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Genetics R - Z

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Terms

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R plasmid
a plasmid containing one or several transposons that bear resistance genes
random genetic drift
changes in allele frequency that result because the genes appearing in offspring do not represent 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)
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
reannealing
spontaneous realignment of two single DNA strands to re-form a DNA double helix that had been denatured
receptor element
a controlling element that can insert into a gene (making it a mutant) and can also exist (thus making the mutation unstable); both functions under the influence of the regulator element
recessive allele
an allele whose phenotype 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 translocation
a translocation in which part of one chromosome is exchanged with a part of a separate nonhomologous chromosome
recombinant
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
RF
recombinant frequency
recombinant frequency (RF)
the proportion (or percentage) of recombinant cells or individuals
recombination (in general)
any process in a diploid or partially diploid cell that generates new gene or chromosomal combinations not found in that cell or in its progenitors
recombination (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
recon
a region of a gene within which there can be 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
regression
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
regulatory genes
genes that are involved in turning on or off the transcription of structural genes
another name for repetitive DNA
redundant DNA
redundant (repetitive) DNA
DNA sequences that are present in many copies per chromosome set
replication fork
the point at which the two strands of DNA are separated to allow replication of each strand
replicon
a chromosomal region under the influence of one adjacent replication-initiation locus
reporter gene
a gene whose phenotypic expression is easy to monitor
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
restriction enzyme
an endonuclease that will recognize specific target nucleotide sequences in DNA and break the DNA chain at those points
restrictive conditions
environmental conditions under which a conditional mutant shows the mutant phenotype
retrovirus
an RNA virus that replicates by first being converted into double-stranded DNA
reverse transcriptase
an enzyme that catalyzes the synthesis of a DNA strand from an RNA template
reversion
the production of a wild-type gene from a mutant gene
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
RNA
ribonucleic acid
rRNA
ribosomal RNA
ribosome
composed of proteins plus rRNA
ribonucleic acid (RNA)
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 polymerase
an enzyme that catalyzes the synthesis of an RNA strand from a DNA template
ribosomal RNA (rRNA)
a class of RNA molecules, coded in the nucleolar organizer, that have an integral (but poorly understood) role in ribosome structure and function
S
Svedberg unit
Svedberg unit (S)
a unit of sedimentation velocity, commonly used to describe molecular units of various sizes
satellite
a terminal section of a chromosome separated from the main body of the chromosome by a narrow constriction
satellite chromosomes
chromosomes that seem to be additions to the normal genome
satellite DNA
DNA that forms a separate band in a density gradient because of its different nucleotide composition
scaffold
the central core of a eukaryotic nuclear chromosome, from which DNA loops extend
SCE
sister-chromatid exchange
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
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
second-site mutation
the second mutation of a double mutation within a gene
sector
an area of tissue whose phenotype is detectably different from the surrounding tissue phenotype
sedimentation
the sinking of a molecule under the opposing forces of gravitation and buoyancy
cytological segregation
the separation of homologous structures
genetic segregation
the production of two separate phenotypes corresponding to two alleles of a gene either in meiotic or mitotic segregation
segregational petite
a petite that in a cross with wild-type produces ½ wild-type progeny; caused by a nuclear mutation
selection coefficient (s)
the proportional excess or deficiency of a population and the mean of the individuals selected to be parents of the next generation
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
self
to fertilize eggs with sperms from the same individual
self-assembly
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
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
the phenotype of individuals heterozygotic for certain types of chromosome aberration; expressed as a reduced number of viable gametes and hence reduced fertility
senDNA
an amplified section of mtDNA found in senescent Podospora cultures; circular and plasmid-like in nature
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 linkage
the location of a gene on a sex chromosome
sexduction
sexual transmission of a donor E. coli chromosomal genes on the fertility factor
shotgun technique
cloning a large number of different DNA fragments as a prelude to selecting one particular clone type for intensive study
shuttle vector
a vector 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
signal sequence
the N-terminal sequence of a secreted protein, which is required for transport through the cell membrane
silent mutation
mutation in which the function of the protein product of the gene is unaltered
site-specific recombination
recombination occurring between two specific sequences that need not be homologous, and 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
the supercoiled arrangement of DNA in eukaryotic nuclear chromosomes
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 mutation
a mutation occurring in a somatic cell
somatic-cell genetics
asexual genetics, involving study of somatic mutation, assortment, and crossing-over, and of cell fusion
somatostatin
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
spacer DNA
repetitive 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
specific-locus test
a system for detecting recessive mutations in diploids; normal individuals treated with mutagen are mated to testers that ar homozygous for the recessive alleles at a number of specific loci; the progeny are then screened for recessive phenotypes
spindle
the set of microtubular fibers that appear to move eukaryotic chromosomes during division
splicing
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
spore
in fungi, somatic cells that are cast off to act either as gametes or as the initial cells for new haploid individuals
sporophyte
the diploid sexual-spore-producing generation in the life cycle of plants, i.e. the stage in which meiosis occurs
stacking
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
statistic
a computed quantity characteristic of a population, such as the mean
statistical distribution
the array of frequencies of different quantitative or qualitative classes in a population
strain
a pure-breeding lineage, usually of haploid organisms, bacteria, or viruses
structural gene
a gene encoding the amino-acid sequence of a protein
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
supercoil
a closed double-stranded DNA molecule that is twisted on itself
superinfection
phage infection of a cell that already harbors a prophage
supersuppressor
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
suppressor mutation
a mutation that counteracts the effects of another mutation; maps at a different site than the mutation it counteracts, either within the gene or at a more distant locus
synapsis
close pairing of homologs at meiosis
synaptonemal complex
a complex structure that unites homologs during the prophase of meiosis
syncytium
a single cell with many nuclei
T
thymine, or thymidine
tandem duplication
adjacent identical chromosome segments
tautomeric shift
the spontaneous isomerization of a nitrogen base to an alternative hydrogen-bonding condition, possibly resulting in a mutation
T-DNA
a portion 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
telophase
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
template
a molecular "mold" that shapes the structure or sequence of another molecule
teratogen
an agent that interferes with normal development
teratoma
a tumor composed of a chaotic array of different tissue types
terminal redundancy
in phage, 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
testcross
a cross of an individual of unknown genotype or a heterozygote (or a multiple heterozygote) to a tester individual
tester
an individual homozygous for one or more recessive alleles; used in a testcross
testicular feminization
the creation of an apparent female phenotype in an XY individual as a result of an X-linked mutation
tetrad
the four haploid product cells from a single meiosis
tetrad analysis
the use of tetrads 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
tetraploid
a cell having four chromosome sets; an organism composed of such cells
tetratype (T)
a tetrad type containing four different genotypes, two parental two and two recombinant
Thr
threonine (an amino acid)
three-point testcross
a testcross involving one parent with three heterozygous gene pairs
thymidine
the nucleoside having thymine as its base
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)
totipotency
the ability of a cell to proceed through all the stages of development and thus produce a normal adult
trans conformation
in a heterozygote involving two mutant sites within a gene or gene cluster
transcription
the synthesis of RNA using a DNA template
transdetermination
a specific change in the fate of an imaginal disk that can occur when the disk is cultured
transduction
the movement of genes from a bacterial donor to a bacterial recipient using a phage as the vector
transformation (first definition)
the directed modification of a genome by the external application of DNA from a cell of different genotype
transformation (second definition)
conversion of normal higher eukaryotic cells in tissue culture to a cancer-like 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
transition
for example, GC -> AT
translation
the ribosome-mediated production of a polypeptide chain whose amino-acid sequence is derived from the codon sequence of an mRNA molecule
translocation
the relocation of a chromosomal segment in a different position in the genome
transmission genetics
the study of the mechanisms involved in 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
transposon
a mobile piece of DNA that is flanked by terminal repeat sequences and typically bears genes coding for transposition functions
transversion
for example, GC -> TA
triplet
the three nucleotide pairs that compose a codon
triploid
a cell having three chromosome sets, or an organism composed of such cells
trisomic
basically, a diploid with an extra chromosome of one type, producing a chromosome number of the form 2n + 1
tritium
a radioactive isotope of hydrogen
tRNA
transfer RNA
transfer RNA (tRNA)
a class of small RNA molecules that bear specific amino acids to the ribosome during translation
Trp
tryptophan (an amino acid)
truncation selection
a breeding technique in which individuals in whom quantitative expression of a phenotype is above or below a certain value are selected as parents for the next generation
Turner's 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, produced by a mitotic crossover in an individual of appropriate heterozygous genotypye
2 µ ( 2 micron) plasmid
a naturally-occurring extragenomic circular DNA molecule; engineered to form the basis for several types of gene vectors in yeast
found in some yeast cells with a circumferance of 2µ
2µ (2 micron) plasmid
Tyr
tyrosine (an amino acid)
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
uracil
a pyrimidine base that appears in RNA in place of thymine found in DNA
URF (unassigned reading frame)
a DNA region, identified by nucleotide sequencing studies, that has an initiation and a termination codon, and therefore presumably represents a gene, but for which no phenotype or function is known
uridine
the nucleoside having uracil as its base
Val
valine (an amino acid)
variable
a property that may have different values in various cases
variance
a measure of the variation around the central class of a distribution; the average squared deviation of the observations from their mean value
variable region
the part of the antibody that binds to the antigen
variant
an individual organism that is recognizably different from an arbitrary standard type in that species
variate
a specific numerical value of a variable
variation
the differences among parents and their offspring or among individuals in a population
variegation
the occurrence within a tissue of sectors with differing phenotypes
vector
in cloning, the plasmid or phage chromosome used to carry the cloned DNA segment
viability
the probability that a fertilized egg will survive and develop into an adult organism
virulent phage
a phage that cannot become a prophage; infection by such a phage always leads to lysis of the host cell
wild-type
the genotype or phenotype that is found in nature or in the standard laboratory stock for a given organism
wobble
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 linkage
the presence of a gene on the X chromosome but not on the Y
X-and-Y linkage
the presence of a gene on both the X and 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 presence of a gene on the Y chromosome but not on the X (rare)
zygote
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
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