7-Methyl guanosine cap
a modification of eukaryotic mRNA, where a methylated guanine nucleotide is attached to the end of the mRNA molecule in a 5′–5′ linkage. The cap appears important in ribosome assembly at the appropriate initiation codon in the mRNA
A site
see aminoacyl-tRNA binding site
Acentric chromosome
a chromosome without a centromere
Acridine dye
a chemical mutagen known to produce shift mutations
Acrocentric chromosome
a chromosome with the centromere near one end
Alkylating agents
mutagens such as nitrogen mustard that produce mutations by adding methyl or ethyl groups to the bases (particularly guanine), causing loss of bases, transitions, or chromosome breaks
Allele
any of the various forms in which a gene can exist (e.g., a1, a2, A)
Allopatric populations
geographically separated populations
Allopolyploidy
a euploid in which the multiple chromosome sets are of different origin, from either different species, races, or populations
Ames test
analysis for detecting mutagenic chemicals using bacterial strains
Aminoacyl-tRNA binding site (A site)
once translation is initiated, the site on a ribosome where an entering tRNA molecule is bound by its anticodon to the mRNA codon
Aminoacyl-tRNA synthetases
enzymes that activate tRNA by attaching the correct amino acid to its specific tRNA molecule
Amniocentesis
a diagnostic procedure for determining genetic abnormalities in utero; involves removal of amniotic fluid containing fetal cells, which are cultured and used to screen for chromosomal aberrations, DNA markers, and certain protein defects
Amphidiploid
or allotetraploid; tetraploid species in which the genome contains the diploid chromosome complement of both parent species
Analysis of variance (ANOVA)
the statistical breakdown of observed variation into its components
Aneuploidy
change in chromosome number involving loss or gain of less than a complete set of chromosomes (for example, 2n + 1, 2n − 1)
Antibody
a protein (immunoglobulin) that reacts with specific substances (antigens) that are foreign to the organism
Anticodon
a three-base sequence of nucleotides on tRNA that complements a codon on mRNA
Antigen
a substance that elicits the production of antibodies when introduced into an animal
Ascospores
sexual spores in the pink bread mold Neurospora crassa, occurring as eight haploid spores confined in a sac called an ascus; result of one meiotic cycle that produces four haploid spores, which then divide by mitosis, producing the eight spores
Assortative mating
a nonrandom mating; see negative assortative mating, disassortative mating, and positive assortative mating
Attached-X chromosome
in Drosophila, refers to two X chromosomes attached by one centromere
Attenuation
control of transcription termination at specific regulatory sequences in bacterial operons
Autopolyploidy
a euploid condition in which the extra chromosome sets are derived from within the same species
Autoradiography
a technique for detecting the presence of radioactivity within a sample by overlaying the sample with a photographic emulsion (e.g., X-ray film) and allowing the emitted radiation to expose the film. The location of the radioactive substance (in a cell, on a chromosome, in a gel, etc.) is revealed by developing the film
Autosome
any chromosome other than the sex chromosomes
Auxotroph mutant
a mutant microorganism that is dependent on specially supplemented media to supply molecules it is unable to synthesize
BAC
see bacterial artificial chromosome
Back mutation
see reverse mutation
Bacterial artificial chromosome (BAC)
an E. coli cloning vector based on the F factor plasmid, widely used in the Human Genome Project due to its stability and ability to accommodate cloned inserts between 100 and 300 kb
Bacteriophage
also termed phage: viruses that infect bacterial cells
Balanced lethal
a method of retaining recessive lethals at high frequencies in a population; a situation in which linkage assures the maintenance of the recessive lethals in a system of permanent heterozygotes, in which only the heterozygote is viable
Balanced polymorphism
the maintenance of several different genotypes in a population, due to different selection pressures on each type; common example: heterozygous individuals have superior fitness
Barr body
a dark-staining body in the nucleus of mammals that is the result of inactivation or heterochromatinization of all but one of the X chromosomes
Base analogue
a chemical that is functionally similar to a base but structurally different; may undergo tautomeric shifts more often than the normal bases
Binomial distribution
a probability distribution in which the expected frequencies of classes are obtained by binomial expansion
Bivalent
a pair of homologous chromosomes during synapsis at prophase I of meiosis
Canalized characters
characters in which the phenotype is normally unaffected by environmental stresses or by underlying genetic variability
Carrier
an individual who is heterozygous for a recessive allele
Catabolite repression
the lowered expression of many bacterial operons when glucose is present in the growth medium; mediated by decreased levels of cyclic AMP
cDNA
DNA that has been enzymatically synthesized from an RNA template (i.e., “complementary” DNA) (see reverse transcriptase)
cDNA library
a collection of numerous cDNAs (spliced into a vector) that is representative of an mRNA population at a given time or in a given tissue during development
CentiMorgan (cM)
the genetic unit of crossing over, equal to 1 percent crossing over
Centric fusion
the translocation of two nonhomologous acrocentric or telocentric chromosomes with the loss of one of the centromeres; leads to reduction in chromosome number
Centriole
a self-replicating cytoplasmic body found in animal cells, located at each of the poles during mitosis and meiosis, and appearing to have spindle fibers radiating from it
Centromere
a chromosomal region to which the spindle fibers attach during mitosis and meiosis
Chi-square test
a statistical test for measuring the significance of the discrepancy between observed and expected results; also written χ2
Chiasma (pl. chiasmata)
the visible evidence of exchange (crossing over), seen in prophase I of meiosis
Chimera
another term for mosaic
Chromatid
a strand of a replicated chromosome that is attached by a centromere to an identical sister strand
Chromatin
the complex of protein and nucleic acids that makes up the eukaryotic chromosome, which may change in state as a result of changes in the division cycle or developmental programming
Chromomere
local coiled regions of the linear DNA thread that give the eukaryotic chromosome a beaded appearance. In Drosophila’s polytene chromosomes, these coils are in register and produce the banded appearance of the chromosomes
Cis-acting factor
a site in DNA, such as a promoter or regulatory region (e.g., Lac operator), that must be adjacent or physically linked to a gene to exert an effect
Cis-configuration
linkage arrangement in which the alleles of two linked genes are on the same homologue, and the recessive alleles are on the other (for example, ab versus AB) or are coupled; opposite is trans-configuration, or noncoupling (Ab versus aB)
Cistron
a sequence of DNA coding for a single polypeptide (or tRNA or rRNA product) as defined by the complementation test
Class intervals
subdivisions arbitrarily made in frequency distributions for a quantitative trait to facilitate presentation and analysis; for example, one-inch class intervals used to subdivide human height
Codominance
type of dominance in which alleles express their product independently in heterozygotes, so that a heterozygote for two such alleles expresses products from both; seen, for example, in blood-group antigens and protein electrophoretic alleles
Codon
a section of mRNA, three bases in length, that codes for an amino acid
Coefficient of coincidence
the ratio of the observed frequency of double crossovers to the expected frequency of double crossovers
Coefficient of regression (b)
the slope of the line of y on x
Coefficient of variation (CV)
a statistic used to compare the variability of different samples; defined as a percentage, 
· 100, where s = standard deviation and 
= the mean; produces an estimate of variability that is independent of the sample measurement
Complementary gene action
refers to the interaction of nonallelic genes in the production of a specific phenotype. Specific alleles from each gene are necessary for the expression of the phenotype
Complementation
the ability of two recessive mutations to restore the normal (wild) phenotype in trans-linkage
Complementation test
test used to identify functional alleles; based on the fact that in trans-linkage a mutant phenotype is produced if two mutants are alleles, but a wild-type phenotype (complementation) is produced if they are functionally different genes
Concatemer
a tandemly repeating nucleotide sequence found in viruses, in which the entire virus genome is multiply repeated
Conditional mutation
an allele that produces a mutant phenotype in certain environments, but a wild phenotype under other conditions. The environment is called permissive if wild-type is produced, and restrictive if it produces a mutant phenotype. The mutants are often lethal (conditional lethals)
Confidence limits
intervals, established around the observed frequency, within which we can be relatively sure that the true frequency is found; set by using the normal distribution curve and standard deviation intervals (usually the 95 percent and 99 percent confidence intervals)
Conjugation (in bacteria)
a unidirectional transfer of genetic information, following the pairing of a donor and a recipient cell
Consanguinity
the sharing of at least one common ancestor; mating between close relatives
Constitutive
the production of gene products continuously, without regard to the cell’s requirements; often referring to an operon in which there is a mutation in one of the control genes, so that the cell no longer responds to repression or to induction
Constitutive heterochromatin
heterochromatin that is identical on a given chromosome pair and forms a permanent structural component of homologous chromosomes
Contingency chi-square test
a statistical test used to compare one set of observations to another set to see whether the results are dependent or independent of the conditions under which the observations were made
Continuous variation
variation among phenotypes, measured on some scale, that cannot be divided into a few distinct categories (see discontinuous variation)
Copy choice
a hypothesis to explain intrachromosomal recombination in which pairing of two parental chromosome sequences allows the copying process to begin on one strand and then switch to the other parental strand
Correlation
when two or more characteristics are dependent on, and vary with, each other; positive correlation: one characteristic increases as the other increases; negative correlation: one characteristic increases as the other decreases
Correlation coefficient
a statistical measure of the relationship between two characteristics; measures the extent to which variations in one variable are related to variations in another
Cosmid
a recombinant DNA vector that can “infect” a bacterial cell packaged within a bacteriophage protein coat, but is capable of replicating as a plasmid after entrance. Vector DNA sequences (cos sites) required for packaging are minimized, permitting large amounts of foreign DNA to be included in phage heads and subsequently introduced into cells
Coupling linkage
see cis-configuration
Covariance
a variance component used in calculating the correlation coefficient
cpDNA
chloroplast DNA
Cri du chat syndrome
a human condition associated with a deletion at the tip of chromosome 5 and characterized by congenital abnormalities and a thin “mewing” cry in newborn infants
Crossing over
the events leading to genetic recombination between linked genes
Cyclical selection
a shifting selection that results in selection in one direction for one or a few generations or seasons, then selection in the opposite direction for one or a few generations or seasons
Cytokinesis
the division of the cytoplasm during cell division
Cytoplasmic determinant
molecules in the egg that are sequestered during cleavage to a particular region of the embryo and then influence the developmental programming of the cells that inherit them
Cytoplasmic inheritance
inheritance of genetic traits from determinants that are not located on the chromosomes: extranuclear inheritance
Degenerate code
a code in which there is more than one codon for the same amino acid
Degrees of freedom (d.f.)
the number of independently variable classes in an experiment (for example, in a typical χ2 analysis, where d.f. equals the number of classes minus one)
Deletion
a loss of a part of a chromosome
Deme
a local inbreeding group in which individuals have an equal chance of mating with all members of the opposite sex
Deoxyribonucleic acid (DNA)
Organic molecule that is the basis of heredity. A double-stranded structure composed of nucleotides (phosphate group, 5-carbon deoxyribose sugar, and purine or pyrimidine base)
Deoxyribose
the 5-carbon sugar found in a DNA nucleotide
Dicentric
a chromosome with two centromeres; often the result of recombination within a heterozygous paracentric inversion
Dideoxyribonucleotides
analogues of the normal DNA precursor molecules that lack 3′ hydroxyl residues. Incorporation of dideoxyribonucleotides into DNA terminates chain elongation and is the basis for the DNA sequencing technology developed by Frederick Sanger
Diester bond
the bond in the DNA backbone between the 3′ carbon of the sugar of one nucleotide and the phosphate attached to the 5′ carbon of the adjacent nucleotide
Diesterase
an enzyme that breaks diester bonds
Differentiation
the developmental process by which apparently unspecialized cells form tissues and structures of adult forms
Dihybrid cross
a cross between two individuals having different alleles at two loci (for example, AaBb)
Diploid
having two sets of chromosomes; found in the somatic cells of most higher organisms; represented as 2n
Directional selection
selection favoring an extreme phenotype
Disassortative mating
the mating of unlike phenotypes or genotypes (see assortative mating)
Discontinuous variation
phenotypic variation that can be divided into a few distinctive classes (for example, brown fur and white fur)
Disruptive selection
selection of two or more different genotypes or phenotypes in a single population or species; results in an increase in phenotypic variance
DNA
see deoxyribonucleic acid
DNA chip
see microarray
DNA sequencing
a technique that permits the base-by-base determination of the linear array of nucleotides within a DNA chain
Dominant
an allele that manifests itself phenotypically in heterozygotes
Dosage compensation
a term used to describe the fact that phenotypic effects of sex-linked genes are similar, whether an individual carries one (for example, X chromosome in a man) or two (X chromosome in a woman) gene doses
Down syndrome
or trisomy 21; human condition associated with aneuploidy for chromosome 21 and characterized by mental retardation; a flat, round face; and simian crease in the palm
Duplication
the presence of a section of a chromosome in excess of the normal amount
Dyad
a chromosome having two sister chromatids attached at a single centromere
E site
exit site; a region on the ribosome where the tRNA (i.e., deacylated tRNA) attaches after its amino acid has been transferred in the P site
Effective population size
the number of mating pairs contributing to the next generation
Electrophoresis
a technique used to detect variation in proteins, involving the use of an electric field to cause the proteins to migrate along a gel and then observing their relative positions on the gel by protein-specific staining reactions
Elongation factors
proteins that are necessary for translation to proceed
Endonuclease
an enzyme that produces an internal cut in the sugar–phosphate backbone of DNA (for example, in repair of thymine dimers, in which a specific endonuclease cuts the backbone at a nucleotide adjacent to a thymine dimer)
Enhancer
cis-acting regulatory sequence that alters chromatin structure of the promoter, thereby increasing the rate of transcription; may be at some distance upstream or downstream from the genes they regulate or can be within the transcribed region (e.g., within an intron)
Epigenesis
the theory that during development new tissues and organs appear that were not present in the original zygote; replaced preformation
Epigenetic change
a change in gene state that can affect phenotype and be transmitted during cell division but does not involve mutation of the DNA; this can be accomplished through reversible modifications of the genetic material, such as DNA methylation
Episome
a circular piece of self-replicating double-stranded DNA, additional to the normal genome of the bacterial cell; can either exist independently in the cytoplasm or become an integral part of the host’s chromosome; there is not full agreement as to the definition of plasmid and episome
Epistasis
the interaction between two nonallelic genes, such that one interferes with the phenotypic expression of the other
EST
see expressed sequence tag
Euchromatin
light-staining chromatin that carries most of the typical genes of an organism
Eugenics
controlled breeding of humans to eliminate unfavored genotypes or select for favored ones
Eukaryote
an organism that has chromosomes composed of DNA and protein, nuclear membranes, and other membrane-bound organelles
Euploidy
a normal chromosome complement or a complete multiple of a haploid set
Exit site
see E site
Exons
the coding regions of a gene that are separated from each other by intervening sequences (introns)
Exonuclease
an enzyme that removes the terminal nucleotide of nucleic acids
Expressed Sequence Tag (EST)
sequence information derived from the recovery of cDNA by either PCR or cloning; since the sequence represents an mRNA molecule, it is a marker for a region of the genome that was transcribed in a particular tissue type
Expressivity
the degree of phenotypic expression in those individuals showing a certain trait
F factor
the sex or fertility episome in bacterial donor cells; gives bacteria that have it the ability to form conjugation tubes
Facultative heterochromatin
heterochromatin that varies in different cell types at different stages of development, or even from one homologous chromosome to another (for example, the Barr body)
Fitness
in the genetic sense, a term referring to the ability to produce offspring in a particular environment; the relative reproductive success of a genotype
Founder principle
a term referring to the small sample of genetic information carried by a group of individuals establishing a new population; one form of genetic drift
Frameshift
a mutation caused by deletion or addition of nucleotides in DNA, which has the effect of changing the reading frame of codons, causing the wrong sequence of amino acids to be produced
Frequency distribution
an arrangement of statistical data that demonstrates the number of occurrences of each value in a data set
G-banding
banding patterns of chromosomes that are produced by treating chromosomes with trypsin and then staining with Giemsa. Most heterochromatin stains darkly, and most euchromatin stains lightly with this technique
Gamete
a haploid cell; in multicellular organisms is usually called the egg or the sperm
Gene
a nucleotide sequence coding for mRNA, tRNA, or rRNA
Gene conversion
the process by which, in a heterozygote, one allele of a gene is lost and is replaced by the other allele; the consequence of partial heteroduplexes and the use of either strand to repair the helix
Gene frequency
a measure of the commonness of an allele in a population; the proportion of each allele of a gene in a population
Gene pool
the total genetic information in a population, as represented, for example, by the genes in the gametes of a population
Genetic drift
changes in gene frequency due to random sampling of the gene pool each generation; caused by populations being limited in size rather than being infinite
Genetic homeostasis
the maintenance of genetic variability in a population in the face of all forces acting to reduce it; probably arises from the necessity for the maintenance of certain levels of heterozygosity to ensure normal development
Genetic load
a complex concept that one might describe as the extent to which a population departs from a genetic constitution producing an optimum phenotype in all members
Genome
the total complement of genes characteristic of a species; has different usages, which should be clear from the context: (1) for viruses and bacteria, the entire set of genes; (2) for eukaryotes, all the genes present in the monoploid set of chromosomes (in diploids, sometimes called the haploid genome): for example, the allopolyploid Raphanobrassica, derived from the radish (Raphanus), and the cabbage (Brassica), which contains both the Raphanus and Brassica (diploid genomes); (3) the genetic makeup of an individual diploid organism
Genomic imprinting
an epigenetic mechanism seen in mammals, where certain genes are selectively inactivated depending on whether they are passed through the maternal or paternal parent; imprinted genes are inherited and transmitted in somatic tissues in an inactivated state but can be reactivated in the germline
Genotype
the total genetic information contained in an organism or the genetic constitution of an individual with respect to the genes under consideration
Gynandromorph
sexual mosaic usually in insects, in which male and female tissues are present in the adult; in Drosophila, usually the result of the loss of an X chromosome in a developing female; in the parasitic wasp Bracon hebetor, the result of the fertilization of one nucleus of an infrequent binucleate egg, producing diploid (female) and monoploid (male) tissues
Gyrase
jargon for a type II topoisomerase that relaxes closed-circle DNA supercoils in E. coli. Important in DNA replication
Haploid
having one set of unpaired chromosomes; found in cells such as gametes; the haploid chromosome number (1n) is the same as the number of linkage groups; also termed monoploid
Haplotype
the genetic makeup of a single chromosome; commonly used to describe the genotype of a set of closely linked genes such as the histocompatibility complex. A genetic pattern that arises from polymorphisms at any given genetic locus or closely linked group of loci, such as differences in restriction fragment sizes due to individual chromosomal variations in the location of restriction enzyme sites
Hardy–Weinberg equilibrium
rule stating that gene frequencies do not change from one generation to the next if no outside forces are acting upon them
Helicase
an enzyme that unwinds the DNA helix ahead of the replicating DNA polymerase
Helix-destabilizing protein (HDP)
a protein that binds to single-stranded DNA at the replication fork and prevents the parental strands from reannealing (single-strand binding protein)
Helix-relaxing protein (gyrase)
relaxes the tension caused by the unwinding of the double helix during replication; acts by breaking a phosphodiester bond in one of the parental strands ahead of the replication fork
Helix-unwinding protein
a protein that causes the unwinding of the DNA strands during replication, creating the replication fork
Hemizygous
genes present only once in the genotype, such as the genes on the X chromosome in male flies or mammals
Heritability
the proportion of phenotypic variation in a given trait in a specified population that can be accounted for by genetic segregation
Heterochromatin
dark-staining chromatin that is believed to be primarily structural and to include condensed, inactivated regions of the chromosome and only a few functioning genetic loci
Heteroduplex
a double strand of DNA in which the bases within the helix are not totally complementary; may be the result of recombination between single strands of the homologues
Heterogametic sex
the sex that produces two kinds of gametes relative to the sex chromosomes: in humans, the male (XY), since his gametes contain either an X or a Y chromosome; in birds, the female
Heterogeneous nuclear RNA (hnRNA)
precursor mRNA, which must be processed before it becomes mRNA and leaves the nucleus; processing involves splicing, capping on the 5′ end, and adding a repeat adenine chain (poly A) on the 3′ end
Heteroplasmy
cell or organism carrying two or more genetically different mitochondrial or chloroplast genomes
Heterosis
superiority of a hybrid to either parental type; superiority of heterozygous genotypes
Heterozygote
an organism having different alleles at a given locus on homologous chromosomes
Hfr (high-frequency recombination)
a strain of E. coli in which the F factor has incorporated into the bacterial chromosome inducing conjugative transfer of the bacterial chromosome
Histocompatibility gene
a gene that belongs to either the major histocompatibility (MHC) system or other minor complexes. The systems are responsible for the histocompatibility molecules (antigens)
Histocompatibility molecules
cell-surface antigens that cause rejection of cells or tissue that does not have them. They are genetically encoded glycoproteins
Histones
basic proteins (of five major types) that complex with the phosphate groups of DNA and are involved in the packing of DNA
hnRNA
heterogeneous RNA; the various forms of RNA as they are processed by intron splicing and other events in the eukaryotic nucleus; the primary transcript, excised introns, and the forms of pre-mRNA
Holandric
a gene that is found only on the Y chromosome, and thus will never be passed from a father to his daughters
Homeobox
a 180 bp DNA region that codes for the DNA binding site, the “homeodomain,” of proteins that regulate cell fates; found near the 3′ end of homeotic genes
Homeotic mutant
mutation in a gene defining developmental fate of specific segments, causing one body part to replace another (e.g., Antennapedia, in which a leg forms in place of the antenna in Drosophila)
Homogametic sex
the sex that produces one kind of gamete relative to the sex chromosomes; contrast with heterogametic sex
Homologous chromosomes
chromosomes that pair during meiosis and contain the same gene loci; diploid organism normally receives one chromosome of the pair from each parent
Homozygote
an organism having the same allele at a given locus on homologous chromosomes
Hox genes
homeobox-containing genes that specify spatial information during development; four clusters of Hox genes in mice are homologous to the single series of homeotic selector genes (i.e., homeobox genes) of Drosophila; the homologous genes in humans are called HOX genes
Hybrid zones
zones where species barriers have broken down and viable fertile hybrids are being produced
Hydrophilic molecules
molecules that interact with water
Hydrophobic molecules
molecules that are insoluble in water
Hypothesis
a generalization made to explain a set of observations; in science, should be heuristic – that is a type that leads to predictions beyond the observations that generated it
Imaginal disc
a small group of cells in insect larvae that are destined to develop into adult parts
In situ hybridization
the ability to detect a specific nucleic acid sequence within a biological structure (e.g., genes on a chromosome, mRNA within a cell) through complementary base pairing with a nucleic acid probe (see nucleic acid hybridization)
Inbreeding
mating between relatives or genotypically similar individuals
Inbreeding coefficient
a value that measures the probability that two genes in a zygote are identical
Inbreeding depression
deterioration of fitness, due to inbreeding
Incipient species
superspecies; a monophyletic group with differences distinct enough to warrant separation above the species level, but not yet reproductively isolated
Incomplete dominance
type of dominance observed when the phenotype of the heterozygote is intermediate between, and distinguishable from, those of the homozygotes
Independent assortment
the random combination of alleles of two or more genes, due to the behavior of nonhomologous chromosomes or effectively unlinked chromosome segments during meiosis, since the orientation of one pair of homologous chromosomes at the equatorial plate in cell division is independent of the orientation of other pairs
Independent event
event whose occurrence or nonoccurrence does not affect the occurrence or nonoccurrence of another event; probability of two or more independent events occurring together calculated by multiplying their individual probabilities
Informosomes
cytoplasmic particles of protein and mRNA found in the egg, which are then activated in some way during differentiation
Initiation factors
enzymes that produce the complex of mRNA, ribosomal subunits, and the start tRNA, thus initiating translation
Insertion sequences (IS)
transposable elements in bacteria that can insert anywhere in the genome: insertion within a coding region may result in a nonfunctioning gene; also can remove themselves from the insertion site, often resulting in reversion of mutations or further mutation such as deletions, or breakage; when excluded from the DNA, form loops called “hairpin loops” or “lollypops,” because of repeat sequences at the ends of the IS element
Interference (chromosomal interference)
the effect that one crossover event has on the occurrence of a second crossover event
Intergenic suppressors
mutations in one gene that suppress other mutations in different genes
Intervening sequence
noncoding nucleotide sequence in eukaryotic DNA, which separates coding portions of the gene; also called intron
Intragenic suppressors
mutations that suppress other mutations in the same gene
Introgressive hybridization
hybridization that results from backcrossing to one of the parental populations
Intron
an intervening sequence
Inversion
a change in gene order within a chromosome, involving two breaks and a turning around of the segment between the breaks
Isoallele
allelic difference that can only be detected under special conditions; for example, elevated temperature, which can uncover thermal lability differences among gene (or allele) products that appear identical under normal conditions
Isochromosome
a chromosome in which two genetically identical arms are attached to the same centromere; arms are mirror images of each other
Isolating mechanisms
mechanisms that prevent gene exchange between populations
Karyotype
the descriptive morphologic characteristics and number of chromosomes of a particular organism; usually, description of chromosomes as they appear in mitotic metaphase
Klinefelter syndrome
a human condition associated with sexual aneuploidy; occurs in individuals bearing one Y chromosome and two or more X chromosomes, who are externally male, although usually sterile and phenotypically feminized
Lethal
a mutation that causes death before reproductive age (may be recessive or dominant, a point mutation, or a chromosomal mutation)
Ligase
an enzyme that joins nucleotides together
LINEs
Long Interspersed Elements; nonviral retrotransposons; in mammals they are from about 1 to 5 kb in length, and the human genome carries from 20,000 to 40,000 copies
Linkage disequilibrium
occurrence of alleles of two genes together more often than predicted by their individual frequencies
Linkage group
genes located on the same chromosome; same in number as the haploid chromosome number
Locus
the physical position of a gene on a chromosome
LOD score
Log of odds, a statistical treatment of pedigree or similar data used to establish the probability of linkage in complex traits
Lysis
the bursting of a cell by the destruction of the cell wall and/or membrane
Maternal effect
type of development seen in offspring that, instead of expressing its own genotype, expresses the genotype of the mother, for example, because of products that are laid down in the egg and that act early in development
Mean (
)
the average value of a set of numerical data; the sum of all values, divided by the total number of data points
Meiosis
the single replication of chromosomes, followed by a sequence of two cell divisions, the first a reduction division and the second similar to a mitotic division; result: four nuclei that are haploid in genetic makeup
Messenger RNA (mRNA)
the RNA that carries the code sequence for polypeptide chains
Metacentric chromosome
a chromosome in which the centromere is located near the center
Microarray
a matrix of defined DNA fragments, usually oligonucleotides, which are attached in a known order to a solid matrix, such as a glass slide; target molecules from complex populations (e.g., labeled mRNAs isolated from a particular tissue type) can be hybridized to the array and the amounts and types of target molecules quantified; gene expression in thousands of targets can be analyzed in the same experiment
Microsatellite
tandemly repeated sequence involving from one to four nucleotides; used as markers for genetic mapping and screening; microsatellite sequences are associated with some disease conditions (e.g., Huntington disease, fragile-X syndrome, nonpolyposis colorectal cancer); also referred to as simple tandem repeat (STR) or simple sequence repeat (SSR)
Minisatellite
a simple DNA sequence (15–100 nucleotides) repeated about 10–50 times and located at more than one chromosomal site. Restriction enzyme polymorphisms surrounding these sites provide the basis for performing DNA typing using hybridization probes complementary to minisatellite sequences
Mitosis
the replication of chromosomes and their orderly division into a pair of nuclei having the same number and kind of chromosomes as the original parent nucleus
Monohybrid cross
a cross between two individuals heterozygous at a single locus
Monoploid
see haploid
Monosomic
a term applied to individuals lacking one of a pair of homologous chromosomes
Morgan unit
a unit of genetic map length, equal to 100 centiMorgans
Mosaic
also termed chimera; gene or chromosomal difference in tissues that occurs after zygote formation
mtDNA
mitochondrial DNA
Multiple-factor inheritance
inheritance in which several genes influence the phe-notype of one quantitative character; also called polygenic inheritance
Mutation
any detectable and heritable change in the genetic material that is not caused by segregation or recombination
Mutation equilibrium (
)
situation in which gene frequencies have become stable; forward and back mutations no longer cause a change in them (see unstable equilibrium)
Mutation frequency
the proportion of alleles in a population that are mutant
Mutation rate
the probability of a particular mutational event taking place per biological entity per generation
Muton
the basic unit of gene mutation; the smallest alterable element of a gene (a nucleotide)
Mutually exclusive event
an event whose occurrence precludes the occurrence of another event (for example, a flipped coin will be either a head or a tail)
Negative assortative mating
the mating of unlike phenotypes or genotypes; experimentally, mating of parents chosen and sorted on the basis of their dissimilarity
Nondisjunction
the irregular distribution of chromosomes or chromatids during anaphase of either mitosis or meiosis; results in either addition or loss of chromosomes
Nonhomologous chromosomes
chromosomes that contain different genes and that do not pair during meiosis
Nonsense codon
codons that do not code for an amino acid; specifically, UAA, UAG, and UGA; result in termination of translation, and hence are also called terminator or stop codons
Normal distribution
also called the normal curve or Gaussian distribution; defined by the equation: 
where is the mean and σ is the standard deviation
Northern blot
the identification of a specific RNA within a biological sample by separating the RNA molecules by gel electrophoresis, transfer of the gel contents to a filter membrane, and detection and quantification of the filter-bound RNA by hybridization to a nucleic acid probe
Nucleic acid hybridization
the detection or characterization of a particular nucleic acid sequence in a biological sample via its ability to base-pair or hybridize to a complementary nucleic acid probe molecule through Watson–Crick hydrogen-bonding interactions
Nucleolus
a dark-staining nuclear organelle of eukaryotes; the location of the genes coding for ribosomal RNA; rich in rRNA
Nucleoside
the combination of a nitrogenous base and a 5-carbon sugar; can be either a ribonucleoside or a deoxyribonucleoside
Nucleotide
the monomeric unit that makes up DNA or RNA, formed by the addition of a phosphate group to a nucleoside
Null hypothesis
a statistical hypothesis that assumes that two samples are drawn from the same population or that a sample fits a specified model (for example, a 3:1 ratio); accumulation of observations or measurements and application of appropriate statistical tests permit the null hypothesis to be rejected
Okazaki fragments
small fragments of DNA produced during replication along one strand of a replication fork because of the requirement that DNA nucleotides be added in the 5′ to 3′ direction; may be found in both strands when there are numerous replicating points
Oocyte
a female sex cell from which the haploid egg is produced
Operator
a regulatory DNA region at one end of an operon; acts as the binding site for repressor proteins
Operon
a set of contiguous genes that transcribe a single mRNA unit; function is controlled by regulatory genes
Overdominance
type of dominance observed when the phenotype of the heterozygote falls outside the range of either homozygote
p arm
the term for the short arm of a human chromosome
P site
see Peptidyl-tRNA binding site
Panmixia (or panmixis)
random mating
Paracentric inversion
an inversion in a chromosome that does not involve the centromere
Parameter
values of a population from which a sample has been taken
Parthenogenesis
the production of offspring from a female gamete without fertilization by a male gamete; occurs in many different forms, some of which involve nongenetic participation by a male
Pedigree
a diagram that shows ancestral relationships among members of a family over several generations
Penetrance
the frequency with which a gene manifests itself in the phenotype of the heterozygote (if dominant) or of the homozygote (if recessive)
Peptide bond
the formation of a covalent bond between the carboxyl end of one amino acid and the amino end of another amino acid
Peptidyl-tRNA binding site (P site)
one of two tRNA binding sites on the ribosome; the growing peptide chain will be transferred from this site to the A site following the formation of a peptide bond by peptidyl transferase, associated with the large ribosomal subunit
Peptidyl transferase
the enzyme that forms a peptide bond between amino acids during translation
Pericentric inversion
an inversion in a chromosome that includes the centromere
Petite mutations
mutations in strains of yeast that result in lack of mitochondrial function
Phenocopy
an alteration of a phenotype by environmental factors such that the phenotype mimics that produced by a specific gene
Phenotype
the observed characteristic of an individual; the result of the interaction of the genotype with the environment
Phosphodiester bond
a covalent bond between two of a phosphate group’s oxygens and two carbon-containing groups. 5′C—O—P—O—C3′
Photoreactivation
repair of thymine dimers, which requires the cells to be exposed to visible light
Plaque
the formation of an area of lysed cells on a confluent lawn of bacteria, resulting from the clonal propagation of a single infection through adjacent host cells in the population
Plasmid
a circular piece of self-replicating DNA that is additional to the normal genome of the bacterial cell and does not become integrated in the host genome (cf. episome)
Pleiotropism
one gene with more than one phenotypic effect
Point mutation
a mutation within a gene, in contrast to chromosomal mutation
Polar body
any of three nuclei produced during meiosis in the female that get little cytoplasm and will not become an egg
Polar mutation
a mutation in one gene that affects the expression of downstream genes in a prokaryotic polycistronic transcription unit
Poly(A) tail
a posttranscriptional modification of eukaryotic mRNA, in which a series of adenine bases are enzymatically added to the 3′ end of the mRNA molecule
Polygene
gene of relatively small effect; contributes, along with environmental influences, to the determination of a quantitative trait phenotype
Polymerase chain reaction (PCR)
the amplification of a unique segment of DNA, often from a heterogeneous DNA population, through progressive cycles of primer annealing, DNA polymerization, and strand denaturation. The specificity of the amplification is controlled by using two oligonucleotide primers complementary to the DNA flanking the target DNA sequence
Polymerases
enzymes that catalyze the assembly of nucleotides into RNA or DNA
Polymorphism
having two or more forms in a population; it can refer to genetic differences (e.g., dominant and recessive alleles, chromosome inversion polymorphism) or to phenotypic differences (e.g., protein electrophoretic mobility, melanism in peppered moths, ABO blood type)
Polymorphic locus
a genetic difference (chromosomal structure, nucleotide, allele, or other heritable condition) in which the most common form is at a frequency of less than 99 percent; some researchers use 95 percent as the cutoff value; this is a frequency that cannot be explained simply by recurrent mutation
Polyploid
any organism having a number of complete chromosome sets that exceeds that of the diploid (for example, 3n, 4n)
Polysome
several ribosomes aligned along a single mRNA strand
Population
(1) Mendelian population: a group of sexually interbreeding or potentially interbreeding individuals in a defined area; (2) statistical population: a large group of potential observations from which a sample is taken
Position effect
a change in the phenotypic expression of a gene, caused by a change in its position in relation to other genes
Positive assortative mating
the mating of like phenotypes or genotypes; experimentally, mating of parents chosen and sorted for similarity
Preformation
theory that there was an exact miniature replica of an adult in either the egg or sperm; supplanted by the epigenetic view in the eighteenth and nineteenth centuries
Pribnow box
in E. coli, a region of the promoter found 10 nucleotides upstream (−10 position) of the transcription start site
Primary oocyte
the original diploid egg cell that produces the polar bodies and the ovum by meiotic divisions
Primary spermatocytes
a group of diploid cells, produced mitotically from spermatogonial cells of the testes, that undergo meiosis to produce the haploid sperm cells
Primase
an RNA polymerase that is part of the primosome. It produces the RNA primers for DNA replication on the “lagging” strand
Primer
a substrate that is necessary for polymerization (for example, during DNA synthesis) and that is similar to, and later replaced by, the subunits of the reaction product (for example, RNA nucleotides replaced by DNA nucleotides)
Primosome
protein complex, including primase, required for the initiation and synthesis of the Okazaki fragments used in DNA replication. Without the primase it is often called a preprimosome
Proband
the affected person who first calls a genetic condition to the attention of a physician; important in pedigree analysis; see propositus, proposita
Prokaryotes
organisms that lack nuclear membranes and do not have histones associated with their DNA (for example, bacteria and blue-green algae)
Promoter
a regulatory region at one end of an operon that acts as the binding site for RNA polymerase
Prophage
the DNA of a temperate bacteriophage that is integrated into the genome of the host bacterium
Proposita
the term for proband, if the person is female
Propositus
the term for proband, if the person is male
Protein primary structure
the sequence of the amino acids in the polypeptide chain
Protein quaternary structure
two or more polypeptide chains bound together by one or more bonds
Protein secondary structure
the configuration imposed on the polypeptide chain by its peptide linkages; e.g., the alpha helix or beta sheet
Protein tertiary structure
the bending and folding of a protein secondary structure due to the interactions of specific amino acids (creating disulfide bonds, for example)
Prototroph
a nutritionally independent cell that can live on a chemically defined minimal medium
Pseudoalleles
genetic loci that are functionally allelic but structurally nonallelic; produce similar phenotypic effects but are capable of recombining with each other
Pseudodominance
type of dominance that results when a recessive allele expresses itself in a heterozygote, caused by a deletion of the dominant allele on the homologous chromosome
Purine
an adenine or guanine nitrogenous base found in RNA and DNA nucleotides
Pyrimidine
nitrogenous base found in RNA and DNA nucleotides; in DNA the two pyrimidines are cytosine and thymine; in RNA they are cytosine and uracil
q arm
the term for the longer arm of a human chromosome
Q-PCR
quantitative PCR, the application of RT-PCR to the detection of small amounts of a particular mRNA in a heterogeneous mRNA population; with gene-specific primers and appropriate controls, this assay can also be used to measure accurately the amounts of particular mRNAs expressed in small amounts of tissue, and it is often used to monitor gene expression or to detect pathogens in biological samples
QTL
quantitative trait locus; see also polygene
Quantitative characters
traits in which the phenotype is measured on a continuous scale, caused by polygenic segregation and environmental influences (for example, height, IQ, or amount of pigment)
Recessive gene
an allele that expresses itself only when it is homozygous (or hemi-zygous)
Reciprocal translocation
exchange of chromosomal material between two non-homologous chromosomes
Recombination
a process occurring during normal meiosis that gives rise to linked combinations of alleles that differ from those found in the parents; see crossing over
Recon
the unit of genetic subdivision at which recombination occurs (between one nucleotide and the next)
Regression
a statistical method for predicting one character by using another that is correlated to it; on a graph, represented by a straight line, drawn with the formula 
, where y and x are values of the correlated characters and b is the slope of the line or coefficient of regression of y on x
Releasing factors
proteins that bring about the cleavage of the translated protein from the tRNA molecule and the ribosome
Repetitive DNA
type of DNA occurring in eukaryotes, with nucleotide sequences in multiple copies ranging from a few short sequences to as many as a million copies; may be composed of a tandem sequence or be dispersed among the unique sequences of the genome
Replication
the process by which the two strands of DNA separate and each serves as a template for the synthesis of a new complementary strand
Repulsion linkage
when nonallelic mutants are on different homologous chromosomes, C f/c F (see cis-configuration)
Restriction endonuclease
an enzyme that internally cleaves a double-stranded DNA molecule after binding to a specific nucleotide recognition sequence
Restriction fragment length polymorphism (RFLP)
variation in the length of a discrete segment of genomic DNA following restriction endonuclease digestion, due to population differences in the distribution of enzyme sites flanking the sequence. Variations are inherited in a codominant manner and are useful in linkage studies (see haplotype) and DNA typing
Reverse mutation
a heritable change in a mutant gene that restores the original nucleotide sequence or the original amino acid
Reverse transcriptase
RNA-dependent DNA polymerase; an enzyme capable of polymerizing a DNA chain in the 5′ to 3′ direction, copying from a single-stranded RNA template
Rho
the factor that binds with RNA polymerase during RNA synthesis, causing the release of a newly formed RNA chain, as well as of the RNA polymerase and the DNA template
Ribose
the 5-carbon sugar found in RNA nucleotides
Ribosomal RNA (rRNA)
any of several forms of RNA that are a part of the structural unit called a ribosome
Ribosomes
cellular organelles composed of complex proteins and rRNA; the site of protein synthesis
Roentgen unit (r)
measurement of ionizing radiation; each roentgen produces 1 electrostatic unit of charge in 1 cubic centimeter of air
Rolling circle replication
the method of replicating circular DNA in which one strand is used as a template and is copied many times, producing concatemers
RT-PCR
reverse transcriptase/polymerase chain reaction, where mRNA is first converted into a single-stranded DNA using reverse transcriptase and a primer, then amplified using a heat-stable DNA polymerase and two specific primers flanking the region of amplification; can be used to produce large amounts of a particular cDNA for cloning without the need for screening cDNA libraries
Sampling error
variation due to the limited size of a sample
Secondary oocyte
a product of first-division meiosis in which reduction has caused one nucleus (the secondary oocyte) to receive most of the cytoplasm and the other to become the first polar body
Secondary spermatocytes
male haploid sex cells produced by meiotic division of the primary spermatocytes
Segregation
the separation of homologous chromosomes (and thus alleles) during the first meiotic division
Segregation distortion
unequal representation of alternate alleles in the gametes, due to abnormalities of segregation during meiosis
Segregational load
also termed balanced load: refers to the situation in which the heterozygote is superior (has higher relative fitness) to both homozygotes; results in the retention of all three genotypes in the population
Selection
the preferential reproduction and survival of some genotypes, along with the elimination of other genotypes
Selection coefficient (s)
a measure of the force acting on each genotype to reduce its adaptive value
Selection pressure (I)
also termed selection intensity; the difference in survival rates between the optimal (so) and suboptimal (ss) phenotypes, multiplied by the frequency of the suboptimal type in the population (fs); I = (so − ss) · fs
Semiconservative replication
refers to DNA replication in which each of the strands in the double helix is used as a template to produce a new complementary strand, so that after replication the two DNA molecules are each composed of one old and one new strand
Semilethal
a lethal mutation causing death before reproductive age in more than 50 percent, but less than 100 percent, of the carriers
Sex-influenced
autosomal characters whose expression is conditional on the sex of the organism. For example, the index finger in humans is shorter than the ring finger. The gene that has the major effect on this trait is recessive in females and dominant in males
Sex-limited
autosomal or sex-linked genes that are present in both sexes but phenotypically express themselves in only one sex, such as testicular feminization or milk production rate in cattle
Sex-linked
genes that are found on the sex chromosomes; usually used interchangeably with the term X-linked, referring to genes on the X chromosome
Sexduction
the incorporation of bacterial genes by sex factors and their transfer by conjugation
Shine–Dalgarno sequence
a specific sequence in prokaryotic mRNA, located just 5′-ward of the start codon, that can base-pair with 16S ribosomal RNA and is important in initiating protein synthesis at the appropriate starting point in translation
Sibling species
species that are morphologically very similar but that normally do not mate or, if forced to mate, usually do not produce viable offspring
Sigma
factor involved in RNA synthesis that helps E. coli RNA polymerase to recognize the initiation point for transcription
Simple tandem repeat (STR)
simple sequence repeat (SSR), see microsatellite
SINEs
Short Interspersed Elements, such as the Alu family that makes up more than 10 percent of the DNA in humans; the nucleotide repeat monomer is usually shorter than 300 bp
Single nucleotide polymorphism (SNP)
DNA marker where a single base pair difference at a particular site in the genome is sufficient to distinguish different individuals; the development of high-density SNP maps in the human genome is aiding the characterization of multifactorial traits
Single-strand binding protein (SSBP)
protein that binds to single-strand DNA during replication. It prevents the reannealing of the DNA molecule
SNP
see single nucleotide polymorphism
snRNPs
small nuclear ribonucleoprotein particles; complexes of small nuclear RNA (snRNA) and proteins; several associate and are required for splicing/processing of primary mRNA transcripts
Somatic cell genetics
a subdivision of genetics that studies asexually reproducing body cells. The field uses cell fusion, somatic cell assortment, and crossing-over techniques. Somatic cell genetic engineering deals with insertion of genes and attempts to correct hereditary defects
Somatic cell genetics
an artificial segregation technique resulting from the loss of human chromosomes from rodent–human hybrid cell fusions. The presence or absence of a human characteristic (protein, enzyme activity, human RFLP) is correlated with the human chromosome complement retained or lost in the hybrid cells
Southern blot
a technique, developed by E. M. Southern, whereby DNA fragments generated by restriction enzyme digestion are separated by electrophoresis, transferred to a filter membrane, and detected by nucleic acid hybridization to a specific nucleic acid probe
Species
groups of phenotypically similar organisms that can interbreed and produce viable offspring
Spermatocyte
germinal cell produced in the testes; see primary spermatocyte and secondary spermatocyte
Spindle
a set of fibers or microtubules that attach to the centromeres during nuclear division and move the chromosomes to opposite poles of the cell
Spliceosome
enzymatic complex involved in the elimination of introns and joining of exons in eukaryotic mRNA
Stabilizing selection
selection against extreme phenotypes, with corresponding selection for the intermediate type
Standard deviation (s.d.)
the square root of the variance; symbols are s.d. or s for a sample, and s for the population from which the sample was taken; in a normally distributed population, 68 percent of the population falls within ±1 sof the mean, 95 percent within ±1.96 s, and 99 percent within ±2.58 s
Standard error (s.e.)
the square root of the variance of the mean: 
where s is the standard deviation and n is the sample size
Start codon
the codon on mRNA that is recognized as the starting point for the formation of a polypeptide chain, AUG, which codes for methionine
Statistics
all values computed from sample data
Structural alleles
two functional alleles (that is, two forms of a given gene) are structural alleles if they involve changes in exactly the same nucleotide
Suppressor mutation
a second mutation, at a different site, that totally or partially restores a function lost as a result of an earlier mutation
Sympatric populations
populations occupying the same geographic area
Synapsis
the pairing of homologous chromosomes during prophase I of meiosis
Synaptonemal complex
a structure present during prophase I of meiosis that produces the pairing of homologous chromosomes
t test
a statistical test used to compare the differences between two distributions
Tandem repeats
short DNA sequences that are duplicated numerous times and lie adjacent to each other
TATA box
in eukaryotes, a region of the promoter found approximately 25 nucle-otides upstream of the start site (–25), with a consensus sequence of TATAAA; also know as Goldberg–Hogness box
Tautomeric shift
a reversible change in the location of a proton in a molecule; for example, in DNA nucleotide in amino form (NH2) is changed to an imino (NH), resulting in mispairing of bases
Telocentric chromosome
a chromosome with the centromere at one end
Telomere
region found at the tips of chromosomes; unique in being “nonsticky” – that is, chromosome pieces will not attach to it
Temperate phage
a bacteriophage that can either incorporate its DNA into the genome of the bacteria or cause the lytic response
Template DNA
DNA that serves as a sort of macromolecular mold for the correct synthesis of a complementary DNA or RNA strand
Terminalization
the movement of the chiasmata toward the ends of the bivalents during prophase I of meiosis
Testcross
a cross between an individual having a dominant phenotype and a homo-zygous recessive individual; used to determine whether the dominant phenotype is homozygous or heterozygous
Tetrad
a group of four associated chromatids of a bivalent; a synapsed pair of dyads
Tetrad analysis
the study of crossing over in organisms that keep the products of a single meiotic division together (the tetrad). Useful in the analysis of crossing over
Tetraploid
organism containing four sets of chromosomes (4n)
Three-point testcross
a genetic cross using three linked marker alleles to map the linkage distance between these genes; involves crossing a heterozygote for the three markers to a homozygous recessive individual
Thymine dimer
two adjacent thymine bases connected by a double covalent bond; often results when DNA is hit by ultraviolet radiation
Time of entry mapping
using the times of transfer of genes from one bacterial cell to another during conjugation as a measure of relative genetic map positions
Trans-acting factor
a diffusible molecule, such as a protein transcription factor, capable of binding to a promoter or enhancer region in DNA and regulating gene expression
Trans-configuration
linkage arrangement in which the mutant alleles of two linked genes are on different homologues (Ab and aB); also termed repulsion (noncoupling), as opposed to cis-configuration, in which the alleles are on the same homologue and demonstrate coupling
Transcription
the process by which RNA is produced from a DNA strand through the action of DNA-directed RNA polymerase
Transcription factor
any of a large number of proteins that bind to DNA regions upstream or in the termination region of a gene and influence transcription
Transduction
the transfer of genes from one cell to another cell by means of a viral vector; generalized transduction: a random fragment of the bacterial chromosome is incorporated into a phage coat and can be transferred; specialized transduction: the result of improper excision of a prophage, so that the defective phage carries a specific fragment of the bacterial chromosome
Transfer RNA (tRNA)
the RNA molecules that carry amino acids to the ribosomes
Transformation
the picking up of extracellular “naked” DNA by a bacterium and the incorporation of this DNA into its genome by recombination
Transition
a base-pair substitution in which the purine–pyrimidine orientation is preserved; that is, a purine is replaced by a purine or a pyrimidine by a pyrimidine
Translation
the process by which an mRNA sequence is used to produce a polypeptide chain
Translocation
the transfer of a section of one chromosome to a nonhomologous chromosome
Transposon
a transposable DNA sequence bounded by insertion sequences
Transversion
a base-pair substitution in which the purine–pyrimidine orientation is not preserved; a purine is replaced by a pyrimidine, or vice versa
Triploid
an organism containing three sets of chromosomes (3n)
Trisomic
an organism that possesses three homologues of one chromosome
True-breeding
homozygous
Turner syndrome
a human condition in which an X chromosome is missing (XO); affected individuals are externally female, although they do not mature sexually
Two-point-testcross
mating of a dihybrid (heterozygous for two genes, AaBb) to the homozygous recessive (aabb)
Unstable equilibrium
a situation in which gene frequencies are not stable; a disturbance will cause the genes to adopt a new frequency or cause one of the genes to go toward fixation (see mutation equilibrium)
Variable number of tandem repeat (VNTR) locus
an individual genetic locus that shows extreme allelic polymorphism in the population due to variations in the number of tandem repeats of a minisatellite core sequence. With appropriate probes and hybridization conditions, these loci can be distinguished from other minisatellite members and used as genetic markers in linkage studies
Variance
a measure of the extent to which values within a distribution depart from the mean; symbol is s2, for a sample, and σ2 or 
2 for the population from which the sample was taken: ![s2=(Σx2−x¯2[n])/n−1](genetic-analysis.files/image416.jpg){kind=small}
where xrepresents each measurement and n is the sample size
Vector
in recombinant DNA technology, a DNA molecule that can replicate independently and is used to propagate a foreign DNA insert in a heterologous prokaryotic or eukaryotic system
Virulent phage
a bacteriophage that provokes a lytic response in which the bacterial cell lyses and releases new virus particles
Western blot
the separation of proteins from one another by gel electrophoresis, the direct transfer of those proteins to a membrane, and the probing of that membrane with a specific antibody to detect and quantify the amount of a particular protein present in a biological sample
Wild type
the typical or normal phenotype or allele; it is the most common representative form of a species
Wobble hypothesis
the idea that certain bases at the 5′ end (third base position) of the anticodon in tRNA are capable of forming hydrogen bonds with several different nitrogen bases at the 3′ end of the codon
Yates’s correction factor
the subtraction of 1/2, or 0.5, from the absolute value of the observed deviation minus the expected deviation in the chi-square test; corrects for continuity and adds to the accuracy of chi-square determination whenever expected classes are small; formula for chi-square thus becomes ![x2=Σ([|O−E|−0.5]2/E)](genetic-analysis.files/image417.jpg){kind=small}
Yeast artificial chromosome (YAC)
a recombinant DNA vector in which a very large foreign DNA insert (> 106 base pairs) is coupled to an origin of replication and a centromere and is flanked by telomeres, allowing clonal propagation in yeast
Zygote
a diploid cell produced by the union of the egg and sperm