Sampling Variance (of mean). The square of the standard deviation of the sample means(that is, standard
error).
Sampling. The process of
selecting a part of the population to represent it as a whole.
Satellite Chromosome. A chromosome
that seems to be an addition to the normalgenome.
Satellite chromosome: Each chromosome with a secondary
constriction; thisconstriction divides a satellite part from the rest of the
chromosome (arm)
Satellite DNA. Any type of
DNA having highly repetitive nucleotide sequence. It wasformerly defined as the
DNA forming a satellite band after cesium chloride densitygradient
centrifugation.
Satellite DNA: A highly repetitive DNA, composed of
repeated hepta- to decanucleotidesequences; DNA of different buoyant density; a
minor DNA fractionthat has sufficiently different base composition from the
bulk of the DNA in orderto separate distinctly during cesium chloride density
gradient centrifugation; itcan derive from nucleus, plastid, or mitochondrial
Satellite. A terminal
section of a chromosome that is separated from the main body of thechromosome
by a narrow constriction.
Saturation Mutagenesis. Induction and
recovery of large numbers of mutations in onearea of the genome or in one
function, in the hope of identifying all the genes in thatarea or affecting
that function.
Saturation mutagenesis: Induction and recovery of large
numbers of mutationsin one area of a genome, or in one biological function, in
order to identify all thegenes in that area, or affecting that function
Savannas. Tropical/sub-tropical
grasslands usually interrupted with scattered patches ofshrubs/trees. These
occur in areas in which warm-rainy season is interrupted by along, cool season
dry spell. This long dry spell favours large seeded annuals as well asplants
with rhizomes, tubers, stolons, bulbs, etc.
Science. The systematic and
real knowledge about a thing. The word has been derivedfrom a Latin word – sciencia
– that means learning or knowledge in its widest sense.The science pursued
for the sake of its own existence is called basic science.However, the
science that becomes a means of producing material necessities of asociety is
referred to as applied science.
Second Cycle selection. A method of
improving inbred lines. It consists of crossing twoinbred lines that complement
each other in desirable attributes accompanied byselection for desirable
recombinations in the segregating generations. Also calledpedigree selection.
Second division restitution (SDR): Due to premature
cytokinesis before thesecond meiotic division takes place; sister chromatids
end up in the same nucleus
Second Division. The second of
the two divisions of a normal meiosis; also calledequation division.
Second Filial Generation. The progeny
generation obtained by selfing or crossing of F1generation plants. It is
designated by F2. Subsequent generations obtained by selfingthis F2 generation
plants are called F3, F4, …., etc.
Second Site Mutation. The second
mutation of a double mutation within a gene. In manycases, the
second-site mutation suppresses the first mutation resulting in
wild-typephenotype of the double mutant.
Secondary Centres of Origin. The areas having appreciable extent of genetic diversityof a plant
species. Notice that these areas are found far removed from the primarycentres
of origin of crop plants. Now the concept of centres of origin has bee revised(see
centres of origin).
Secondary Gene Pool. Genetic
resources that include biological species having somebarriers of crossability
with the cultivated species (primary gene pool). Chromosomepairing at meiosis
is not normal. However, special techniques (e.g., embryo rescue)can overcome
barriers of crossability and lead to normal seed set.
Secondary gene pool: Species in the secondary gene pool
include those fromwhich genes can be transferred to the cultivated species,
however, with moredifficulties as compared to species
Secondary mapping populations Populations
developed by crossing two lines/individuals selected from a mapping population
and created mainly for fine mapping of the genomic region of interest.
Secondary Species. (See digenomic
species).
Secondary Structure (of protein). A spiral or zigzag arrangement of the polypeptidechain.
Secondary Trisomics. Trisomics with
a secondary or isochromosome as the additionalone (2n + one isochromosome).
Total number of secondary trisomics is equal to thediploid chromosome number of
the species. Only a few trisomics has been reported inbarley.
Secondary Triticales. See Triticales.
Sector. An area of tissue with
a distinct phenotype from that of the rest of the tissue.
Sectorial. Of a chimera
in which the genetically distinct tissues are cross-sectionallyarranged as
sectors of a circle.
Sedimentation. The sinking of
a molecule under the opposing forces of gravitation andbuoyancy.
Seed certification: A procedure developed as a means of
assuring that seedshave a high standard of purity and quality
Seed. A mature ovule with
normal coverings. A seed consists of the seed coat, embryo,and in certain
plants, an endosperm.
Seedling Resistance. A category of
resistance that is expressed in the early stage ofplants and may persist even
in the later growth stages.
Segment. A portion of a
chromosome taken as a unit under given circumstances.
Segregating population: A population, generally the progeny of
a cross, in which genetic differences are detectable, thus permitting
identification ofindividuals having a desired trait and
Segregation distortion A significant
deviation of the observed segregation ratio for a marker locus from the
expected ratio.
Segregation. Separation of
paternal from maternal chromosomes at meiosis andconsequent separation of
different alleles from each other leading to the possibility ofrecombination in
the offspring. It may also be the production of two separatephenotypes,
corresponding to two alleles of a gene, either in different individuals(meiotic
segregation) or in different tissues of the same individual
(mitoticsegregation).
Segregation: The process whereby alleles are
separated from one another as aresult of meiosis. Segregation can only occur
with respect to genes which are in theheterozygous state in the parental plant
Selectable marker: A gene, often encoding resistance to
an antibiotic or anherbicide, introduced into a group of cells to allow
identification of those cells thatcontain the gene of interest from the cells
that do not. Selectable markers are usedin genetic engineering to facilitate
identification of cells that have incorporatedanother desirable trait that is
not easy to identify in individual cells.
Selection (field): The process of discriminating among
geneticvariability to advance a fraction to the next generationor breeding
cycle.
Selection (in vitro): A method to retain specific cells
(orclones of cells) expressing a specific trait, such as antibioticor herbicide
resistance, while killing off all other cells that donot express that trait.
Selection bias In case of SIM,
the tendency to detect only large effect QTLs, leading to an upward bias in the
effect size estimates for the detected QTLs.
Selection Coefficient. A quantitative
measure of intensity of selection, designated by ‘s’.It refers to proportional
reduction in the gametic contribution of a particular genotypecompared to the
standard one, usually most favoured. The contribution of the most fitgenotype
is taken as unity and the contribution of the genotype selected against is “1-s”.
Selection Differential Reproduction
rates for different genotypes.
Selection Differential. The average phenotypic
superiority of the selected individualsover the base population of their
origin. The s.d., which is symbolised by S, is ameasure of the selection
applied.
Selection differential: In artificial selection, the
difference in mean phenotypicvalue between the individuals selected as parents
of the following generation andthe whole population
Selection index A numerical
score that combines information on all the traits associated with the dependent
variable, usually, yield.
Selection Intensity. The percentage
of the population selected.
Selection intensity: The ratio of the number of genotypes
selected divided bythe number of genotypes tested
Selection Limit. The point at
which there is no selection response. The limit is attainedafter the selection
has exhausted entire genetic variability.
Selection nucleotide An arbitrary
nucleotide added to the 30 end of the primers to reduce the number of different
fragments amplified by PCR.
Selection Pressure. The intensity
of selection usually measured by the change of genefrequency per generation
under the influence of selection. Selection pressure isintense if the intensity
of selection is the highest.
Selection Progress. See selection
response.
Selection Response. The difference
between the mean of the progeny of the selectedindividuals and the mean of the
original population. Quantitatively, it is expressed as:Rs = Sd×h2 = k×σP×h2.
Numerically, it is equal to the genetic gain. It is also calledselection
progress.
Selection response: The difference between the mean of the
individuals selectedto be parents and the mean of their offspring; it is
expressed by the formula R = h2× S (h = heritability, S = selection
coefficient, R = phenotypic difference between themean of all selected
fractions and the mean of total population)
Selection. A process in which
individuals with certain characteristics are favoured inreproduction. It is the
discrimination among individuals in the number of offspringcontributed to the
next generation. It is thus a non-random process that causesindividuals with
different genotypes to be represented unequally in subsequentgenerations.
Natural selection operates only for traits that are necessarily related to
thesurvival and reproduction of the individuals. Artificial selection, which is
practised byplant breeders, on the other hand, may act mostly in contrary to
the natural selection.However, it is a purposeful process with goals set by
breeders. In both evolution andplant breeding, selection is a primary force
that alters the genetic composition of thepopulation. It is also the result of
the process, that is, a plant, line, or strain thatoriginated by a selection
process is referred to as selection. Selection, whether naturalor
artificial, acts through differential reproductive rates (fitness) of the
genotypes.Suppose a locus with two alleles in a diploid species. Three possible
genotypes areAA, Aa, and aa (and WAA, WAa, Waa are their
fitness, respectively). Frequencies ofalleles A and a are
represented by p and q, respectively. After one generation
ofselection, the frequency of A becomes p′. Thus p′ = p (WA/W), whereas WA is
meanfitness of A alleles (= pWAA+ qWAa), and W is the mean fitness of
the population (=p2WAA + 2pqWAa + q2Waa = pWA + qWa). Therefore, if (WA/W) >
1.0, p′ is largerthan p and vice-versa. Thus, the change in the gene frequency
after one generation ofselection can be given by p = (p′- p) =
[pq (WA-Wa)/W]. This change can be zero, ifeither p or q is zero or there is no
difference between mean fitness of the two alleles(WA-Wa=0). Selection
increases the frequency of superior allele(s); therefore, it is aprocess
that increases mean fitness of the population.
Selection: The process determining the relative
share allotted individuals ofdifferent genotypes in the propagation of a
population; natural selection occurs ifzygotic genotypes differ with regard to
fitness
Selective Advantage. The relative
advantage of a genotype when it competes for survivaland reproduction against
another in the same population. In artificial selection, s.a. isconferred by
the breeder on some genotypes because they contain some desiredcharacteristics.
For example, modern semi-dwarf varieties of wheat and rice are poorcompetitors
in mixture, but have been propagated by careful nurturing since they arehigh
yielders in isolation.
Selective advantage: An advantage for survival of a
genotype in a population and for production of viable progeny as compared to
other genotypes, which mayshow a selective disadvantage with respect to fitness
and viability.
Selective breeding: Making deliberate crosses or matings
of organisms so theoffspring will have particular desired characteristics
derived from one or both of the parents.
Selective disadvantage: Inferior
fitness of one genotype compared to others inthe population
Selective genotyping An extension of
the BSA approach; the population is phenotyped for the trait of interest, 30–50
plants/lines with extreme high and a similar number of plants with extreme low
phenotypic values for the trait are selected, plants/lines are subjected to
precision phenotyping and genotyping for a large number of markers, and the
data from the two groups are analyzed to identify the markers linked to the
target trait.
Selective mapping A small sample
is drawn from the population on the basis of chromosomal breakpoints, and the
individuals are genotyped with a large number of new markers, which are
assigned to appropriate bins.
Selective Neutrality. A situation in
which different alleles of a gene are adaptivelyneutral, that is, each allele
confers equal fitness to the corresponding genotypes. Forexample, under
situation of selective neutrality, both homozygotes (AA and aa)
andthe heterozygote (Aa) will have equal fitness. This condition gives
rise to a passive orneutral equilibrium instead of a stable one
such that an allele frequency p is as goodas any other q.
Therefore, the presence of a widespread polymorphism is notnecessarily the
evidence for superior heterozygotes.
Selective System. An
experimental technique that enhances the recovery of specific butusually rare
genotypes.
Selective Value. The relative
survival value and the reproductive ability of a genotype ina population in any
given environment; also called adaptive value, fitness, biologicalfitness
or Darwinian fitness.
Self Assembly. The ability of
certain multimeric biological structures to assemble fromtheir component parts
through random movements of the molecules and formation ofweak chemical bonds
between surfaces with complementary shapes.
Self Pollination. The transfer
of pollen grains from an anther of a flower to the stigma ofthe same flower or
of different flowers of the same plant or clone. Its geneticconsequence is the
same if different plants with the same genetic make-up are crosspollinated(e.g.,
cross pollinating members of a pure line, of a heterozygote, Aa, andof a
clone).
Self-fertile. Capable of
setting seed following fertilization after self-pollination.
Self-Fertility. Capability of
producing seed upon self-fertilization.
Self-Fertilization. Fusion of male
and female gametes from the same individual.
Self-Incompatibility. Genetically
controlled physiological hindrance to self-fruitfulness.
Self-incompatibility: Controlled physiological hindrance to
self-fertilization; inability to set seed
Selfish DNA: A segment of the genome with no
apparent function other than to ensure its own replication
Self-pollination: The transfer of pollen from anther to
stigma of the same plant
Self-sterile. Incapable to
set seed due to failure either to produce functional gametes or tocomplete
fertilization after self-pollination.
Self-sterility: The inability
of some hermaphrodites to form viable offspring byself-fertilisation
Semiconservative Replication. The established model of DNA replication in which eachof the two
parental strands acts as a template for the synthesis of a new strand so
thateach daughter DNA molecule has one parental strand and one newly
polymerisedstrand.
Semiconservative replication: Replication of DNA in which the
moleculedivides longitudinally, each half being conserved and acting as a
template for theformation of a new strand
Semigamy. An abnormal
type of fertilisation in which a male gamete enters into the eggcell but there
is no karyogamy (no fusion of nuclei). Both sperm and egg nuclei
divideindependently so that the embryo contains separate sectors of paternal
and maternaltissues. It can be utilised to produce haploids in cultivated
cotton.
Semi-isolation. Incomplete
isolation between two populations, allowing incidental geneflow.
Semisterility. A condition in
which plants are characterised by having reduced number ofviable gametes and
hence reduced fertility. Plants heterozygotic for certain types ofchromosome
aberrations (e.g., inversion/translocation heterozygotes) usually showthis
condition. Nevertheless, plants show sterility for male gametes more
frequentlythan for female ones.
Sense mutation: A mutation that changes a termination
(stop) codon into onethat codes for anSense strand DNA: The DNA strand
with the same sequence as Mrna
Sequence capture A targeted SNP
discovery strategy applied to specific genomic regions.
sequence characterised amplified
region (SCAR): A molecular
marker; theDNA amplified as a PCR derived marker (eg. RAPD) is sequenced; the
sequenceis used to develop a new PCR marker comprising a single DNA species,
unique tothe target organism or genotype
Sequence characterized amplified regions A marker
derived from an RAPD marker; a pair of forward and reverse primers, usually, of
20–24 nt long, specific for the two terminal sequences of the RAPD marker, are
used for PCR amplification.
Sequence tagged site (STS): Short (200 to 500 base pairs) DNA
sequencethat has a single occurrence in the human genome and whose location and
basesequence are known. Detectable by polymerase chain reaction, STSs are
usefulfor localising and orienting the mapping and sequence data reported from
manydifferent laboratories and serve as landmarks on the developing physical
map ofthe human genome. Expressed sequence tags (ESTs) are STSs derived from
cDNAs.
Sequence Tagged Site. A relatively
small sequenced region of a cloned genomicfragment that can be used by a
computer to align the cloned fragment into a contig.
Sequence-related amplified polymorphism A marker based
on amplification of open reading frames (ORFs); one primer targets exons and
the other targets introns and promoters.
Sequence-specific amplification polymorphism A modification
of AFLP; the restriction fragments are amplified using one AFLP primer and one
primer based on a conserved sequence of a transposable element (TE). Syn.,
transposon display procedure.
Sequence-tagged microsatellite profiling
A modification of AFLP; one AFLP primer and one primer based on a SSR sequence
(anchored at its 30 end) are used for amplification of the restriction
fragments.
Sequence-tagged site A locus that
can be unambiguously defined by a pair of primer sequences that are used for
its amplification.
Sequence-tagged-microsatellites (STMs)
marker: Primers
constructed fromthe flanking regions of microsatellite DNA which can be used in
PCR reactions toamplify the repeat region
Sequencing by synthesis DNA sequencing
based on DNA synthesis.
Sequencing depth For a specific
nucleotide, the total number of all reads, in which the given genomic position
or base pair is represented. For the whole genome, the average number of times
each base of the entire genome of an individual has been sequenced.
Sequencing: Determination of the order of
nucleotides (base sequences) in aDNA or RNA molecule or the order of amino
acids in a protein.
Sex Cell. A gamete.
Sex chromatin: A condensed mass of chromatin
representing an inactivated Xchromosome
Sex Chromosome. A chromosome
whose presence or absence is correlated with the sexof the bearer owing to its
role in the sex determination, for example, XX-XY systemin humans or WW-WZ
system in birds.
Sex chromosomes: The X and Y chromosomes in human
beings that determinethe sex of an individual. Females have two X chromosomes
in diploid cells; males have an X and a Y chromosome. The sex chromosomes
comprise the 23rdchromosome pair in a karyotype. Compare autosome.
Sex Controlled Traits. Hereditary
traits that appear more frequently in one sex than inthe counterparts. For
example, pattern baldness occurs more frequently in men than inwomen. The gene
is located in the autosomes. Its expression is influenced by thespecific
hormone. Also called sex-influenced traits.
Sex Cycle. The
alternation of karyogamy and meiosis in eukaryotes.
Sex Determination. Any process by
which sex of an individual is established. Themechanism or process may be
chromosomal, genetic or environmental.
Sex Limited Traits. Traits that
are limited to one sex only. However, genes conditioningsuch traits are located
on the autosomes of both sexes (e.g., genes influencing themilk yield in dairy
cattle).
Sex linkage. The location
of a gene on a sex chromosome, for example, X-linkage.
Sex linkage: Genes located and inherited on a sex
chromosome
Sex Linked Traits. Traits
governed by the genes carried on the specific sex chromosome(usually X-chromosome),
colour blindness, for example.
Sex Mosaic. Gynander/gynandromorph.
Sex Ratio. The number of
males per 1000 females at zygote formation (primary sexratio), at birth
(secondary sex ratio), or at sexual maturity (tertiary sex ratio).
Sex. A complex
developmental character that refers to the state of being male and female.It
can be the sum total of contrasting and complementary characteristics exhibited
bymale and female (as in humans and other higher animals and a few higher plants),
+or – (in case of physiological differences), donor or recipient (as in
prokaryotes)individuals or clones within the same species and their capacity to
performamphimixis and genetic recombination through reproduction. The
biologicaldivergence of sex may be inherent (that is, genetic/chromosomal) or
impressed on itby its environment.
Sexual Polyploidization (Mendiburu and Peloquin 1970s). A process of obtaining apolyploid via fusion of unreduced female
and male gametes. If the fusion occursbetween an unreduced gamete (2n)
and a normal one, it is called unilateral sexual217polyploidization.
In case, both the fusing gametes are unreduced ones, the process istermed bilateral
sexual polyploidization. Both types have been reported in theprogeny
derived from crosses between Solanum spp. Sexual polyploidization via
2ngametes is a distinctly separable process from somatic doubling. It has
the advantageof providing for heterosis, variability, minimal inbreeding, high
fertility andmaximum heterozygosity (new intra- and inter-locus interactions).
For example,tetraploids derived from diploids via somatic doubling with
colchicine have the sameor low yield compared to their undoubled counterparts
and were inferior to tetraploidsderived from bilateral sexual polyploidization
in potato.
Sexual. Of all processes
involving both meiosis and fertilization by means of whichsexual reproduction
and genetic recombination are achieved.
Shattering. Falling out
and the loss of seeds from a plant before harvest or duringharvesting operations.
Resistance to shattering is important to prevent the loss incrops like
soybeans, mungbeans, and many other crops.
Shift. A chromosomal
structural change resulting in the transposition of a chromosomesegment to
another place in the same chromosome. It may be homo- orheterobranchial
depending on whether transposition occurs within the same ordifferent arms of
the chromosome.
Shotgun method: A way of preparing DNA for genetic
studies; random-cutfragments of DNA are cloned into a vector; the fragments
resemble a clone library; using this collection of clones, several molecular
techniques are applicable
Shotgun sequencing In NGS,
sequencing of one end of each DNA fragment.
Shotgun Technique. Cloning a
large number of different DNA fragments as a prelude toselecting one particular
clone type for intensive study.
Shuttle Breeding. A breeding
approach that aims at speeding up advancement ofsegregating materials by moving
the germplasm rapidly from place to place. Althoughthe primary objective is to
get two or more generations per year, a valuable side effectresults in terms of
incorporation of insensitivity to photoperiod in the selectedmaterials. This
system was initially followed in breeding for stem rust resistance. Itled to
selection of photo-insensitive wheat genotypes besides permitting
wide-spreadadoption of CIMMYT cultivars. Thus wide-testing strategy of CIMMYT
led not onlyto the development of many widely grown wheat varieties, but also
led to the conceptof megaenvironment and the use of very wide testing in many
other crop species.
Shuttle breeding: A program that shuttles seed between
two (or more)locations to be grown at each; this method was introduced by N.
Borlaug and is aproven contributor to the success of CIMMYT (Mexico); shuttle
breeding allowstwo breeding cycles per year instead of one - for example, a
winter cycle in thenorthern desert of Sonora, and a summer crop in the central
Mexican highlands;this not only fast-forwards selection, but also exposes test
varieties to radicallydifferent day lengths, temperatures, altitudes, and
diseases; resulting plants arebroadly adapted; they grow well in numerous
environments
Shuttle Vector. A vector (for
instance, a plasmid) constructed in such a way that it canreplicate in at least
two different host species, allowing a DNA segment to be testedor manipulated
in several cellular settings.
Shuttle vector: A vector constructed in such a way
that it can replicate in atleast two different host species recombined into
such a vector can be tested ormanipulated in several cell types
Sib Mating. The
intercrossing of the progeny of the same parents, that is,
brother-sistermating. It is the closest form of inbreeding possible in
self-incompatible species suchas alfalfa.
Sib mating: A form of inbreeding in which progeny
of the same parents(siblings) are crossed
Sibs. Progeny of the same
parents derived from different gametes. Also called full sibs.Half-sibs are
progeny with one parent in common. Also called siblings.
Sickle-cell Anaemia. A potentially
lethal human disease caused by a mutation in a genethat codes for the
oxygen-transporting molecule, haemoglobin. Haemoglobin, whichis a conjugated
protein and an integral part of red blood cells, is made up of fourpolypeptide
chains (2α and 2β chains). A single gene mutation causes alteration at 6thamino
acid position in β-globin chain wherein amino acid valine is substituted
forglutamic acid [codon change: GAG (glutamic acid)→ GTG
(valine)]. The alteredmolecule causes red blood cells to be sickle-shaped. This
trait is inherited as anautosomal recessive.
Silencer. A cis-regulatory
sequence that can reduce levels of transcription from anadjacent promoter.
Silent mutation. Mutation in
which the amino acid sequence and function of the proteinis unaltered. For
example, a single gene mutation can alter the nucleotide sequence ofDNA in such
a way that the altered codon encodes the same amino acid in apolypeptide chain
(CGU↔CGA; both
coding for the same amino acid, arginine). Asamino acid sequence does
not change, the mutation goes on unnoticed, that is,silently.
Silent mutation: Mutation in which the function of the
protein product of thegene is unaltered.
Simple interval mapping A systematic
one-dimensional search for one QTL at a time treating each marker interval
independent of other intervals.
Simple Recurrent Selection. A system of breeding in which the population for nextcycle of selection
is synthesized from intercrosses made in all possible combinationsfrom a number
of desirable genotypes (or their selfed progeny) of the originalpopulation.
Since testcrosses are not made, the effective use of simple recurrentselection
is restricted to characters with high heritability that can be judged visually
orby simple breeding tests (e.g. resistance to leaf blight in corn).
Simple sequence repeat markers A special
version of STS markers; a microsatellite locus is amplified using a pair of
specific primers based on the unique sequences flanking the locus. Syn.,
sequence-tagged microsatellite site markers, simple sequence length
polymorphisms, microsatellite markers.
SINE. Short Interspersed Element; a type of small repetitive DNA sequence foundthroughout the eukaryotic
genome.Single Cross. An F1 hybrid between two different inbreds. The
number of possiblesingle crosses involving n inbreds will be [n.(
n-1 ) / 2 ]. The current trend in breedinghybrid maize is to develop
single cross hybrid as it displays heterosis twice as muchas double cross
hybrids.
Single Factor Experiments. Experiments
with varying levels of a single factor while allothers are kept constant. For
example, most crop variety trials are single-factorexperiments because the
variety is a single factor and the factor levels (that is,treatments) are the
different genotypes/varieties. Only the variety differs from oneexperimental
plot to another while all management factors are applied uniformly to allplots.
Single feature polymorphism Allelic
variation detected between pairs of lines of a species by using gene
sequence-based highdensity oligonucleotide microarrays for hybridization with
their genomic fragments/ cDNAs.
Single marker analysis Each marker is
separately tested for its association with the target trait.
Single nucleotide polymorphism Variation in
single base pairs of DNA.
Single Pod Descent Method. A method of
handling segregating generations in which F2plants and their progenies are
advanced rapidly in succeeding generations by seedsfrom a single pod.
Evaluation of progeny rows start in F6 generation. (Also see singleseed
descent method).
Single primer amplification reaction Markers
generated by non-anchored ISSR primers.
Single Seed Descent Method (Goulden 1939). A modification of bulk populationbreeding in which F3 and succeeding
generations are advanced from single seeds ofeach harvested plant of the
previous generations followed by progeny evaluation fromF6/F7. The objective is
to maintain the descendants from a maximum number of F2plants during
segregating generations. It is usually accepted that this method is
quiteconvenient, timesaving, and economical and is able to maintain
considerablevariability. However, the inherent drawback attached to this method
is that a numberof superior worthwhile genotype may be lost at any stage of
advancement.
Single sequence repeat (SSR) DNA
marker technique: A genetic
mappingtechnique that utilises the fact that microsatellite sequences repeat
(appearrepeatedly in sequence within the DNA molecule) in a manner enabling
them tobe used as markers
Single Test. Evaluation of
genotypes performed only once. This does not provide anaccurate picture of the
genetic variance as it contains various interaction variances inaddition.
However, progressively more and more of the interaction variances can
beidentified and removed as more and more locations and times are sampled.
Single-base extension (SBE): A method of SNP
genotyping; the primer is extended by a single nucleotide only.
Single-gene disorder: Hereditary disorder caused by a mutant
allele of a singlegene (e.g., Duchenne muscular dystrophy, retinoblastoma,
sickle cell disease).Compare polygenic disorders.
Single-nucleotide polymorphisms
(SNPs): Variations (in
individualnucleotides) that occur within DNA at the rate of approximately one
in every1,300 bp in most organisms; SNPs usually occur in the same genomic
location indifferent individuals
Single-seed descent (SSD): Derivation of plants by a selection
procedure inwhich F2 plants and their progeny are advanced by single
seeds until geneticpurity is achieved; single-seed descent methods
(single-seed, single-hill, multipleseed)are easy ways to maintain populations
during inbreeding; natural selectioncannot influence the population, unless
genotypes differ in their ability to produceviable seeds; artificial selection
is based on the phenotype of individual plants, noton the progeny performance
Single-strand conformation profile/polymorphism A marker
system, in which detection is based on differential movement of singlestranded
DNA molecules representing identical genomic regions from different
individuals.
Single-strand conformational
polymorphism (SSCP): This feature
relies onsecondary and tertiary structural differences between denatured and
rapidlycooled amplified DNA fragments that differ slightly in their DNA
sequences; different SSCP alleles are resolved on non-denaturing acrylamide
gels, usuallyat low temperatures; the ability to resolve alleles depends on the
conditions ofelectrophoresis and this requires DNA sequence data
Sire. The male parent in
mammals.
Sister Chromatid Exchange. An event
similar to crossing-over that can take placebetween sister chromatids at
mitosis or meiosis. It has been detected in harlequinchromosomes.
Sister Chromatids. Chromatids
derived from the same chromosome through duplication.
Site Directed Mutagenesis. A process of
introducing specific mutations into thesequences of a cloned gene in order to
assay the resulting phenotype to make itpossible to ascertain the function of
the gene. Site directed mutagenesis based onpolymerase chain reaction is the
most popular.
Site Specific Recombination. Recombination between two specific sequences that neednot be homologous.
It is mediated by a specific recombination system.
Site. The smallest unit in a
gene capable of independent mutation. The entity is a singlenucleotide. Also
called mutational site.
Site-specific mutagenesis: The use of recombinant DNA technology
to createspecific deletions, insertions, or substitutions in vitro in a
particular gene; thetechnique allows the production of proteins having any
desired amino acid at anyposition
Site-specific recombination: A crossover event that requires
homology ofonly a very short region and uses an enzyme specific for that
recombination; recombination occurring between two specific sequences that need
not behomologous, mediated by a specific recombination system
Skewness. The departure
from symmetrical spread of the frequency curve around thecentral value.
Positive skewness enhances value of the constant of selection intensity(>
2.06 at 5% SI) whereas the negative skewness retards it. Therefore, the data
mustbe tested for normal distribution while making prediction for genetic
advance. In caseof skewness, genetic advance is overestimated (or
underestimated).Soma. The body of an organism aside from the germ line
cells.
Small interfering RNAs (siRNAs): Models of PTGS indicate that
21–23nucleotide dsRNAs mediate PTGS; introduction of siRNAs can induce
PTGS,e.g. in mammalian cells; siRNAs are apparently produced in vivo by
cleavage ofdsRNA introduced directly or via a transgene or virus;
amplification by an RNAdependentRNA polymerase (RdRP) may occur in some
organisms; siRNAsare incorporated into the RNA-induced silencing complex
(RISC), guiding thecomplex to the homologous endogenous mRNA where the complex
cleaves thetranscript
SNP calling: Determination of the genomic
positions at which single nucleotide polymorphisms occur.
SNP index : The ratio of the number of
short reads with the mutant allele at a SNP locus to the total number of short
reads covering the SNP locus.
SNP locus : A specific position in the
genome, at which different nucleotides occur in the same DNA strand of
different individuals of the species.
SNP mining SNP discovery by analysis of
genomic and/or EST sequences of different individuals of a species available in
the databases.
SNP validation Evaluation that
a discovered SNP is a true SNP and not a product of sequencing error, faulty
read alignment, etc.; that its alleles represent homologous genomic regions;
and that it segregates in a typical Mendelian fashion.
SNP: An abbreviation for “single nucleotide
polymorphism”, pronounced“snip”. A SNP which distinguishes two sequences can be
used as a genetic marker
Somaclonal Variation (Larkin and Scowcroft 1983). Tissue culture induced variationoccurring within a clone of cells. Such
variations may be developmental or heritable(stable). If stable, it may provide
for broadening the genetic base and thus obtainingincremental improvement in
the commercial cultivars, more particularly in clonallypropagated crops. In
India, Pusa Jaikisan (a somaclone of Varuna variety of
Indianmustard) and B-13 (a somaclone of a medicinal plant, Citronella
java) have alreadybeen released to farmers for general commercial
cultivation.
Somaclonal variation Heritable
variation generated in cells and tissues grown in vitro.
Somaclonal variation: Variation among the tissues or plant
derived from thein vitro somatic cell culture i.e., callus or suspension
cultures.
Somatic Cell Genetics. The genetics
of somatic or body cells. It is the asexual genetics,including the study
of somatic mutation, assortment, crossing-over, and cell fusion.
Somatic Cell. A “body cell”
that is not destined to become a gamete. Its genes will not bepassed on
to future generations.
Somatic cell: Cells in the body that are not
involved in sexualreproduction (that is, not germ cells).
Somatic crossing-over: Crossing-over during mitosis of
somatic cells such thatparent cells heterozygous for a given allele, instead of
giving rise to two identicalheterozygous daughter cells, give rise to
non-identical daughter cells, one of whichis homozygous for one of these
alleles, the other being homozygous for the otherallele
Somatic Embryo. The embryo of
somatic origin. It differs from zygotic one in lacking awell-defined hypocotyl.
It also does not experience desiccation and dormancy; theseproperties seem to
be imposed by accumulation of abscisic acid that, in turn, ispresumably
provided by the maternal tissues (in the zygotic embryo).
Somatic embryo: An organised embryonic structure
morphologically similarto a zygotic embryo but initiated from somatic cells;
somatic embryos developinto plantlets in vitro through developmental
processes that are similar to those ofzygotic embryos
Somatic Embryogenesis. The genesis of
embryos from somatic cells through tissueculture technology. Such embryos are
desiccated and encapsulated to form artificial orsomatic seeds.
Somatic hybridisation: The fusion of genetically different
somatic cells bydifferent means, usually to overcome natural crossing
(incompatibility) barriers
Somatic Mutation. A mutation in
a somatic or body cell.
Somatic mutation: A mutation occurring in a somatic
cell; if the mutated cellcontinues to divide, the individual will develop a
patch of tissue with a genotypedifferent from the cells of the rest of the body
Somatic Seed. A somatic
embryo derived through tissue culture and encapsulated in asuitable matrix
(gel) with nutrients, growth regulators, and antibiotics necessary forthe development
of a complete plant. It is also called synthetic or artificial seed.
It canbe stored for several years and can be sown like natural ones.
Somatic Segregation. The
segregation at mitosis of genes giving rise to variegation orsectoring.
Somatic. Referring to diploid
body cells (2n), normally with one set of chromosomescoming from the
male parent and the other from the female one.
Somatoplastic Sterility. A condition of
embryo abortion following self-fertilization dueto disturbances in
embryo-endosperm relationship (e.g., alfalfa). This conditionobviously favours
heterozygosity.
Sorts. The different
botanical forms of a species.
SOS Repair. Save our Soul Repair; a process or mechanism of repairing damage toDNA. It is an error-prone
process whereby gross structural DNA damage iscircumvented by allowing
replication to proceed past the damage through imprecisepolymerisation. A
biological system resorts to this mechanism of DNA repair whenall other such
systems fail. As it is an error-prone and imprecise mechanism,phenotypic
expression of repaired DNA may be altered.
Southern Blot. Transfer of
electrophoretically separated fragments of DNA from a gel toan absorbent sheet
such as paper. This sheet is then immersed in a solution containinga labelled
probe that will bind to a fragment of interest.
Southern blotting: Transfer by absorption of DNA
fragments separated inelectrophoretic gels to membrane filters for detection of
specific base sequences byradiolabeled complementary probes. SPERM
(abbreviation of Spermatozoon) Themale reproductive cell carrying 23
chromosomes.
Southern hybridization: A DNA, DNA hybridization procedure
named after E.M. Southern, involves restriction digestion of DNA followed by
gel electrophoresis, blotting, hybridization with a labeled probe, and
detection of the bands hybridized with the probe.
Spacer. The nucleotide
sequence of DNA found between genes (compare it with intronsor intervening
sequences, which are present within a eukaryotic gene). It may
betranscribed, but is never translated. Its function is still unknown.
Speciation. The formation
of a new species including the development of speciesdiversity. It refers to
the origin of a group of individuals capable of living in a newway and at the
same time acquiring some barriers to genetic exchange with thespecies from
which it arose. Once speciation gets completed, each newly formedspecies will
take an independent evolutionary course; inevitably the species willbecome
increasingly different as time passes. It occurs when gene flow within
thecommon gene pool is interrupted by an isolating mechanism. When isolation
occursthrough geographical separation of populations descended from a common
ancestor,allopatric (different fatherland) speciation may result that is more
common in birds. Ifisolation occurs through ecological or genetic means within
the same area, sympatric(joint fatherland) speciation is a possibility as
occurs more commonly in parasites. Inplants and in a few group of animals, a
different form of speciation is effected throughpolyploidy (instantaneous
speciation), the doubling of chromosomes, becausepolyploid individuals are
immediately isolated reproductively from their parents.
Species. A natural biological
unit tied together by the sharing of a common gene pool. Insexually reproducing
organisms, a species is a group of potentially interbreedingpopulations that
are reproductively isolated from any other such groups. Species areoften
subdivided into subspecies, races, ecotypes, varieties, or other
subspecificcategories.
Specific Combining Ability. The deviation in performance of a cross from average ofall crosses
possible among n number of parents. For an accurate measure of SCA,
thetests must be conducted in more seasons and more locations compared to GCA.
Specific Locus Test. A test for
detecting recessive mutations in diploids. Normalindividuals treated with a
mutagen are mated to testers that are homozygous for therecessive alleles at a
number of specific loci. If markers are related to the seed,recessive phenotype
(in case of mutation) can be visible in the mating-generationitself [white
endosperm colour (ccc) in Indian corn; cc × CC (mutagenised)].Otherwise,
screening is done for recessive phenotypes in the next generation.
Specific Resistance. Host plant
resistance to specific race or biotype of a pathogen. Suchresistance reaction
operates usually on a gene (for host resistance)-for-gene (pathogenavirulence)
basis.
Sperm. A male gamete. In
animals and plants, it is called spermatozoon andspermatozoid,
respectively.
Spermatid. Any of the
four haploid cells arising by meiosis from a single spermatocyte.Each spermatid
gives rise to a spermatozoon without further nuclear division.
Spermatocyte. Any of the
sperm mother cells, derived from spermatogonium, which giverise to spermatids.
Spike. An inflorescence with
a more or less elongated axis, along which flowers aresessile or nearly so.
Spikelet. A unit of
inflorescence in the grasses, composed of the glumes, the rachilla, andthe
florets.
Spindle Poison. Any mitotic
poison affecting the formation or function of the spindle andblocking
karyokinesis at metaphase. The prototype of spindle poison is colchicine.
Spindle. The set of
microtubular fibres that seem to move eukaryotic chromosomesthrough their
attachment to centromere during cell division. Its main constituent is
aprotein, called tubulin.
Splicing. The reaction
that removes introns and joins together exons to form functionalmRNA transcript
in eukaryotes.
Splicing: The removal of the intron (noncoding
sequence) from an mRNAgene sequence and the fusion of the exons (coding
sequence) during the mRNAprocessing
Split Plot Design. A design
specifically suited for a two-factor experiment that has moretreatments than
can be accommodated by a complete block design. In this design, afactor whose
precision of effect could be sacrificed is allotted to the main plot. Thenthe
main plot is sub-divided into sub-plots to which the second factor is
assigned.Thus each main plot becomes a block for the sub-plot treatments.
Therefore,measurement of the main effect of the sub-plot factor and its
interaction with themain-plot factor is more precise than that obtainable with
a randomized completeblock design. Degree of precision, relative size of the
main effects, and managementpractices required by a factor are considered while
assigning a factor to the main orsub-plot.
Split plot design: Allows the testing of two factors in
combination; one factor(the main effect) serves as a replication for the second
factor (the split effect); thereare many split plot design options, but the
basic principle involves assigning oneset of treatments to the main plots that
are arranged in randomised completeblocks; the second set of treatments is
assigned to subplots within each main plot;the statistical analysis is similar
to that used with the randomized complete blockdesign, i.e., an ANOVA set up
for a split plot is followed by a means comparisontest such as the Dunn’s test
or Duncan’s
Spontaneous Mutation. A mutation
occurring in the absence of mutagens, usually due toerrors in the normal
functioning of cellular enzymes (which are normally required forDNA
replication).
Spontaneous mutation: A naturally occurring mutation, as
opposed to oneartificially induced by chemicals or irradiation; usually such
mutations are due toerrors in the normal functioning of cellular enzymes
Spore. A haploid cell
destined to form or act either as a gamete or as the initial cell for anew
haploid individual. In plants and fungi, sexual spores are the haploid
cellsproduced by meiosis. In certain other fungi, asexual spores are somatic
cells that mayact as gametes or initiate the development of a new haploid
individual.
Sporocyte. A diploid cell
undergoing meiosis to form haploid spores; a spore mothercell.
Sporophyte. The diploid sexual-spore-producing
generation or phase in the life cycle ofplants, that is, the stage in which
meiosis occurs.
Sporophytic Incompatibility. A kind of mating wherein incompatibility reaction of thepollen is
governed by the genotype of the parent. The gene for incompatibility
hasnumerous alleles that may show dominance, individual action or competition
in eitherpollen or styles. This complex incompatibility reaction leads to (a)
frequent reciprocaldifferences, (b) occurrence of female sterility, and (c)
homozygosity at the locusconcerned (for example, members of crucifereae).
Sporophytic self-incompatibility: Self-incompatibility is based on
thegenotypic and phenotypic relationship between the female and male
reproductivesystem; alleles in cells of the pistil determine its receptivity to
pollen; the phenotypeof the pollen, expressed as its inability to effect
fertilization, may be determined bythe maternal plant, referred to as
sporophytic incompatibility
Sport. In plant and animal
breeding, an aberrant individual resulting from mutation. Budsport is a somatic
mutation occurring in a bud.
Spotted microarrays DNA fragments
representing different genes of an organism spotted onto a suitable solid
support.
Spurious Correlation. Associations
between two variables, which are purely accidentalor arise due to chance
factors. The two variables have no connection whatever.(Population increase in
India and England after 1947, for example).
Square. An unopened flower bud
in cotton with its accompanying bracts.
SSD procedure One seed is harvested from each
plant of the F2, and the later generations and seeds from all the plants are
composited and planted to raise the next generation.
SSLP. Short Sequence Length Polymorphism; the presence of different numbers ofshort repetitive elements (mini- and
microsatellite DNA) at one particular locus indifferent homologous chromosomes.
Heterozygotes for such SSLP represent usefulmarkers for genome mapping.
Stability Analysis. An analysis of
consistency in performance of genotypes over a widerange of environments based
on certain statistical parameters. If a cultivar showsconsistent performance
over a set of environments, it is released for wider cultivation.For stability
analysis, a number of stability models such as Finley and Wilkinsonmodel (1963),
Eberhart and Russell model (1966), Perkins and Jinks model (1968),Freeman and
Perkins model (1971), and the like is available.
Stability In case of PBR, the new variety
must be stable in appearance and its clonal characteristics over successive
generations under the specified environment.
Stability. The
consistency in the performance of a genotype(s) across environments withrespect
to a characteristic(s). If genotype-environment interaction is zero, it means
thegenotype is stable. Terms such as homeostasis and buffering have also been
used todescribe stability. The mean yield of a variety is the most common
description of itsperformance. Multiline varieties, bulk populations,
composites, or F3/F4 derived linesvarieties are usually more stable than pure
line varieties. Stable genotypes, however,usually have below average yields.
Several models have been used to study stabilityof varieties. For example,
Finley and Wilkinson (1963) used regression as a parameterof stability.
However, in the Eberhart and Russel (1966) model (the most commonlyused model),
regression is termed response index and mean square deviations fromregression
is referred to as the parameter of stability.
Stabilizing Selection (Van der Plank 1963, ’68). The reduction in the fitness of thepathogen races that carry unnecessary
virulence genes. Thus stabilizing selectionshould favour simple races, that is,
races with least number of unnecessary virulencegenes. In nature, however,
there are many cases that contradict the concept ofstabilizing selection. For
example, Flor (1971) reported several races of pathogens(Puccinia sp.)
carrying virulent genes not needed to parasitise the host crop.
Overall,stabilizing selection probably is not an important factor in preventing
thedevelopment of complex races.
Stabilizing selection: A type of selection that removes
individuals from apopulation (i.e., from both ends of a phenotypic distribution
divided by deviationof a sample of means)
Stable Feedback System. A system that
promotes retention of a large number offavourably interacting genotypes in a
heavily inbreeding population. This system isnecessary in such a population as
it leads to continuous release of genetic variabilitythrough chance crossing
(among the existing genotypes) followed by segregation andrecombination.
Stable QTL: The phenotypic effect of such a
QTL is little affected by the environment so that it is detected across
environments.
Stacked traits: Term used to describe seed products or
GMO’s with more thanone biotechnology trait. These GMO’s have had more than one
gene from anotherorganism or organisms transferred into their genome, thus
creating a GMO withstacked “biotech” traits.
Staggered Cuts. The cleavage
of two opposite strands of duplex DNA at points near oneanother.
Stamen. The pollen-bearing
organ in the flower composed of an anther and a filament.
Staminate flower. A flower
bearing stamens but no pistil.
Standard Deviation (Fisher 1913). A measure of variability. This statistic ‘s’ [orparameter ‘σ’] is
defined as the square root of the average squared deviations of theindividual
observations from their mean [di2/N]½. This
statistic is used morefrequently in plant breeding experiments. It quantifies
the variability of a character inthe same unit in which observations themselves
are measured. Thus it is more usefulthan variance, which is square of standard
deviation.
Standard deviation (SD): A measure of the
variability in a population of items,i.e., a set of n measurements x1,
x2, x3, ..., xn is equal to the positive square root ofthe variance of the
measurements
Standard error (SE): A measure of variation of
a population of means
Standard Error. A statistic,
which is the estimate of the standard deviation (parameter).It is the
standard deviation of the sample means.
Standard Type. An arbitrarily
specified gene or genotype taken as the basis ofcomparison for genetic
investigations. The standard genes are designated as ‘+’irrespective of whether
being dominant or recessive. However, the designatedstandard types are supposed
to be the most frequent types in nature. Also called wildtypes.
Staple Length. Also called fibre
length; the normal length of a typical portion of thefibres of a cotton
sample.
Start codon-targeted marker Markers based
on single primers designed on the basis of the sequence of the short conserved
region around the start codon, ATG, of plant genes.
Statistic. Any function
based on sample values. It is the estimate of a parameter madefrom a sample. It
strictly applies to a sample whereas a parameter to a population.
Statistical analysis: Analysing collected data for the
purposes of summarisinginformation to make it more usable and/or making
generalizations about apopulation based on a sample drawn from that population
Statistical Distribution. The array of
frequencies of different qualitative or quantitativeclasses in a population.
Statistical significance: The degree to which a value is greater
or smaller thanwould be expected by chance; typically, a relationship is
considered statisticallysignificant when the probability of obtaining that
result by chance is less than 5%if there were, in fact, no relationship in the
population
Statistics. A branch of
mathematics, which deals with collection, analysis andinterpretation of
numerical data.
Statistics: A quantity that is calculated from a
sample of data in order to giveinformation about unknown values in a
corresponding population
Stem Cell. The germ cell;
an undifferentiated cell capable of giving rise to specializeddaughter cells.
Stepwise regression It treats
marker effects as fixed and fits the markers into the model either singly or in
small groups.
Sterility. A condition in
which plant fails to produce functional gametes. It may becaused by chromosomal
aberrations (structural or numerical), gene action orcytoplasmic effects which
may, in turn, lead to abortion or modification of entireflower-buds, stamens,
pistils or may disrupt the development of pollen, embryo sac,embryo or
endosperm.
Stigma. The portion of the
pistil that receives the pollen.
Stolon. A trailing stem, capable
of forming roots and shoots from its nodes.
Stoma. A minute opening in
the epidermis of a plant organ (as in leaf) through whichgaseous exchanges
occur.
Strain Building. A broad term
used by early forage breeders to include any system ofmating by which a strain
is built up from crossing carefully selected plants. Thus itincludes a number
of breeding procedures varying from simple mass selection tomethods that
resemble recurrent selection.
Strain. A group of similar
individuals from a common origin. It is generally a morenarrowly defined group
than a variety. It may also be the progeny of a single plantirrespective of the
mode of pollination. It can also be a pure-breeding lineage, usuallyof haploid
organisms, bacteria, or viruses.
Stratified mass selection: Mass selection in which the population
is split intosubpopulations that are grown under different environmental
conditions (i.e., indifferent fields or in different parts of a field); plants
for next generation seed areselected from the different subpopulations
Strength of a QTL The proportion
of total phenotypic variance for the target trait explained by the QTL.
Stress An environmental factor that limits the
performance of a crop genotype.
Strigol. A stimulant produced
by roots of host species (sorghum, sugar cane, bajra,maize, etc.) of
Striga (S. asiatica, S. densiflora, and other species). Strigol
producedby such host plants stimulates germination of seeds of Striga spp. This
obligateparasite, after germination, grows towards the host roots, penetrates,
and thus useshost photosynthates. However, host varieties differ in strigol
production. Hostcultivars that do not produce this stimulant can avoid Striga
establishment. Hostvarieties that have hard roots (due to highly
sclenchymatised endodermal tissuesalong with deposition of silicon) are also
likely to be protected. Certain Strigaresistant sorghum varieties have been
found to possess phenolic acids in their roots.
Stringency of distinctness criterion
In case of PBR, the number of bands required to differ between a pair of
varieties/lines for them to be accepted as distinct varieties/lines.
Strip Plot Design. An
experimental design wherein each block is divided into verticaland horizontal
strips. The design is specifically suited for a two-factor experiment inwhich
the prime object is to attain a higher degree of precision for measurement
ofinteraction effects of two factors than that for measuring the main effect of
either oneof the two factors. The vertical and horizontal strips are always
perpendicular to eachother; however, there is no relationship between their
sizes, unlike the case of mainplotand sub-plot of the split-plot design.
The interaction plot is, of course, thesmallest. Thus, in this design, the
degrees of precision associated with the maineffects of both factors are
sacrificed in order to improve the precision of theinteraction effect.
Structural Gene. A gene
encoding amino acid sequence of a protein.
Structural genomics Determination
of the complete genome sequence and the complete set of proteins produced by an
organism.
Structural Genomics. Characterising
and locating the entire set of genes in a genome.
Structural Heterozygosity. Heterozygosity
for a chromosomal structural change.Structural heterozygotes are usually
characterized by a reduction in fertility due toproduction of genotypically
defective gametes and unbalanced zygotes.Style. The stalk connecting the
ovary and the stigma.
Structural proteomics Mapping of the
3-D structure and nature of protein complexes present specifically in a
particular cell/organelle.
Structured association model An AM model
designed to tackle the problems due to population structure.
SubBin In case of MAPMAN tool, a subdivision
of a Bin.
Subcloning In chromosome walking, cloning
of a small segment representing one end of the DNA fragment being analyzed.
Subcloning: Transplantation of a piece of DNA from
one vector to another
Subculture: The aseptic transfer of a part of a
stock culture to a fresh medium
Subsampling. The process of
selecting a random sample at successive stages. The deviceis extensively used
in sampling on account of the ease in selection and economy oflabour (as in
wheat, for example).
Subsistence Agriculture. Agricultural
activities performed by resource poor farmers justto produce food for
themselves, usually in harsh, erratic and unpredictable climaticconditions. In
subsistence agriculture, yield at farmers’ fields is many fold less thanwhat is
realized in well-managed agricultural fields; low level of yield results
fromlow or negligible use of inputs as well as less than ideal agricultural
practices.Resource poor farmers manage subsistence agriculture through
strategies based onspreading of risk through interspecific (inter- or mixed
cropping) and intraspecific(heterogeneous cultivars) diversity.
Subspecies. An aggregate
of breeding populations of a given species that inhabit ageographical
subdivision of the species range. It differs from any other such groupsboth
taxonomically and with respect to certain gene pool characteristics.
Substitution line: A line in which one or more
chromosomes are replaced by one or more chromosomes of a donor variety or
species; it is used for genetic analysis and gene transfer
Subvital Gene. A gene that
causes the death of some proportion (but not all) of theindividuals that
express it. It possibly reduces the fitness of the individual carryingand
expressing it.
Subvital. Of a gene or
genotype that reduces the viability of its carriers compared to thestandard
type.
Sum Rule. A probability
rule. The probability of either one of two mutually exclusiveevents occurring
is the sum of their individual probabilities. For example, with twodice,
probability of either two-fours or two-fives is 1/18 (1/36 + 1/36 = 1/18).
Supercoil. A closed and
double-stranded DNA molecule that is twisted on itself.
Supergene. A cluster of
mutually cooperating genes; a group of genes held togethermechanically by
various mechanisms and inherited as a single unit. If co-adapted,such genes,
even though not necessarily functionally related, may cooperate toproduce some
adaptive characteristic.
Superinfection. Phage
infection of a cell that already harbours a prophage.
Supernumerary chromosomes: Chromosomes present, often in
varyingnumbers, in addition to the characteristic invariable complement of
chromosomes
Supersuppressor. A mutation
that can suppress a variety of other mutations. It istypically a nonsense
suppressor.
Super-vital. A gene or
genotype which, when expressed, increases the viability of itscarrier compared
to the standard type (heterotic effects, for example).
SupF markerA tRNA gene with a mutated
anticodon that recognizes a polypeptide chain termination codon generated by a
suppressor-sensitive mutation within a gene.
Suppression: Changes that eliminate the effects of
a mutation without reversingthe original change in DNA.
Suppressor Mutation. A mutation
that cancels the effects of another mutation. Asuppressor maps at a different
site from the mutation it counteracts, either within thesame gene or at a more
distant locus. Notice that different suppressors act in differentways.
Suppressor mutation: A mutation that counteracts the
effects of anothermutation. A suppressor maps at a different site than the
mutation it counteracts,either within the same gene or at a more distant locus.
Different suppressors actin different ways.
Suppressor. A secondary
mutation that can cancel the effect of a primary mutation,resulting in the
wild-type phenotype.
Surrogate measurement It determines
the level of some other trait that shows a reliable and predictable
relationship with the target trait.
Susceptibility. The inability
or incapacity of plants to interfere the normal growth anddevelopment of pests
or pathogens.
Susceptible. The
characteristic of a host plant such that it is incapable of suppressing
orretarding an injurious pathogen or other factor.
Sustainability. Improving the
quality of human life while living within the carryingcapacity of supporting
ecosystem.
Sustainable Agriculture. A form of
farming which produces sufficient food to meet theneeds of the present
generation without eroding the ecological assets and theproductivity of
life-supporting system of the future generations.
Sweet Corn. Any type of
corn (Zea mays saccharata) that is consumed in immaturestage. It
contains relatively high sucrose content (e.g., Madhuri).
SWOT Analysis. An analysis
vis-à-vis strength, weaknesses, opportunities, and threatsof a breeding
programme.
Symbiont Hypothesis. A hypothesis
that mitochondria and chloroplasts might haveoriginated as prokaryotic intracellular
parasites that established a symbioticrelationship with the eukaryotic cell.
Mitochondria could be the result of a bacteriumparasite, and chloroplasts, the
result of a blue green alga (having chlorophyll). Inevolutionary terms, it is
possible that a symbiotic relationship could have evolved intothe present
situation, in which these organelles have only a certain degree ofautonomy.
Symbiosis. A kind of
inter-species interaction in which both the interacting partnersbenefit from
each other. Symbiosis literally means living together. Contrary toprotocooperation,
this kind of interaction is obligatory, Rhizobium-legumeassociation, for
example. Instances of mutualism (symbiosis) are more likely todevelop between
organisms with widely different requirements whereas organismswith similar
needs are more likely to get involved in competition. Each step
ofsymbiosis is controlled and/or influenced genetically.
Sympatric Species. Species
formation in the absence of geographical isolation byacquiring mechanisms of
reproductive isolation within a panmictic unit.
Sympatric. Of two or more
populations occupying the same geographical area. Suchpopulations may remain
completely separated by different habitat preferences, or bydifferent habits.
Syn-0, Syn-I, Syn-II. Symbols for
designating the original synthetic population, firstsynthetic generation
(progeny of Syn-0), and second synthetic generation (progeny ofsyn-I)
Synapsis. Close-pairing
(point-to-point pairing) between homologous chromosomes atzygotene
sub-stage during meiosis I.
Synaptic Genes. Genes
influencing the extent of meiotic pairing.
Synaptonemal complex (SC): Ribbonlike structures
observed in electronmicrographs of nuclei in the synaptic stages of meiosis;
the ribbon represents asystem that promotes synapsis of homologous chromosomes;
the SC seems to bethe essential prerequisite for homologous chromosome pairing
and crossing-over
Synaptonemal Complex. A complex
structure that unites homologs during prophase ofmeiosis-I. A protein plus a
small amount of DNA constitutes the complex structure.
Syncytium. A single cell
with many nuclei.
Syndrome. A set of
specific characteristics (symptoms) of an abnormal geneticcondition. A genetic
syndrome may be transmitted to the offspring as a unit.
Syngamy. Sexual reproduction;
the union of male and female gametes leading eventuallyto zygote formation.
Syn-II Generation. The panmictic
generation following synthesis by crossing a numberof lines having proven
combining ability (analogous to F2 generation).
Synonymous Substitution. Mutational
replacement of an amino acid with one of similarchemical properties.
Syntenic markers Markers located
in the same chromosome.
Synteny. The description of DNA
segments in which the gene order is identical indifferent related species. Sufficient
mapping of the same genes from mouse andhumans has been done to observe that
many blocks of closely linked genes in humansare in closely linked blocks in
the mouse as well. Such blocks of genes are said to besyntenic. A
synteny map shows how the chromosomal locations of the block in onespecies
relate to the comparable blocks in another. The study of synteny shows
thatchromosomal rearrangements have been instrumental in evolution at the
chromosomelevel.
Synthetic Seed. See somatic
seed.
Synthetic Variety. A variety
maintained by open-pollination in isolation following itssynthesis by crossing
in all possible combinations a number of genotypes havingproven general
combining ability. The farmers are usually given this open-pollinatedseed (Syn
II generation), which may be used for 3/4 generations without deteriorationin
performance.
Synthetic variety: A variety produced by crossing inter
se a number ofgenotypes selected for good combining ability in all possible
hybrid combinations,with subsequent maintenance of the variety by open
pollination; usually the firstgeneration of a synthetic variety is obtained by
a polycross involving a certainnumber of components with a good general
combining ability; the components aremaintained by identical reproduction, either
by vegetative propagation (clones) orby continued sib mating (inbred
populations)
System. The regularly
interacting and interdependent components forming a unifiedwhole; a set of
mutual relationships constituting an identifiable entity, real orpostulational.
Systematic Inbreeding. Deliberate
mating between relatives generation after generation.For example, in
experimental genetics (especially in plant breeding), systematicselfing,
full-sib, parent-offspring, or any other form of mating between relatives maybe
used to increase homozygosity.
Systemic Pressure. Any of the
non-random evolutionary pressure (selection, mutation,migration, etc.).
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