosome 21; more common in babies born to older mothers.


See random genetic drift.

Duchenne muscular dystrophy

A lethal muscle disease in humans caused by mutation in a huge gene that encodes the muscle protein dystrophin; inherited as an X-linked recessive phenotype.

duplicate genes

Two identical allele pairs in one diploid individual.


More than one copy of a particular chromosomal segment in a chromosome set.


A pair of sister chromatids joined at the centromere, as in the first division of meiosis.



A molting hormone in insects.

ectopic expression

The occurrence of gene expression in a tissue in which it is normally not expressed. Such ectopic expression can be caused by the juxtaposition of novel enhancer elements to a gene.

ectopic integration

In a transgenic organism, the insertion of an introduced gene at a site other than its usual locus.


A technique for separating the components of a mixture of molecules (proteins, DNAs, or RNAs) in an electric field within a gel.

embryonic (or tissue) polarity

The production of axes of asymmetry in a developing embryo or tissue primordium.

embryonic stem cells

Cultured cell lines that are established from very early embryos and that are essentially totipotent; that is, these cells can be implanted into a host embryo and populate many or all tissues of the developing animal, most importantly including the germ line. Manipulations of these embryonic stem cells (ES cells) are used extensively in mouse genetics to produce targeted gene knockouts.

endocrine system

The organs in the body that secrete hormones into the circulatory system.


See merozygote.


An enzyme that cleaves the phosphodiester bond within a nucleotide chain.


An increase in the number of chromosome sets caused by replication without cell division.


Triploid tissue in a seed, formed from the fusion of two haploid female nuclei and one haploid male nucleus.

enforced outbreeding

Deliberate avoidance of mating between relatives.


A cis-regulatory sequence that can elevate levels of transcription from an adjacent promoter. Many tissue-specific enhancers can determine spatial patterns of gene expression in higher eukaryotes. Enhancers can act on promoters over many tens of kilobases of DNA and can be 5′ or 3′ to the promoter that they regulate.

enhancer trap

A transgenic construction inserted in a chromosome and used to identify tissue-specific enhancers in the genome. In such a construct, a promoter sensitive to enhancer regulation is fused to a reporter gene, such that expression patterns of the reporter gene identify the spatial regulation conferred by nearby enhancers.

enucleate cell

A cell having no nucleus.


The combination of all the conditions external to the genome that potentially affect its expression and its structure.

environmental variance

The variance due to environmental variation.


A protein that functions as a catalyst.

epigenetic inheritance

Processes by which heritable modifications in gene function occur but are not due to changes in the base sequence of the DNA of the organism. Examples of epigenetic inheritance are paramutation, X-chromosome inactivation, and parental imprinting.


A genetic element in bacteria that can replicate in the cytoplasm or can be inserted into the main bacterial chromosome and replicate with the chromosome.


A situation in which the differential phenotypic expression of genotypes at one locus depends on the genotype at another locus.


The part of an antigen molecule that is recognized by a specific immunoglobulin.

equational division

A nuclear division that maintains the same ploidy level of the cell.

equivalence group

A set of immature cells that all have the same developmental potential. In many cases, cells of an equivalence group end up adopting different fates from one another.

ES cells

See embryonic stem cells.

EST (expressed sequence tag)

A sequence-tagged site derived from a cDNA clone; used to position and identify genes in genomic analysis. See also sequence-tagged site.


A molecule that can intercalate into a DNA double helix when the helix is under torsional stress.


A chromosomal region that stains normally; thought to contain the normally functioning genes.


Controlled human breeding based on notions of desirable and undesirable genotypes.


An organism having eukaryotic cells.

eukaryotic cell

A cell containing a nucleus.


A cell having any number of complete chromosome sets or an individual composed of such cells.

excision repair

The repair of a DNA lesion by removal of the faulty DNA segment and its replacement with a wild-type segment.


A female bacterial cell that has just been in conjugation with a male and that contains a fragment of male DNA.


See merozygote.


Any nonintron section of the coding sequence of a gene; together, the exons constitute the mRNA and are translated into protein.


An enzyme that cleaves nucleotides one at a time from an end of a polynucleotide chain.

expression library

A library in which the vector carries transcriptional signals to allow any cloned insert to produce mRNA and ultimately a protein product.

expression vector

A vector with the appropriate bacterial regulatory regions located 5′ to the insertion site, allowing transcription and translation of a foreign protein in bacteria.


The degree to which a particular genotype is expressed in the phenotype.


F cell

In E. coli, a cell having no fertility factor; a female cell.

F+ cell

In E. coli, a cell having a free fertility factor; a male cell.

F factor

See fertility factor.


factor  A fertility factor into which a part of the bacterial chromosome has been incorporated.

F1 generation

The first filial generation, produced by crossing two parental lines.

F2 generation

The second filial generation, produced by selfing or intercrossing the F1.

facultative heterochromatin

Heterochromatin located in positions that are composed of euchromatin in other individuals of the same species or even in the other homolog of a chromosome pair.


Fluorescence-activated chromosome sorting. The use of specific fluorescence signals of stained chromosomes in droplets to activate deflector plates that sort them into individual tubes of uniform types.

familial trait

A trait common to members of a family.

family selection

A breeding technique of selecting a pair on the basis of the average performance of their progeny.

fate map

A map of an embryo showing areas that are destined to develop into specific adult tissues and organs.

feedback loop

See autoregulatory loop.

fertility factor (F factor)

A bacterial episome whose presence confers donor ability (maleness).

filial generations

Successive generations of progeny in a controlled series of crosses, starting with two specific parents (the P generation) and selfing or intercrossing the progeny of each new (F1, F2, . . . ) generation.

filter enrichment

A technique for recovering auxotrophic mutants in filamentous fungi.


The characteristic spot pattern produced by electrophoresis of the polypeptide fragments obtained through denaturation of a particular protein by a proteolytic enzyme.

first-division segregation pattern

A linear pattern of spore phenotypes within the ascus for a particular allele pair, produced when the alleles go into separate nuclei at the first meiotic division, showing that no crossover has occurred between that allele pair and the centromere.


Fluorescent in situ hybridization. In situ hybridization using a probe coupled to a fluorescent molecule.


See Darwinian fitness.

fixed allele

An allele for which all members of the population under study are homozygous, so no other alleles for this locus exist in the population.

fixed breakage point

According to the heteroduplex DNA recombination model, the point from which unwinding of the DNA double helix begins, as a prelude to the formation of heteroduplex DNA.

flow sorting


fluctuation test

A test used in microbes to establish the random nature of mutation or to measure mutation rates.


See formylmethionine.

focus map

A fate map of areas of the Drosophila blastoderm destined to become specific adult structures that is based on the frequencies of specific kinds of mosaics.

foreign DNA

DNA from another organism.

formylmethionine (fMet)

A specialized amino acid that is the very first one incorporated into the polypeptide chain in the synthesis of proteins.

forward mutation

A mutation that converts a wild-type allele into a mutant allele.

founder effect

A difference in allele frequencies between a donor population and a small colony derived from it that occurs as a result of sampling in the first generation of colonization.

frame-shift mutation

The insertion or deletion of a nucleotide pair or pairs, causing a disruption of the translational reading frame.

frequency-dependent fitness

Fitness differences whose intensity changes with changes in the relative frequency of genotypes in the population.

frequency-dependent selection

Selection in which the fitnesses of genotypes depend on their relative frequency in the population.

frequency histogram

A “step curve” in which the frequencies of various arbitrarily bounded classes are graphed.

frequency-independent fitness

Fitness that is not dependent on interactions with other individuals of the same species.

frequency-independent selection

Selection in which the fitnesses of genotypes are independent of their relative frequency in the population.

fruiting body

In fungi, the organ in which meiosis takes place and sexual spores are produced.

functional complementation

The use of a cloned fragment of wild-type DNA to transform a mutant into wild type; used in identifying a clone containing one specific gene.

functional genomics

The study of patterns of gene expression and interaction in the genome as a whole.



Guanine or guanosine.

G0 phase

The resting phase that can occur during G1 of interphase of the cell cycle.

G1 phase

The part of interphase of the cell cycle that precedes S phase.

G2 phase

The part of interphase of the cell cycle that follows S phase.

gain-of-function mutation

A mutation that results in a new functional ability for a protein, detectable at the phenotypic level.


A specialized haploid cell that fuses with a gamete from the opposite sex or mating type to form a diploid zygote; in mammals, an egg or a sperm.


The haploid gamete-producing stage in the life cycle of plants; prominent and independent in some species but reduced or parasitic in others.

gap gene

In Drosophila, the class of cardinal genes that are activated in the zygote in response to the anterior–posterior gradients of positional information. Through regulation of the pair-rule and homeotic genes, the patterns of expression of the various gap gene products lead to the specification of the correct number and types of body segments. Gap mutations cause the loss of several adjacent body segments.


The first process of movements and infoldings of the cell sheet in early animal embryos, usually immediately following blastula (or blastoderm).


The fundamental physical and functional unit of heredity, which carries information from one generation to the next; a segment of DNA, composed of a transcribed region and a regulatory sequence that makes possible transcription.

gene amplification

The process by which the number of copies of a chromosomal segment is increased in a somatic cell.

gene conversion

A meiotic process of directed change in which one allele directs the conversion of a partner allele into its own form.

gene disruption

Inactivation of a gene by the integration of a specially engineered introduced DNA fragment.

gene dose

The number of copies of a particular gene present in the genome.

gene family

A set of genes in one genome all descended from the same ancestral gene.

gene frequency

See allele frequency.

gene fusion

The accidental joining of the DNA of two genes, such as can occur in a translocation. Gene fusions can give rise to hybrid proteins or to the misregulation of the transcription unit of one gene by the cis-regulatory elements (enhancers) of another.

gene interaction

The collaboration of several different genes in the production of one phenotypic character (or related group of characters).

gene locus

The specific place on a chromosome where a gene is located.

gene map

A linear designation of mutant sites within a gene, based on the various frequencies of interallelic (intragenic) recombination.

gene mutation

A point mutation that results from changes within the structure of a gene.

gene pair

The two copies of a particular type of gene present in a diploid cell (one in each chromosome set).

gene rearrangement

The process of programmed changes in the DNA structure of somatic cells, leading to changes in gene number or in the structural and functional properties of the rearranged gene.

gene replacement

The insertion of a genetically engineered transgene in place of a resident gene; often occurs by a double crossover.

gene therapy

The correction of a genetic deficiency in a cell by the addition of new DNA and its insertion into the genome.

generalized transduction

The ability of certain phages to transduce any gene in the bacterial chromosome.

genetic code

The set of correspondences between nucleotide pair triplets in DNA and amino acids in protein.

genetic dissection

The use of recombination and mutation to piece together the various components of a given biological function.

genetic markers

Alleles used as experimental probes to keep track of an individual, a tissue, a cell, a nucleus, a chromosome, or a gene.

genetic variance

Phenotypic variance resulting from the presence of different genotypes in the population.


The study of genes.

The study of inheritance.


The entire complement of genetic material in a chromosome set.

genomic library

A library encompassing an entire genome.


The cloning and molecular characterization of entire genomes.


The specific allelic composition of a cell—either of the entire cell or, more commonly, for a certain gene or a set of genes.

geographical race

A population of a species that occupies a different geographical area from other populations of the species and that differs in its allele frequencies from other such populations.

germ line

The cell lineage in a multitissued eukaryote from which the gametes derive.

germinal mutations

Mutations in the cells that are destined to develop into gametes.


Glutamine (an amino acid).


Glutamate (an amino acid).


Glycine (an amino acid).


A member of a family of proteins that contribute to signal transduction through protein–protein interactions that occur when the G-protein binds GTP but not when the G-protein binds GDP.


A gradual change in some quantitative property over a specific distance.

ground state

The developmental state of a cell (or group of cells) in the absence of the activation of a developmental regulatory switch.

growth factors

Paracrine signaling molecules, usually secreted polypeptides, that induce cell division in cells receiving these signals.


A purine base that pairs with cytosine.


The nucleoside having guanine as its base.


An individual that is a mosaic of male and female structures. The underlying cause is frequently sexchromosome mosaicism, such that some cells are chromosomal females, whereas others are chromosomal males.


half-chromatid conversion

A type of gene conversion that is inferred from the existence of nonidentical sister spores in a fungal octad showing a non-Mendelian allele ratio.


A cell having one chromosome set or an organism composed of such cells.


Production of a haploid from a diploid by progressive chromosome loss.


Description of a gene that, in a diploid cell, cannot promote wild-type function in only one copy (dose).


Description of a gene that, in a diploid cell, can promote wild-type function in only one copy (dose).

Hardy-Weinberg equilibrium

The stable frequency distribution of genotypes A/A, A/a, and a/a, in the proportions p2, 2pq, and q2, respectively (where p and q are the frequencies of the alleles A and a), that is a consequence of random mating in the absence of mutation, migration, natural selection, or random drift.

harlequin chromosomes

Sister chromatids that stain differently, so that one appears dark and the other light.


An enzyme that breaks hydrogen bonds in DNA and unwinds it during movement of the replication fork.

helix-loop-helix (HLH) protein

A protein, a part of which forms two α-helices separated by a loop (the HLH domain) that acts as a sequence-specific DNA-binding domain. HLH proteins are thought to act as transcription factors.

hemizygous gene

A gene present in only one copy in a diploid organism—for example, X-linked genes in a male mammal.

hemoglobin (Hb)

The oxygen-transporting blood protein in most animals.


A human disease in which the blood fails to clot, caused by a mutation in a gene coding for a clotting protein; inherited as an X-linked recessive phenotype.

hereditary nonpolyposis colorectal cancer (HNPCC)

One of the most common predispositions to cancer.


The biological similarity of offspring and parents.

heritability in the broad sense

See broad heritability.

heritability in the narrow sense (h2)

The proportion of phenotypic variance that can be attributed to additive genetic variance.


A plant species in which male and female organs are present in the same flower of a single individual (compare monoecious plant).

An animal with both male and female sex organs.


Densely staining condensed chromosomal regions, believed to be for the most part genetically inert.


A DNA double helix formed by annealing single strands from different sources; if there is a structural difference between the strands, the heteroduplex may show such abnormalities as loops or buckles.

heteroduplex DNA model

A model that explains both crossing-over and gene conversion by assuming the production of a short stretch of heteroduplex DNA (formed from both parental DNAs) in the vicinity of a chiasma.

heterogametic sex

The sex that has heteromorphic sex chromosomes (for example, XY) and hence produces two different kinds of gametes with respect to the sex chromosomes.

heterogeneous nuclear RNA (HnRNA)

A diverse assortment of RNA types found in the nucleus, including mRNA precursors and other types of RNA.


A culture of cells composed of two different nuclear types in a common cytoplasm.

heterokaryon test

A test for cytoplasmic mutations, based on new associations of phenotypes in cells derived from specially marked heterokaryons.

heteromorphic chromosomes

A pair of chromosomes with some homology but differing in size, shape, or staining properties.


A cell containing a mixture of genetically different cytoplasms, generally different mitochondria, or different chloroplasts.

heterothallic fungus

A fungus species in which two different mating types must unite to complete the sexual cycle.


A measure of the genetic variation in a population; with respect to one locus, stated as the frequency of heterozygotes for that locus.


An individual having a heterozygous gene pair.

heterozygous gene pair

A gene pair having different alleles in the two chromosome sets of the diploid individual—for example, A/a or A1/A2.


A cell having six chromosome sets or an organism composed of such cells.

high-frequency recombination (Hfr) cell

In E. coli, a cell having its fertility factor integrated into the bacterial chromosome; a donor (male) cell.


A mammalian temperature-dependent coat phenotype, generally albino with pigment only at the cooler tips of the ears, feet, and tail.


Histidine (an amino acid).

histocompatibility antigens

Antigens that determine the acceptance or rejection of a tissue graft.

histocompatibility genes

The genes that encode the histocompatibility antigens.


A type of basic protein that forms the unit around which DNA is coiled in the nucleosomes of eukaryotic chromosomes.

HLH protein

See helix-loop-helix protein.

homeobox (homeotic box)

A family of quite similar 180- base-pair DNA sequences that encode a polypeptide sequence called a homeodomain, a sequence-specific DNA-binding sequence. Although the homeobox was first discovered in all homeotic genes, it is now known to encode a much more widespread DNA-binding motif.


A highly conserved family of sequences 60 amino acids in length found within a large number of transcription factors that can form a helix-turn-helix structure and bind DNA in a sequence-specific manner.

homeologous chromosomes

Partly homologous chromosomes, usually indicating some original ancestral homology.


The replacement of one body part by another. Homeosis can be caused by environmental factors leading to developmental anomalies or by mutation.

homeotic gene

Genes that control the fate of segments along the anterior–posterior axis of higher animals.

homeotic mutation

A mutation that can change the fate of an imaginal disk.

homogametic sex

The sex with homologous sex chromosomes (for example, XX).


A member of a pair of homologous chromosomes.

homologous chromosomes

Chromosomes that pair with each other at meiosis or chromosomes in different species that have retained most of the same genes during their evolution from a common ancestor.

homothallic fungus

A fungus species in which a single sexual spore can complete the entire sexual cycle (compare heterothallic fungus).


An individual having a homozygous gene pair.

homozygous gene pair

A gene pair having identical alleles in both copies—for example, A/A or A1/A1.


A molecule that is secreted by an endocrine organ into the circulatory system and that acts as a long-range signaling molecule by activating receptors on or within target cells.

hormone response element (HRE)

For hormones that act by binding to receptors that can act as transcription factors, an HRE is a cis-regulatory DNA sequence that is a binding site for a hormonereceptor complex.

host range

The spectrum of strains of a given bacterial species that a given strain of phage can infect.

hot spot

A part of a gene that shows a very high tendency to become a mutant site, either spontaneously or under the action of a particular mutagen.

Huntington disease

A lethal human disease of nerve degeneration, with late-age onset. Inherited as an autosomal dominant phenotype; new mutations rare.


A heterozygote.

A progeny individual from any cross between parents of differing genotypes.

hybrid dysgenesis

A syndrome of effects including sterility, mutation, chromosome breakage, and male recombination in the hybrid progeny of crosses between certain laboratory and natural isolates of Drosophila.

hybridization in situ

Finding the location of a gene by adding specific radioactive probes for the gene and detecting the location of the radioactivity on the chromosome after hybridization.


To form a hybrid by performing a cross.

To anneal nucleic acid strands from different sources.

hydrogen bond

A weak bond in which an electron is shared with a hydrogen atom; hydrogen bonds are important in the specificity of base pairing in nucleic acids and in the determination of protein shape.


A form of calcium phosphate that binds double-stranded DNA.


Aneupolid containing a small number of extra chromosomes.

hypervariable region

The part of a variable region that determines the specificity of an antibody.

hypha (hyphae)

A threadlike structure (composed of cells attached end to end) that forms the main tissue in many fungus species.


Aneuploid with a small number of chromosomes missing.


Ig (immunoglobulin)

See antibody.


Isoleucine (an amino acid).


An adult insect.

immune system

The animal cells and tissues that recognize and attack foreign substances within the body.


See antibody.


The use of antibodies or antisera as histological tools for identifying patterns of protein distribution within a tissue or an organism. An antibody (or mixture of antibodies) that binds to a specific protein is tagged with an enzyme that can convert a substrate into a visible dye. The tagged antibody is incubated with the tissue, unbound antibody is washed off, and the enzymatic substrate is then added, revealing the pattern of protein (antigen) localization.

in situ

“In place”; see hybridization in situ.

in vitro

In an experimental situation outside the organism (literally, “in glass”).

in vitro mutagenesis

The production of either random or specific mutations in a piece of cloned DNA. Typically, the DNA will then be repackaged and introduced into a cell or an organism to assess the results of the mutagenesis.

in vivo

In a living cell or organism.


Mating between relatives.

inbreeding coefficient

The probability of homozygosity that results because the zygote obtains copies of the same ancestral gene.

incomplete dominance

The situation in which a heterozygote shows a phenotype quantitatively (but not exactly) intermediate between the corresponding homozygote phenotypes. (Exact intermediacy is no dominance.)

independent assortment

See Mendel’s second law.


An environmental agent that triggers transcription from an operon.


The relief of repression for a gene or set of genes under negative control.

inductive interaction

The interaction between two groups of cells in which a signal passed from one group of cells causes the other group of cells to change their developmental state (or fate).

infectious transfer

The rapid transmission of free episomes (plus any chromosomal genes that they may carry) from donor to recipient cells in a bacterial population.


A rare base that is important at the wobble position of some tRNA anticodons.

insertion sequence (IS)

A mobile piece of bacterial DNA (several hundred nucleotide pairs in length) that is capable of inactivating a gene into which it inserts.

insertional translocation

The insertion of a segment from one chromosome into another nonhomologous one.

intercalating agent

A chemical that can insert itself between the stacked bases at the center of the DNA double helix, possibly causing a frame-shift mutation.

interchromosomal recombination

Recombination resulting from independent assortment.


A measure of the independence of crossovers from each other, calculated by subtracting the coefficient of coincidence from 1.

intermediate filaments

A heterogeneous class of cytoskeletal elements characterized by an intermediate cable diameter larger than that of microfilaments but smaller than that of microtubules.


The cell cycle stage between nuclear divisions, when chromosomes are extended and functionally active.

interrupted mating

A technique used to map bacterial genes by determining the sequence in which donor genes enter recipient cells.

interstitial region

The chromosomal region between the centromere and the site of a rearrangement.

intervening sequence

An intron; a segment of largely unknown function within a gene. This segment is initially transcribed, but the transcript is not found in the functional mRNA.

intrachromosomal recombination

Recombination resulting from crossing-over between two gene pairs.


See intervening sequence.


A chromosomal mutation in which a chromosome segment is removed, rotated through 180 degrees, and reinserted in the same location.

inverted repeat (IR) sequence

A sequence found in identical (but inverted) form—for example, at the opposite ends of a transposon.


See inverted repeat sequence.


See insertion sequence.

isoaccepting tRNAs

The various types of tRNA molecules that carry a specific amino acid.


One of several forms of an atom having the same atomic number but differing atomic masses.



The entire chromosome complement of an individual or cell, as seen in mitotic metaphase.


See kilobase.

kilobase (kb)

1000 nucleotide pairs.


A complex of proteins to which a nuclear spindle fiber attaches.

Klinefelter syndrome

An abnormal human male phenotype with an extra X chromosome (XXY).


Inactivation of one specific gene. Same as gene disruption.


lagging strand

In DNA replication, the strand that is synthesized apparently in the 3′-to-5′ direction, by ligating short fragments synthesized individually in the 5′-to-3′ direction.

λ (lambda) phage

One kind (“species”) of temperate bacteriophage.


A λ phage carrying a gal bacterial gene and defective (d) for some phage function.

lampbrush chromosome

A large chromosome found in amphibian eggs, with lateral DNA loops that produce a brushlike appearance under the microscope.

lateral inhibition

The signal produced by one cell that prevents adjacent cells from acquiring its fate.


A continuous layer of bacteria on the surface of an agar medium.


An untranslated segment at the 5′ end of mRNA between the transcriptional and the translational start sites.

leader sequence

At the 5′ end of an mRNA, the sequence that is not translated into protein.

leading strand

In DNA replication, the strand that is made in the 5′-to-3′ direction by continuous polymerization at the 3′ growing tip.

leaky mutant

A mutant (typically, an auxotroph) that results from a partial rather than a complete inactivation of the wild-type function.

leaky mutation

A mutation that confers a mutant phenotype but still retains a low but detectable level of wild-type function.


A damaged area in a gene (a mutant site), a chromosome, or a protein.

lethal gene

A gene whose expression results in the death of the individual expressing it.


Leucine (an amino acid).


A collection of DNA clones obtained from one DNA donor.

ligand–receptor interaction

The interactions between a molecule (usually of an extracellular origin) and a protein on or within a target cell. One type of ligand–receptor interaction can be between steroid hormones and their cytoplasmic or nuclear receptors. Another can be between secreted polypeptide ligands and transmembrane receptors.


An enzyme that can rejoin a broken phosphodiester bond in a nucleic acid.


A group of identical pure-breeding diploid or polyploid organisms, distinguished from other individuals of the same species by some unique phenotype and genotype.


Long interspersed element; a type of large repetitive DNA segment found throughout the genome.

linear tetrad

A tetrad that results from the occurrence of the meiotic and postmeiotic nuclear divisions in such a way that sister products remain adjacent to one another (with no passing of nuclei).


The association of genes on the same chromosome.

linkage group

A group of genes known to be linked; a chromosome.

linkage map

A chromosome map; an abstract map of chromosomal loci, based on recombinant frequencies.

locus (loci)

See gene locus.


Lysine (an amino acid).


The rupture and death of a bacterial cell on the release of phage progeny.


See lysogenic bacterium.

lysogenic bacterium

A bacterial cell capable of spontaneous lysis due, for example, to the uncoupling of a prophage from the bacterial chromosome.


M phase

The mitotic phase of the cell cycle.


See megabase.


A large polymer such as DNA, a protein, or a polysaccharide.


Tailless phenotype in cats, caused by an autosomal dominant mutation that is lethal when homozygous.

map unit (m.u.)

The “distance” between two linked gene pairs where 1 percent of the products of meiosis are recombinant; a unit of distance in a linkage map.

mapping function

A formula expressing the relation between distance in a linkage map and recombinant frequency.

Marfan syndrome

A human disorder of the connective tissue expressed as a range of symptoms, including very long limbs and digits as well as heart defects; inherited as an autosomal dominant phenotype.


See genetic markers.

marker retention

A technique used in yeast to test the degree of linkage between two mitochondrial mutations.

maternal effect

The environmental influence of the mother’s tissues on the phenotype of the offspring.

maternal effect lethal

A mutation that is viable in zygotes, but mothers having the mutation produce inviable offspring.

maternal inheritance

A type of uniparental inheritance in which all progeny have the genotype and phenotype of the parent acting as the female.

maternally expressed gene

A gene that contributes to the phenotype of an offspring on the basis of its expression in the mother.

mating types

The equivalent in lower organisms of the sexes in higher organisms; the mating types typically differ only physiologically and not in physical form.

matroclinous inheritance

Inheritance in which all offspring have the nucleus-determined phenotype of the mother.


The arithmetic average.


Any material on (or in) which experimental cultures are grown.

Megabase (Mb)

One million nucleotide pairs.


Cell in which meiosis takes place.


Two successive nuclear divisions (with corresponding cell divisions) that produce gametes (in animals) or sexual spores (in plants and fungi) having one-half of the genetic material of the original cell.


Cell that is one of the products of meiosis in plants.


Denaturation of DNA.

Mendelian ratio

A ratio of progeny phenotypes in accord with the application of Mendel’s laws.

Mendel’s first law

The two members of a gene pair segregate from each other in meiosis; each gamete has an equal probability of obtaining either member of the gene pair.

Mendel’s second law

The law of independent assortment; unlinked or distantly linked segregating gene pairs assort independently at meiosis.


A partly diploid E. coli cell formed from a complete chromosome (the endogenote) plus a fragment (the exogenote).

messenger RNA

See mRNA.


Methionine (an amino acid).


The chemical reactions that take place in a living cell.

metacentric chromosome

A chromosome having its centromere in the middle.


A segmental repeat unit in higher animals.


An intermediate stage of nuclear division when chromosomes align along the equatorial plane of the cell.


Modification of a molecule by the addition of a methyl group.


The smallest-diameter cable system of the cytoskeleton. Microfilament cables are composed of actin polymers.

microsatellite DNA

A type of repetitive DNA based on very short repeats such as dinucleotides.

microtubule organizing center

The part of the microtubule cytoskeleton in which all of the minus ends of the microtubules are clustered. Ordinarily, this cluster is near the center of the cell.


The largest-diameter cable system of the cytoskeleton. Microtubules are composed of polymerized tubulin subunits forming a hollow tube.

midparent value

The mean of the values of a quantitative phenotype for two specific parents.

minimal medium

A medium containing only inorganic salts, a carbon source, and water.

minisatellite DNA

A type of repetitive DNA sequence based on short repeat sequences with a unique common core; used for DNA fingerprinting.


The occurrence of gene expression in a tissue in which the gene is normally not expressed. Such ectopic expression can be caused by the juxtaposition of novel enhancer elements to a gene.

missense mutation

A mutation that alters a codon so that it encodes a different amino acid.

mitochondrial cytopathies

Human disorders caused by point mutations or deletions in mitochondrial DNA; inherited maternally.


A eukaryotic organelle that is the site of ATP synthesis and of the citric acid cycle.


A type of nuclear division (occurring at cell division) that produces two daughter nuclei identical with the parent nucleus.

mitotic crossover

A crossover resulting from the pairing of homologs in a mitotic diploid.

mobile genetic element

See transposable genetic element.


The single class in a statistical distribution having the greatest frequency.

modifier gene

A gene that affects the phenotypic expression of another gene.

molecular clock

A constant rate of change in DNA or protein sequences as a consequence of random mutation and genetic drift of unselected molecular variants.

molecular genetics

The study of the molecular processes underlying gene structure and function.

monocistronic mRNA

An mRNA that encodes one protein.

monoecious plant

A plant species in which male and female organs are found on the same plant but in different flowers (for example, corn).

monohybrid cross

A cross between two individuals identically heterozygous at one gene pair—for example, A/a × A/a.


A cell having only one chromosome set (usually as an aberration) or an organism composed of such cells.


Refers to a cell or individual that is basically diploid but that has only one copy of one particular chromosome type and thus has chromosome number 2n − 1.


A molecule that can induce the acquisition of different cell fates according to the level of morphogen to which a cell is exposed.


A chimera; a tissue containing two or more genetically distinct cell types or an individual composed of such tissues.

motor protein

A protein that is able to move unidirectionally along a specific type of cytoskeletal cable. Kinesins and dyneins are microtubule-based and myosins are microfilament-based motor proteins. By attaching to other subcellular components, motor proteins are capable of directed movement of these components within the cell.

mRNA (messenger RNA)

An RNA molecule transcribed from the DNA of a gene and from which a protein is translated by the action of ribosomes.


Mitochondrial DNA.


See map unit.

mu phage

A kind (“species”) of phage with properties similar to those of insertion sequences, being able to insert, transpose, inactivate, and cause rearrangements.

multimeric structure

A structure composed of several identical or different subunits held together by weak bonds.

multiple allelism

The existence of several known alleles of a gene.

multiple cloning site

See polylinker.

multiple-factor hypothesis

A hypothesis to explain quantitative variation by assuming the interaction of a large number of genes (polygenes), each with a small additive effect on the character.

multiple-hit hypothesis

The proposal that a single cell must receive a series of mutational events to become malignant or cancerous.

multiplicity of infection

The average number of phage particles that infect a single bacterial cell in a specific experiment.


An agent that is capable of increasing the mutation rate.


An organism or cell carrying a mutation.

mutant allele

An allele differing from the allele found in the standard, or wild type.

mutant hunt

The process of collecting different mutants showing abnormalities in a certain structure or in a certain function, as a preparation for mutational dissection of that function.

mutant site

The damaged or altered area within a mutated gene.


The process that produces a gene or a chromosome set differing from the wild type.

The gene or chromosome set that results from such a process.

mutation breeding

Use of mutagens to develop variants that can increase agricultural yield.

mutation event

The actual occurrence of a mutation in time and space.

mutation frequency

The frequency of mutants in a population.

mutation rate

The number of mutation events per gene per unit of time (for example, per cell generation).

mutational dissection

The study of the components of a biological function through a study of mutations affecting that function.


The smallest part of a gene that can take part in a mutation event; now known to be a nucleotide pair.


A cancer of the bone marrow.



10−9 meters.

narrow heritability

See heritability in the narrow sense.

negative assortative mating

Preferential mating between phenotypically unlike partners.

negative control

Regulation mediated by factors that block or turn off transcription.


A human disease with tumors of nerve cells and café au lait spots, both in the skin. The allele generally arises from germinal mutation, but it is inherited as an autosomal dominant.


A pink mold, commonly found growing on old food.

neutral evolution

Nonadaptive evolutionary changes that occur because of random genetic drift.

neutral mutations

Mutations to alleles that are not different enough from each other in their physiological effects to be affected by natural selection.

neutral petite

A petite that produces all wild-type progeny when crossed with wild type.


See selective neutrality.


Nuclease action to sever the sugar-phosphate backbone in one DNA strand at one specific site.

nitrocellulose filter

A type of filter used to hold DNA for hybridization.

nitrogen bases

Types of molecules that form important parts of nucleic acids, composed of nitrogen-containing ring structures; hydrogen bonds between bases link the two strands of a DNA double helix.


See autonomous phenotype.


The failure of homologs (at meiosis) or sister chromatids (at mitosis) to separate properly to opposite poles.

nonlinear tetrad

A tetrad in which the meiotic products are in no particular order.

non-Mendelian ratio

An unusual ratio of progeny phenotypes that is not in accord with the simple application of Mendel’s laws; for example, mutant:wild ratios of 3:5, 5:3, 6:2, or 2:6 in tetrads indicate that gene conversion has occurred.

nonparental ditype (NPD)

A tetrad type containing two different genotypes, both of which are recombinant.

nonsense codon

A codon for which no normal tRNA molecule exists; the presence of a nonsense codon causes termination of translation (ending of the polypeptide chain). The three nonsense codons are called amber, ocher, and opal.

nonsense mutation

A mutation that alters a gene so as to produce a nonsense codon.

nonsense suppressor

A mutation that produces an altered tRNA that will insert an amino acid in translation in response to a nonsense codon.

nonsynonymous substitution

Mutational replacement of an amino acid with one of different chemical properties See also replacement substitutions.

norm of reaction

The pattern of phenotypes produced by a given genotype under different environmental conditions.

Northern blot

Transfer of electrophoretically separated RNA molecules from a gel onto an absorbent sheet, which is then immersed in a labeled probe that will bind to the RNA of interest.


See nonparental ditype.

nu body

See nucleosome.


An enzyme that can degrade DNA by breaking its phosphodiester bonds.


A DNA mass within a chloroplast or mitochondrion.

nucleolar organizer

A region (or regions) of the chromosome set physically associated with the nucleolus and containing rRNA genes.


An organelle found in the nucleus, containing rRNA and amplified multiple copies of the genes encoding rRNA.


A nitrogen base bound to a sugar molecule.


A nu body; the basic unit of eukaryotic chromosome structure; a ball of eight histone molecules wrapped about by two coils of DNA.


A molecule composed of a nitrogen base, a sugar, and a phosphate group; the basic building block of nucleic acids.

nucleotide pair

A pair of nucleotides (one in each strand of DNA) that are joined by hydrogen bonds.

nucleotide-pair substitution

The replacement of a specific nucleotide pair by a different pair; often mutagenic.

null allele

An allele whose effect is either an absence of normal gene product at the molecular level or an absence of normal function at the phenotypic level.

null mutation

A mutation that results in complete absence of function for the gene.


Refers to a cell or individual with one chromosomal type missing, with a chromosome number such as n − 1 or 2n − 2.

nurse cells

The sister cells of the oocyte in insects. The nurse cells produce the bulk of the cytoplasmic contents of the mature oocyte.


ocher codon

The codon UAA, a nonsense codon.


An ascus containing eight ascospores, produced in species in which the tetrad normally undergoes a postmeiotic mitotic division.

Okazaki fragment

A small segment of single-stranded DNA synthesized as part of the lagging strand in DNA replication.


A short segment of synthetic DNA.


A gene that contributes to the production of a cancer. Oncogenes are generally mutated forms of normal cellular genes.

opal codon

The codon UGA, a nonsense codon.

open reading frame

See ORF.


A DNA region at one end of an operon that acts as the binding site for repressor protein.


A set of adjacent structural genes whose mRNA is synthesized in one piece, plus the adjacent regulatory signals that affect transcription of the structural genes.

ORF (open reading frame)

A section of a sequenced piece of DNA that begins with a start codon and ends with a stop codon; it is presumed to be the coding sequence of a gene.


A subcellular structure having a specialized function—for example, the mitocondrion, the chloroplast, or the spindle apparatus.


The production of organ systems during animal embryogenesis.

origin of replication

The point of specific sequence at which DNA replication is initiated.


A phenotypic relation in which the phenotypic expression of the heterozygote is greater than that of either homozygote.


P element

A Drosophila transposable element that has been used as a tool for insertional mutagenesis and for germ-line transformation.

PAC (P1-based artificial chromosome)

A derivative of phage P1 engineered as a cloning vector for carrying large inserts.

pair-rule gene

In Drosophila, a member of a class of zygotically expressed genes that act at an intermediary stage in the process of the establishment of the correct numbers of body segments. Pair-rule mutations have half of the normal number of segments, owing to the loss of every other segment.

paracentric inversion

An inversion not involving the centromere.

paracrine signaling

The process by which a secreted molecule binds to receptors on or within nearby cells, thereby inducing a signal transduction pathway within the receiving cell.

paralogous genes

Two genes in the same species that have evolved by gene duplication.


An epigenetic phenomenon in plants, in which the genetic activity of a normal allele is heritably reduced by virtue of that allele having been heterozygous with a special “paramutagenic” allele.

parental ditype (PD)

A tetrad type containing two different genotypes, both of which are parental.

parental imprinting

An epigenetic phenomenon in which the activity of a gene is dependent on whether the gene was inherited from the father or the mother. Some genes are maternally imprinted, others paternally.


The production of offspring by a female with no genetic contribution from a male.

partial diploid

See merozygote.

particulate inheritance

The model proposing that genetic information is transmitted from one generation to the next in discrete units (“particles”), so the character of the offspring is not a smooth blend of essences from the parents (compare blending inheritance).


An organism that causes disease in another organism.

patroclinous inheritance

Inheritance in which all offspring have the nucleus-based phenotype of the father.

pattern formation

The processes by which the complex shape and structure of higher organisms develop.


See parental ditype.


A “family tree,” drawn with standard genetic symbols, showing inheritance patterns for specific phenotypic characters.


The proportion of individuals with a specific genotype who manifest that genotype at the phenotype level.


See amino acid.

peptide bond

A bond joining two amino acids.

pericentric inversion

An inversion that involves the centromere.

permissive conditions

Those environmental conditions under which a conditional mutant shows the wild-type phenotype.


A yeast mutation producing small colonies and altered mitochondrial functions. In cytoplasmic petites (neutral and suppressive petites), the mutation is a deletion in mitochondrial DNA; in segregational petites, the mutation is in nuclear DNA.


See bacteriophage.


Phenylalanine (an amino acid).


An environmentally induced phenotype that resembles the phenotype produced by a mutation.


The form taken by some character (or group of characters) in a specific individual.

The detectable outward manifestations of a specific genotype.

phenotypic sex determination

Sex determination by nongenetic means.

phenylketonuria (PKU)

A human metabolic disease caused by a mutation in a gene encoding a phenylalanine-processing enzyme, which leads to mental retardation if not treated; inherited as an autosomal recessive phenotype.

Philadelphia chromosome

A translocation between the long arms of chromosomes 9 and 22, often found in the white blood cells of patients with chronic myeloid leukemia.

phosphodiester bond

A bond between a sugar group and a phosphate group; such bonds form the sugar-phosphate backbone of DNA.

phyletic evolution

Heritable change over time in a continuous line of descent.

physical mapping

Mapping the positions of cloned genomic fragments.


A mammalian phenotype in which patches of skin are unpigmented because of lack of melanocytes; generally inherited as an autosomal dominant.

pilus (pili)

A conjugation tube; a hollow hairlike appendage of a donor E. coli cell that acts as a bridge for transmission of donor DNA to the recipient cell during conjugation.

plant breeding

The application of genetic analysis to the development of plant lines better suited for human purposes.


A clear area on a bacterial lawn, left by lysis of the bacteria through progressive infections by a phage and its descendants.


Autonomously replicating extrachromosomal DNA molecule.


A flat dish used to culture microbes.

To spread cells over the surface of solid medium in a plate.

pleiotropic mutation

A mutation that has effects on several different characters.


The number of chromosome sets.

point mutation

A mutation that can be mapped to one specific locus.

Poisson distribution

A mathematical expression giving the probability of observing various numbers of a particular event in a sample when the mean probability of an event on any one trial is very small.


A slow-growing mitochondrial mutant in Neurospora.

polar gene conversion

A gradient of conversion frequency along the length of a gene.

polar granules

Cytoplasmic granules localized at the posterior end of a Drosophila oocyte and early embryo. These granules are associated with the germ-line and posterior determinants.

polar mutation

A mutation that affects the transcription or translation of the part of the gene or operon on only one side of the mutant site—for example, nonsense mutations, frame-shift mutations, and IS-induced mutations.


A material used to make electrophoretic gels for the separation of mixtures of macromolecules.

poly (A) tail

A string of adenine nucleotides added to mRNA after transcription.

polycistronic mRNA

An mRNA that encodes more than one protein.


More than five fingers or toes or both. Inherited as an autosomal dominant phenotype.


See multiple-factor hypothesis.


A vector DNA sequence containing multiple unique restriction-enzyme-cut sites, convenient for inserting foreign DNA.

polymerase chain reaction (PCR)

A method for amplifying specific DNA segments that exploits certain features of DNA replication.


The occurrence in a population (or among populations) of several phenotypic forms associated with alleles of one gene or homologs of one chromosome.


A chain of linked amino acids; a protein.


A cell having three or more chromosome sets or an organism composed of such cells.


A biological polymer composed of sugar subunits—for example, starch or cellulose.

polytene chromosome

A giant chromosome produced by an endomitotic process in which the multiple DNA sets remain bound in a haploid number of chromosomes.

position effect

Used to describe a situation in which the phenotypic influence of a gene is altered by changes in the position of the gene within the genome.

position-effect variegation

Variegation caused by the inactivation of a gene in some cells through its abnormal juxtaposition with heterochromatin.

positional information

The process by which chemical cues that establish cell fate along a geographic axis are established in a developing embryo or tissue primordium.

positive assortative mating

A situation in which like phenotypes mate more commonly than expected by chance.

positive control

Regulation mediated by a protein that is required for the activation of a transcription unit.

postzygotic isolation

Failure to exchange genes between species because zygotes that are formed either cannot develop to adulthood or, if they develop, are sterile in one or both sexes.

prezygotic isolation

Failure to exchange genes between species because zygotes cannot be formed. The inability to form zygotes can be a consequence of temporal or ecological differences between the species, behavioral barriers to mating, mechanical lack of fit between the genitalia of the two sexes, or incompatibility of gametes.

primary structure of a protein

The sequence of amino acids in the polypeptide chain.


An enzyme that makes RNA primers during DNA replication.


A short single-stranded RNA or DNA that can act as a start site for 3′ chain growth when bound to a single-stranded template.

primer walking

The use of a primer based on a sequenced area of the genome to sequence into a flanking unsequenced area.


Proteinaceous infectious particle that causes degenerative disorders of the central nervous system, such as “scrapie” in sheep and Creutzfeldt-Jacob disease in humans.


Proline (an amino acid).


Defines a nucleic acid segment that can be used to identify specific DNA molecules bearing the complementary sequence, usually through autoradiography.

product of meiosis

One of the (usually four) cells formed by the two meiotic divisions.

product rule

The probability of two independent events occurring simultaneously is the product of the individual probabilities.


A mutagen that tends to produce frame-shift mutations.


An organism composed of a prokaryotic cell, such as bacteria and blue-green algae.

prokaryotic cell

A cell having no nuclear membrane and hence no separate nucleus.


A regulator region a short distance from the 5′ end of a gene that acts as the binding site for RNA polymerase.


A phagechromosome” inserted as part of the linear structure of the DNA chromosome of a bacterium.


The early stage of nuclear division during which chromosomes condense and become visible.


In a human pedigree, the person who first came to the attention of the geneticist.

protein kinase

An enzyme that phosphorylates specific amino acid residues on specific target proteins. One major class of protein kinases phosphorylates tyrosines, and the other phosphorylates serines and threonines on target proteins.


The complete set of protein-encoding genes in a genome.


The normal cellular counterpart of a gene that can be mutated to become a dominant oncogene.


A plant cell whose wall has been removed.


A strain of organisms that will proliferate on minimal medium (compare auxotroph).


A virus “chromosome” integrated into the DNA of the host cell.


The sudden appearance of a recessive phenotype in a pedigree, owing to the deletion of a masking dominant gene.


An inactive gene derived from an ancestral active gene.


See chromosome puff.

pulse-chase experiment

An experiment in which cells are grown in radioactive medium for a brief period (the pulse) and then transferred to nonradioactive medium for a longer period (the chase).

pulsed-field gel electrophoresis

An electrophoretic technique in which the gel is subjected to electrical fields alternating between different angles, allowing very large DNA fragments to “snake” through the gel and hence permitting efficient separation of mixtures of such large fragments.

Punnett square

A grid used as a graphic representation of the progeny zygotes resulting from different gamete fusions in a specific cross.

pure-breeding line or strain

A group of identical individuals that always produce offspring of the same phenotype when intercrossed.


A type of nitrogen base; the purine bases in DNA are adenine and guanine.


A type of nitrogen base; the pyrimidine bases in DNA are cytosine and thymine.


QTL (quantitative trait locus)

The position of a gene whose variant alleles contribute to quantitative variation for some trait (character).

quantitative variation

The existence of a range of phenotypes for a specific character, differing by degree rather than by distinct qualitative differences.

quaternary structure of a protein

The multimeric constitution of the protein.


R plasmid

A plasmid containing one or several transposons that bear resistance genes.

radiation hybrid

A type of human–mouse hybrid cell in which human chromosomes have been fragmented by radiation to determine which markers are inherited together and therefore linked.

random genetic drift

Changes in allele frequency that result because the genes appearing in offspring are not a perfectly representative sampling of the parental genes.

random mating

Mating between individuals where the choice of a partner is not influenced by the genotypes (with respect to specific genes under study).


Randomly amplified polymorphic DNA. A set of several genomic fragments amplified by a single PCR primer; somewhat variable from individual to individual; +/− heterozygotes for individual fragments can act as markers in genome mapping.

reading frame

The codon sequence that is determined by reading nucleotides in groups of three from some specific start codon.

realized heritability

The ratio of the single-generation progress of selection to the selection differential of the parents.


Spontaneous realignment of two single DNA strands to re-form a DNA double helix that had been denatured.


See ligand–receptor interaction.

receptor element

A controlling element that can insert into a gene (making it a mutant) and can also excise (thus making the mutation unstable); both of these functions are nonautonomous, being under the influence of the regulator element.

receptor tyrosine kinase

A transmembrane receptor whose cytoplasmic domain includes a tyrosine kinase enzymatic activity. In normal situations, the kinase is activated only on binding of the appropriate ligand to the receptor.

recessive allele

An allele whose phenotypic effect is not expressed in a heterozygote.

recessive phenotype

The phenotype of a homozygote for the recessive allele; the parental phenotype that is not expressed in a heterozygote.

reciprocal crosses

A pair of crosses of the type genotype A ♀× genotype B ♂ and genotype B ♀ × genotype A ♂.

reciprocal translocation

A translocation in which part of one chromosome is exchanged with a part of a separate nonhomologous chromosome.


An individual or cell with a genotype produced by recombination.

recombinant DNA

A novel DNA sequence formed by the combination of two nonhomologous DNA molecules.

recombinant frequency (RF)

The proportion (or percentage) of recombinant cells or individuals.


In general, any process in a diploid or partly diploid cell that generates new gene or chromosomal combinations not found in that cell or in its progenitors.

At meiosis, the process that generates a haploid product of meiosis whose genotype is different from either of the two haploid genotypes that constituted the meiotic diploid.

recombinational repair

The repair of a DNA lesion through a process, similar to recombination, that uses recombination enzymes.


A region of a gene within which there can be no crossing-over; now known to be a nucleotide pair.

reduction division

A nuclear division that produces daughter nuclei each having one-half as many centromeres as the parental nucleus.

redundant DNA

See repetitive DNA.


A term coined by Galton for the tendency of the quantitative traits of offspring to be closer to the population mean than are their parents’ traits. It arises from dominance, gene interaction, and nongenetic influences on traits.

regression coefficient

The slope of the straight line that most closely relates two correlated variables.

regulator element

See receptor element.

regulatory genes

Genes that have roles in turning on or off the transcription of structural genes.

regulatory region

Upstream (5′) end of the gene to which bind various proteins that cause transcription of the gene at the correct time and place.

repetitive DNA

Redundant DNA; DNA sequences that are present in many copies per chromosome set.

replacement substitutions

Nucleotide changes in the encoding part of a gene that result in a change in the amino acid sequence of the encoded protein.


DNA synthesis.

replication fork

The point at which the two strands of DNA are separated to allow replication of each strand.


A chromosomal region under the influence of one adjacent replication-initiation locus.

reporter gene

A gene whose phenotypic expression is easy to monitor and is used to study tissue-specific promoter and enhancer activities in transgenes.

repressor protein

A molecule that binds to the operator and prevents transcription of an operon.

repulsion conformation

Two linked heterozygous gene pairs in the arrangement A b/a B.

resolving power

The ability of an experimental technique to distinguish between two genetic conditions (typically discussed when one condition is rare and of particular interest).

restriction enzyme

An endonuclease that recognizes specific target nucleotide sequences in DNA and breaks the DNA chain at those points; a variety of these enzymes are known, and they are extensively used in genetic engineering.

restriction map

A map of a chromosomal region showing the positions of target sites of one or more restriction enzymes.

restrictive conditions

Environmental conditions under which a conditional mutant shows the mutant phenotype.


A childhood cancer of the human retina.


A transposable element that utilizes reverse transcriptase to transpose through an RNA intermediate.


An RNA virus that replicates by first being converted into double-stranded DNA.

reverse genetics

The experimental procedure that begins with a cloned segment of DNA or a protein sequence and uses it (through directed mutagenesis) to introduce programmed mutations back into the genome to investigate function.

reverse transcriptase

An enzyme that catalyzes the synthesis of a DNA strand from an RNA template.


The production of a wild-type gene from a mutant gene.


See recombinant frequency.


Restriction fragment length polymorphism. At some chromosomal locations a probe sometimes detects different sizes or different numbers of restriction fragments (often as a result of presence and absence of restriction sites), and this situation is an RFLP. If an individual is heterozygous for such a chromosomal difference, that region can be used as a marker in chromosome mapping.

RFLP mapping

A technique in which DNA restriction fragment length polymorphisms are used as reference loci for mapping in relation to known genes or other RFLP loci.


A protein factor required to recognize certain transcription termination signals in E. coli.

ribonucleic acid

See RNA.

ribosomal RNA

See rRNA.


A complex organelle that catalyzes the translation of messenger RNA into an amino acid sequence. Composed of proteins plus rRNA.


RNAs with enzymatic activities—for instance, the self-splicing RNA molecules in Tetrahymena.

RNA (ribonucleic acid)

A single-stranded nucleic acid similar to DNA but having ribose sugar rather than deoxyribose sugar and uracil rather than thymine as one of the bases.

RNA in situ hybridization

A technique that is used to identify the spatial pattern of expression of a particular transcript (usually an mRNA). In this technique, the DNA probe is labeled, either radioactively or by chemically attaching an enzyme that can convert a substrate into a visible dye. A tissue or organism is soaked in a solution of single-stranded labeled DNA under conditions that allow the DNA to hybridize to complementary RNA sequences in the cells; unhybridized DNA is then removed. Radioactive probe is detected by autoradiography. Enzyme-labeled probe is detected by soaking the tissue in the substrate; the dye develops in sites where the transcript of interest was expressed.

RNA polymerase

An enzyme that catalyzes the synthesis of an RNA strand from a DNA template. In eukaryotes, there are several classes of RNA polymerase. Structural genes for proteins are transcribed by RNA polymerase II.

rRNA (ribosomal RNA)

A class of RNA molecules, encoded in the nucleolar organizer, that have an integral (but poorly understood) role in ribosome structure and function.


S (Svedberg unit)

A unit of sedimentation velocity commonly used to describe molecular units of various sizes (because sedimentation velocity is related to size).

S phase

The part of interphase of the cell cycle in which DNA synthesis takes place.


Scaffold attachment regions; the positions along DNA where it is anchored to the central scaffold of the chromosome.


A terminal section of a chromosome separated from the main body of the chromosome by a narrow constriction.

satellite chromosome

A chromosome that seems to be an addition to the normal genome.

satellite DNA

Any type of highly repetitive DNA; formerly defined as DNA forming a satellite band after cesium chloride density gradient centrifugation.

saturation mutagenesis

Induction and recovery of large numbers of mutations in one area of the genome or in one function in the hope of identifying all the genes in that area or affecting that function.


The central framework of a chromosome to which the DNA solenoid is attached as loops; composed largely of topoisomerase.


See sister-chromatid exchange.

secondary sexual characteristics

The sex-associated phenotypes of somatic tissues in sexually dimorphic animals.

secondary structure of a protein

A spiral or zigzag arrangement of the polypeptide chain.

second-division segregation pattern

A pattern of ascospore genotypes for a gene pair showing that the two alleles separate into different nuclei only at the second meiotic division as a result of a crossover between that gene pair and its centromere; can be detected only in a linear ascus.

second-site mutation

The second mutation of a double mutation within a gene; in many cases, the second-site mutation suppresses the first mutation, so the double mutant has the wild-type phenotype.


An area of tissue whose phenotype is detectably different from the surrounding tissue phenotype.


The sinking of a molecule under the opposing forces of gravitation and buoyancy.

segment-polarity gene

In Drosophila, a member of the class of genes that contribute to the final aspects of establishing the correct number of segments. Segment-polarity mutations cause a loss of a comparable part of each of the body segments.


The process by which the correct number of segments is established in a developing segmented animal.


Cytologically, the separation of homologous structures.

Genetically, the production of two separate phenotypes, corresponding to two alleles of a gene, either in different individuals (meiotic segregation) or in different tissues (mitotic segregation).

segregational petite

A petite that, in a cross with wild type, produces 1/2 petite and 1/2 wild-type progeny; caused by a nuclear mutation.

selection coefficient (s)

The proportional excess or deficiency of fitness of one genotype in relation to another genotype.

selection differential

The difference between the mean of a population and the mean of the individuals selected to be parents of the next generation.

selection progress

The difference between the mean of a population and the mean of the offspring in the next generation born to selected parents.

selective neutrality

A situation in which different alleles of a certain gene confer equal fitness.

selective system

An experimental technique that enhances the recovery of specific (usually rare) genotypes.


To fertilize eggs with sperm from the same individual.


The ability of certain multimeric biological structures to assemble from their component parts through random movements of the molecules and formation of weak chemical bonds between surfaces with complementary shapes.

semiconservative replication

The established model of DNA replication in which each double-stranded molecule is composed of one parental strand and one newly polymerized strand.

semisterility (half sterility)

The phenotype of individuals heterozygotic for certain types of chromosome aberration; expressed as a reduced number of viable gametes and hence reduced fertility.

sequence-tagged site

A relatively small, sequenced region of a cloned genomic fragment that can be used by a computer to align the cloned fragment into a contig.


Serine (an amino acid).

sex chromosome

A chromosome whose presence or absence is correlated with the sex of the bearer; a chromosome that plays a role in sex determination.

sex determination

The genetic or environmental process by which the sex of an individual is established.

sex linkage

The location of a gene on a sex chromosome.

sex reversal

A syndrome known in humans and mice in which chromosomally XX individuals develop as males. In some cases, sex reversal is now known to be due to the translocation of the testis-determining region of the Y chromosome to the tip of the X chromosome in such individuals.


Sexual transmission of donor E. coli chromosomal genes on the fertility factor.

sexual spore

See spore.

shotgun technique

The cloning of a large number of different DNA fragments as a prelude to selecting one particular clone type for intensive study.

shuttle vector

A vector (for example, a plasmid) constructed in such a way that it can replicate in at least two different host species, allowing a DNA segment to be tested or manipulated in several cellular settings.

sickle-cell anemia

Potentially lethal human disease caused by a mutation in a gene encoding the oxygen-transporting molecule hemoglobin. The altered molecule causes red blood cells to be sickle shaped. Inherited as an autosomal recessive.

signal sequence

The N-terminal sequence of a secreted protein, which is required for transport through the cell membrane.

signal transduction cascade

A series of sequential events, such as protein phosphorylations, that pass a signal received by a transmembrane receptor through a series of intermediate molecules until final regulatory molecules, such as transcription factors, are modified in response to the signal.

silent mutation

Mutation in which the function of the protein product of the gene is unaltered.


Short interspersed element. A type of small, repetitive DNA sequence found throughout a eukaryotic genome.

sister-chromatid exchange (SCE)

An event similar to crossing-over that can occur between sister chromatids at mitosis or at meiosis; detected in harlequin chromosomes.

site-specific recombination

Recombination between two specific sequences that need not be homologous; mediated by a specific recombination system.

S-9 mix

A liver-derived supernatant used in the Ames test to activate or inactivate mutagens.

solenoid structure

In eukaryotic nuclear chromosomes, the supercoiled arrangement of DNA produced by coiling the continuous string of nucleosomes.

somatic cell

A cell that is not destined to become a gamete; a “body cell,” whose genes will not be passed on to future generations.

somatic-cell genetics

Asexual genetics; the study of somatic mutation, assortment, and crossing-over, and of cell fusion.

somatic mutation

A mutation occurring in a somatic cell.


A human growth hormone.

SOS repair

The error-prone process whereby gross structural DNA damage is circumvented by allowing replication to proceed past the damage through imprecise polymerization.

Southern blot

Transfer of electrophoretically separated fragments of DNA from the gel to an absorbent sheet such as paper. This sheet is then immersed in a solution containing a labeled probe that will bind to a fragment of interest.

spacer DNA

DNA found between genes; its function is unknown.

specialized (restricted) transduction

The situation in which a particular phage will transduce only specific regions of the bacterial chromosome.


The process of forming new species by the splitting of an old species into two or more new species incapable of exchanging genes with each other.


A group of organisms that are biologically capable of exchanging genes with each other but are incapable of exchanging genes with other such groups.

specific-locus test

A system for detecting recessive mutations in diploids. Normal individuals treated with mutagen are mated to testers that are homozygous for the recessive alleles at a number of specific loci; the progeny are then screened for recessive phenotypes.


The set of microtubular fibers that appear to move eukaryotic chromosomes in division.


The reaction that removes introns and joins together exons in RNA.

spontaneous mutation

A mutation occurring in the absence of mutagens, usually due to errors in the normal functioning of cellular enzymes.


In plants and fungi, sexual spores are the haploid cells produced by meiosis.

In fungi, asexual spores are somatic cells that are cast off to act either as gametes or as the initial cells for new haploid individuals.


The diploid sexual-spore-producing generation in the life cycle of plants—that is, the stage in which meiosis occurs.


Short sequence length polymorphisms; the presence of different numbers of short repetitive elements (mini- and microsatellite DNA) at one particular locus in different homologous chromosomes; heterozygotes represent useful markers for genome mapping.


The packing of the flattish nitrogen bases at the center of the DNA double helix.

staggered cuts

The cleavage of two opposite strands of duplex DNA at points near one another.

standard deviation

The square root of the variance.


A computed quantity, such as the mean, characteristic of a population.

statistical distribution

The array of frequencies of different quantitative or qualitative classes in a population.

stem cell

See blast cell.

steroid hormones

A class of hormones synthesized by glands of the endocrine system that, by virtue of their nonpolar nature, are able to pass directly through the plasma membrane of a cell. Steroid hormones act by binding to and activating transcription factors called steroid hormone receptors.

steroid hormone receptors

A family of related proteins that act as transcription factors when bound to their cognate hormones. Not all members of this family actually bind to steroids; the name derives from the first family member that was discovered, which was indeed a steroid hormone receptor.


A pure-breeding lineage, usually of haploid organisms, bacteria, or viruses.

structural gene

A gene encoding the amino acid sequence of a protein.

structural genomics

Characterizing and locating the entire set of genes in a genome.

subvital gene

A gene that causes the death of some proportion (but not all) of the individuals that express it.

sum rule

The probability that one or the other of two mutually exclusive events will occur is the sum of their individual probabilities.


A closed, double-stranded DNA molecule that is twisted on itself.


Phage infection of a cell that already harbors a prophage.


A mutation that can suppress a variety of other mutations; typically a nonsense suppressor.

suppressive petite

A petite that, in a cross with wild type, produces progeny of which variable non-Mendelian proportions are petite.


A secondary mutation that can cancel the effect of a primary mutation and result in wild-type phenotype.

suppressor mutation

A mutation that counteracts the effects of another mutation. A suppressor maps at a different site from the mutation that it counteracts, either within the same gene or at a more distant locus. Different suppressors act in different ways.


Close pairing of homologs at meiosis.

synaptonemal complex

A complex structure that unites homologs during the prophase of meiosis.

syncytial blastoderm

In insects, the stage of blastoderm preceding the formation of cell membranes around the individual nuclei of the early embryo.


A single cell with many nuclei.

synonymous substitutions

Nucleotide changes in the encoding part of a gene that do not result in a change in the amino acid sequence of the encoded protein.


Description of DNA segments in which the gene order is identical in different related species.



Thymine, or thymidine.

See tetratype.


The use of a piece of foreign DNA or a transposon to tag a gene so that a clone of that gene can be identified readily in a library.

tandem duplication

Adjacent identical chromosome segments.

targeted gene knockout

The introduction of a null mutation in a gene by a designed alteration in a cloned DNA sequence that is then introduced into the genome through homologous recombination and replacement of the normal allele.

tautomeric shift

The spontaneous isomerization of a nitrogen base to an alternative hydrogen-bonding condition, possibly resulting in a mutation.


A part of the Ti plasmid that is inserted into the genome of the host plant cell.

telocentric chromosome

A chromosome having the centromere at one end.


An enzyme that adds repetitive units to the ends of linear chromosomes to prevent shortening after replication, using a special small RNA as a template.


The tip (or end) of a chromosome.


The late stage of nuclear division when daughter nuclei re-form.

temperate phage

A phage that can become a prophage.

temperature-sensitive mutation

A conditional mutation that produces the mutant phenotype in one temperature range and the wild-type phenotype in another temperature range.


A molecular “mold” that shapes the structure or sequence of another molecule; for example, the nucleotide sequence of DNA acts as a template to control the nucleotide sequence of RNA during transcription.


An agent that interferes with normal development.

terminal redundancy

In a phage, refers to a linear DNA molecule with single-stranded ends that are longer than is necessary to close the DNA circle.

tertiary structure of a protein

The folding or coiling of the secondary structure to form a globular molecule.


A cross between an individual of unknown genotype or a heterozygote (or a multiple heterozygote) and a tester individual.


An individual homozygous for one or more recessive alleles; used in a testcross.

testicular feminization syndrome

A human condition, caused by a mutation in a gene encoding androgen receptors, in which XY males develop into phenotypic females.


Four homologous chromatids in a bundle in the first meiotic prophase and metaphase.

The four haploid product cells from a single meiosis.

tetrad analysis

The use of tetrads (definition 2) to study the behavior of chromosomes and genes during meiosis.

tetraparental mouse

A mouse that develops from an embryo created by the experimental fusion of two separate blastulas.


A cell having four chromosome sets; an organism composed of such cells.

tetratype (T)

A tetrad type containing four different genotypes, two parental and two recombinant.


Threonine (an amino acid).

three-point testcross

A testcross in which one parent has three heterozygous gene pairs.


The nucleoside having thymine as its base.


A pyrimidine base that pairs with adenine.

thymine dimer

A pair of chemically bonded adjacent thymine bases in DNA; the cellular processes that repair this lesion often make errors that create mutations.

Ti plasmid

A circular plasmid of Agrobacterium tumifaciens that enables the bacterium to infect plant cells and produce a tumor (crown gall tumor).

tissue-specific gene expression

The expression of a gene in a specific and reproducible subset of tissues and cells during the development of a higher eukaryote.


An enzyme that can cut and re-form polynucleotide backbones in DNA to allow it to assume a more relaxed configuration.


The ability of a cell to proceed through all the stages of development and thus produce a normal adult.


An untranslated segment at the 3′ end of mRNA.

trans conformation

In a heterozygote involving two mutant sites within a gene or gene cluster, the arrangement a1 +/+ a2.


The synthesis of RNA with the use of a DNA template.

transcription factor

A protein that binds to a cis-regulatory element (for example, an enhancer) and thereby, directly or indirectly, affects the initiation of transcription.


The movement of genes from a bacterial donor to a bacterial recipient with the use of a phage as the vector.


The process by which exogenous DNA in solution is introduced into cultured cells.


A gene that has been modified by externally applied recombinant DNA techniques and reintroduced into the genome by germ-line transformation.

transmembrane receptor

A protein that spans the plasma membrane of a cell, with the extracellular part of the protein having the ability to bind to a ligand and the intracellular part having an activity (such as that catalyzed by protein kinase) that can be induced on ligand binding.

transfer RNA

See tRNA.


The directed modification of a genome by the external application of DNA from a cell of different genotype.

Conversion of normal higher eukaryotic cells in tissue culture into a cancerlike state of uncontrolled division.

transgenic organism

One whose genome has been modified by externally applied new DNA.

transient diploid

The stage of the life cycle of predominantly haploid fungi (and algae) during which meiosis occurs.


A type of nucleotide-pair substitution in which a purine replaces another purine or a pyrimidine replaces another pyrimidine—for example, GC → AT.


The ribosome-mediated production of a polypeptide whose amino acid sequence is derived from the codon sequence of an mRNA molecule.


The relocation of a chromosomal segment to a different position in the genome.

transmission genetics

The study of the mechanisms of the passage of a gene from one generation to the next.

transposable genetic element

A general term for any genetic unit that can insert into a chromosome, exit, and relocate; includes insertion sequences, transposons, some phages, and controlling elements.


See translocation.


A mobile piece of DNA that is flanked by terminal repeat sequences and typically bears genes encoding transposition functions.


A type of nucleotide-pair substitution in which a purine replaces a pyrimidine or vice versa—for example, GC → TA.


The three nucleotide pairs that compose a codon.

triplet expansion

The expansion of a 3-bp repeat from a relatively low number of copies to a high number of copies, which is responsible for a number of genetic diseases, such as fragile X syndrome.


A cell having three chromosome sets or an organism composed of such cells.


Basically a diploid with an extra chromosome of one type, producing a chromosome number of the form 2n + 1.


A radioactive isotope of hydrogen.

tRNA (transfer RNA)

A class of small RNA molecules that bear specific amino acids to the ribosome during translation; the amino acid is inserted into the growing polypeptide chain when the anticodon of the tRNA pairs with a codon on the mRNA being translated.


Tryptophan (an amino acid).

true-breeding line or strain

See pure-breeding line or strain.

truncation selection

A breeding technique in which individuals in whom quantitative expression of a phenotype is above or below a certain value (the truncation point) are selected as parents for the next generation.

tumor-suppressor gene

A gene encoding a protein that suppresses tumor formation. The wild-type alleles of tumorsuppressor genes are thought to function as negative regulators of cell proliferation.

tumor virus

A virus that is capable of inducing a cancer.

Turner syndrome

An abnormal human female phenotype produced by the presence of only one X chromosome (XO).

twin spot

A pair of mutant sectors within wild-type tissue that are produced by a mitotic crossover in an individual of appropriate heterozygous genotype.

two-hybrid system

A pair of baker’s yeast vectors used for detecting protein–protein interaction. Each vector carries the gene for a different foreign protein under test; if the genes unite physically, a reporter gene is transcribed.

2-μ m (2-micrometer) plasmid

A naturally occurring extragenomic circular DNA molecule with a circumference of 2 μm, found in some yeast cells. Engineered to form the basis for several types of gene vectors in yeast.


Tyrosine (an amino acid).


Uracil or uridine.


A phenotypic relation in which the phenotypic expression of the heterozygote is less than that of either homozygote.

unequal crossover

A crossover between homologs that are not perfectly aligned.

uniparental inheritance

The transmission of certain phenotypes from one parental type to all the progeny; such inheritance is generally produced by organelle genes.

unstable mutation

A mutation that has a high frequency of reversion; a mutation caused by the insertion of a controlling element whose subsequent exit produces a reversion.


A pyrimidine base that appears in RNA in place of thymine, found in DNA.


Unassigned reading frame. An open reading frame (ORF) whose function has not yet been determined.


The nucleoside having uracil as its base.



Valine (an amino acid).


A property that may have different values in various cases.

variable region

In an immunoglobulin molecule, a region that shows many sequence differences between antibodies of different specificities. The variable regions of the light and heavy chains of an immunoglobulin bind antigen.


A measure of the variation around the central class of a distribution; the average squared deviation of the observations from their mean value.


An individual organism that is recognizably different from an arbitrary standard type in that species.


A specific numerical value of a variable.


The differences between parents and their offspring or among individuals in a population.


The occurrence within a tissue of sectors with differing phenotypes.


In cloning, the plasmid or phage chromosome used to carry the cloned DNA segment.


The probability that a fertilized egg will survive and develop into an adult organism.

viral transforming gene

A gene within a viral genome that can induce abnormal proliferation of cells in culture and, similarly, can induce tumors in infected whole animals.

virulent phage

A phage that cannot become a prophage; infection by such a phage always leads to lysis of the host cell.

VNTR (variable number tandem repeat)

A chromosomal locus at which a particular repetitive sequence is present in different numbers in different individuals or in the two different homologs in one diploid individual.


Western blot

Membrane carrying an imprint of proteins separated by electrophoresis; can be probed with a labeled antibody to detect a specific protein.

wild type

The genotype or phenotype that is found in nature or in the standard laboratory stock for a given organism.


The ability of certain bases at the third position of an anticodon in tRNA to form hydrogen bonds in various ways, causing alignment with several possible codons.


X:A ratio

The ratio between the X chromosome and the number of sets of autosomes.

X-chromosome inactivation

The process by which the genes of an X chromosome in a mammal can be completely repressed as part of the dosage compensation mechanism. See also dosage compensation; Barr body.

X hyperactivation

In Drosophila, the process by which the structural genes of the male X chromosome are transcribed at the same rate as the two X chromosomes of the female combined.

X linkage

The inheritance pattern of genes found on the X chromosome but not on the Y.

X-and-Y linkage

The inheritance pattern of genes found on both the X and the Y chromosomes (rare).

X-ray crystallography

A technique for deducing molecular structure by aiming a beam of X rays at a crystal of the test compound and measuring the scatter of rays.


Y linkage

The inheritance pattern of genes found on the Y chromosome but not on the X (rare).



The cell formed by the fusion of an egg and a sperm; the unique diploid cell that will divide mitotically to create a differentiated diploid organism.

zygotic induction

The sudden release of a lysogenic phage from an Hfr chromosome when the prophage enters the F cell, and the subsequent lysis of the recipient cell.

zygotically acting gene

A gene whose product is expressed only in the zygote and not included in the maternal contribution to the oocyte.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, W. H. Freeman and Company.
Bookshelf ID: NBK21878


Recent Activity

Your browsing activity is empty.

Activity recording is turned off.

Turn recording back on

See more...