D2-Statistic (Mahalanobis 1928). A statistic used to determine the degree of divergenceamong populations/
breeding lines/ genotypes in terms of generalized group distance.Numerically,
it is the sum of squares of differences between any two-populations/genotypes
for each of the uncorrelated variables/traits. (Correlated variables
aretransformed to uncorrelated ones by pivotal condensation method). It is
useful inPlant Breeding as it facilitates in making choice of divergent parents
for hybridisationpurposes.
Dalton. A unit of molecular
weight. One Dalton equals one-twelfth the mass of C12.
Dark-adapted plant: A plant kept in
the dark for 10–15 min.
Darwinian Fitness. The relative
probability of survival and reproduction for a genotype;also called biological
fitness or fitness.
Darwinism. Evolutionary
concept (descent with modification) as propounded by CharlesDarwin. The four
main premises are: (a) life is constantly changing, and all forms oforganisms
are transformed over time, (b) all organisms evolved from a commonancestor, (c)
species multiply by splitting into descendent sibling species whenisolated in
geographically or ecologically divergent space, and (d) changes occurgradually
over time. Evolution under Darwinian and Mendelian principles isessentially a
two phase process: (i) production of variation, and (ii) biological fitnessof
variants in the struggle for existence.
Database: A systematized
collection of vast amounts of information on a specific topic, e.g., nucleotide
sequence, protein sequence, etc., in an electronic environment.
Daughter Cells. Two identical
cells formed by asexual division of a cell.
Daughter Chromosomes. The sister
chromatids at the anaphase of mitosis or anaphase-IIof meiosis.
Dee-geo-woo-gen. A semi-dwarf
variety of rice discovered in Taiwan. It was semi-dwarf,stiff strawed, erect
leafed and high yielding. The semi-dwarf trait appears to bemonogenic recessive
(sd1). The gene reduces plant height; however, it does not reducepanicle
length.
Deep Water Rice. Rice grown in
varying depths of water (1.0-5.0 metres). Such ricevarieties are usually
photosensitive. Two independent recessive genes (duplicaterecessive) govern
their elongation ability with rising levels of water. Gene(s) forsemi-dwarfness
can be incorporated, since it is non-allelic to those for floating habit.
Deficiency. Terminal loss
of a segment from a chromosome. Also see deletion.
Degenerate Code. A genetic code
in which two or more codons encode the same aminoacid.
Degenerate code: A genetic code in which some amino
acids may be encodedby more than one codon each.
Degree of freedom
(DF): The
number of items of data that are free to varyindependently; in a set of
quantitative data, for a specified value of the mean, only (n − 1) items
are free to vary, since the value of the nth item is then determined
bythe values assumed by the others and by the mean
Degree of Freedom. The number of
independent comparisons that can be made in a setof data. Suppose, there are
three genotypes namely, A, B, and C; we have nowfreedom to
compare each one with the rest two ones, that is, A with B and C
or B withA and C, and so on. Thus we have lost our
freedom by one degree (i.e., d.f. = n-1).
Dehiscence. Splitting open
of a fruiting structure or anther.
Deleterious. Of mutations
that impair the traits considered desirable by the breeder in thecultivated
species.
Deletion mapping:
The use of
overlapping deletions to localise the position ofan unknown gene on a
chromosome or linkage map
Deletion
mutation: A mutation in
which one or more bases are removed fromthe DNA sequence of a gene
Deletion. The loss of a
segment from a chromosome or from both members of ahomologous pair. The term is
applied usually to the intercalary loss from achromosome. A deletion may occur
spontaneously or be produced artificially byhighly energetic mutagens (X-ray,
γ-ray). The loss of terminal segment causesterminal deletion (deficiency);
this is a rare phenomenon, however. Two breaks in achromosome can produce an intercalary
or interstitial deletion. Deletion can bedetected genetically by the
lack of revertability, pseudodominance and recessivelethality and cytologically
by the presence of a deletion loop (in a deletionheterozygote). Deletion
homozygotes are lethal since there is loss of biologicalfunctions. It causes
semisterility in deletion heterozygotes.
Deletion: The loss of a chromosomal segment from
a chromosome set; the sizemay vary from a single nucleotide to sections
containing several genes
Deme. A local interbreeding
group; a panmictic unit.
Demographic historyChanges over
time in population size, development of subgroups within a population, etc., in
natural populations of species.
Denaturation. The loss of
the native configuration of a macromolecule resulting fromvarious causes (heat
treatment, extreme pH changes, chemical treatments, etc.) andusually
accompanied by loss of biological activity of the molecule in question. It
maybe the separation of the two strands of a DNA double helix or the severe
disruption ofthe structure of any complex molecule without breaking the major
bonds of its chains.
Denaturation: The separation of the two strands of a
DNA double helix, or thesevere disruption of the structure of any complex
molecule without breaking themajor bonds of its chains.
Denaturing/temperature gradient
gel electrophoresis: It reveals differences in the movement of
double-stranded DNA molecules from the same genomic regions of different
individuals when they are subjected to electrophoresis in a gel in which the
denaturing conditions increase with the distance from the loading well.
Density gradient
centrifugation: The separation of
macromolecules orsubcellular particles by sedimentation through a gradient of
increasing densityunder the influence of a centrifugal force; the density
gradient may either be formedbefore the centrifugation run by mixing two
solutions of different density (Eg. insucrose density gradients), or it can be
formed by the process of centrifugationitself (eg. in CsCl2 and Cs2SO4 density
gradients) derived from a common ancestor
Deoxyribonucleic
acid (DNA): The molecule
that encodesgenetic information. DNA is a double-stranded molecule heldtogether
by weak bonds between base pairs of nucleotides. The four nucleotides in DNA
contain the bases: adenine(A), guanine (G), cytosine (C), and thymine (T). In
nature, base pairs form only between A and T and between G and C; thus the base
sequence of each single strand can be deducedfrom that of its partner.
Depth of coverage of a SNP: The number of
sequence reads containing a given SNP locus.
Derived cleaved amplified
polymorphic sequence
A restriction enzyme recognition site is introduced into the PCR product by one
of the primers so that the product yields a CAPS marker. Syn., mismatch
PCR–RFLP.
Design of Experiment. Various forms
of plot arrangements (treatments) to suit therequirements of a particular
problem. The underlying principle of all the experimental59designs is the same;
that is, they seek to provide by means of randomisation andreplication an
unbiased comparison of treatments against their standard errors, andaim at
reducing these errors with the help of replication and local control.
Thus,experiments are designed with the object to measure experimental error for
decidingthe basis of difference between two treatments. A larger treatment
differencecompared to the experimental error implies varietal difference.
Estimate of error,control of error and proper interpretation of results are the
three essential componentsof any design of experiment.
Designer crops: Varieties
developed by any methodology, including MAS, that exhibit a specified
phenotype.
Desynapsis. The falling
apart during diplotene of meiosis-I of chromosomes, whichpaired normally at
zygotene. (Differentiate it with asynapsis, which refers to the totalfailure of
pairing between homologues during meiosis-I). It may be a geneticallygoverned
mechanism and one or a few recessive genes may control it. Such gene(s)may
impair chiasma formation leading ultimately to formation of univalents.
Desynaptic. Of genes that
affect the maintenance of chromosome pairing betweensynapsed chromosomes.
Detassel. The removal of
immature tassels to facilitate controlled crossing for producinghybrid maize.
Determinate. Descriptive of
an inflorescence in which the terminal bud opens first, thusarresting the
growth of a plant or prolongation of the floral axis. The trait is
ordinarilyrecessive, and is usually governed by a single gene. Plants with such
growth habit areearly maturing compared to their counterparts as exemplified by
peas, beans,fenugreek, etc.
Detrimental. Of mutation
lowering the viability of affected individuals.
Development. The process
whereby a single cell becomes a differentiated organism. It isa process of
regulated growth and differentiation that results from the interaction ofgenome
with its immediate environment (cytoplasm and internal cellularenvironments)
and external environment. It can be considered a programmedsequence of
phenotypic changes that is normally irreversible. The sum total of thesemodifications
constitutes the life cycle of an organism.
Developmental Genetics. A branch of
genetics dealing with the manner in which genescontrol or modulate the process
of development.
Developmental Homeostasis. The capacity
of the developmental pathways of individualsto produce a normal phenotype in
spite of developmental or environmentaldisturbances. Also called individual buffering.
It has been shown that heterozygousindividuals, such as F1 hybrids, are more
stable than their homozygous parents. Thestability of heterozygous individuals
seems to be related to their ability to performunder stress conditions compared
to homozygous parents.
Developmental Pathway. The chain of
molecular events that take a set of equivalentcells and produce the assignment
of different fates among those cells.
Deviation. Departure of
an observation from its expected value.
Dhurrin. A cyanogenetic
glucoside contained in the green leaves of sorghum. Upon itshydrolysis in the
presence of an enzyme, it releases hydrocyanic or prussic acid in therumen of
the animals. At higher concentration, it can cause fatal poisoning toruminants
that graze it. The potential HCN problem may be reduced by grazingmanagement
practices.
Diallel cross: The crossing in all possible
combinations of a series of genotypes
Diallel Selective Mating (Jensen 1970). A set of diallel selective mating procedures toserve as a supplement to
conventional breeding systems for autogamous crops. Thesystem uses multiple
parent input into a central gene pool (population) which, throughselective
mating of individuals, is advanced through successive generations. Mass
andrecurrent selection procedures are employed throughout on both the plant and
seedportions of each plant generation. The diallel and selective mating aspects
of thissystem infuse simultaneously multiple genotypes into a few central
populations.According to Jensen (1970), it involves a planning phase and an
implementationphase. The latter has four stages: a basic parent series of
crosses (1) which set up theF1 diallel series of crosses (2) which when
composited form P2; spaced P2 andsubsequent P3, etc. provide the populations
within which selective mating (3) ispracticed on mass and recurrent selection
principles. The final stage (4) is standardline selection from the various F5
composite populations. Specifically, the proposedarrangements provide for broad
use of germplasm, simultaneous input of parents,creation of persistent gene
pools, breaking of linkage blocks, freeing of geneticvariability, and general
fostering of genetic recombination.
Diallel. A set of crosses
possible among a series of genotypes. Thus crossing n lines inall
possible combinations yields n2 progeny families (including cross of
each line toitself). The set of crosses is called full diallel. If the selfs
are omitted (on account ofmale-sterility or self-incompatibility), only n(n-1)
crosses are possible. Furthermore,in absence of maternal effects, there is
no need to include reciprocal crosses.Therefore, practically only ½ n(n-1)
crosses are required to complete half of thediallel table. The evaluation
of such crosses is called single cross/diallel test or diallelanalysis; it
provides information for both GCA of a line, and SCA of particular pairsof
clones or lines. In addition, nature and magnitude of several genetic
parameters canalso be assessed.
Dicentric Chromosome. A chromosome
with two centromeres.
Dichogamous. Of flowers
whose anthers and pistil mature at different times.
Dichogamy: The condition in which male and female
parts of a flower matureat different times
Dichotomous Classification. The classification applying to an event that can occur inone of the two
possible ways.
Diclinous. Having male
and female germ cells in separate flowers either on the sameplant (monoecious)
or on different plants (dioecious).
Differential
selection: The difference
between a selected plant, family, or clone,and the average of the population
from which it is taken
Differentiation. The changes in
cell shape and physiology associated with the productionof the final cell types
of a particular organ or tissue. It is the origin of differencesduring
embryonic development between spatial parts of an originally homogeneouswhole.
In higher plants and animals, it is one of the spectacular aspects
ofdevelopment.
Digenomic Species. Species
containing two distinct genomes. Such species areinvariably amphidiploids.
These are also called secondary species because they areconvergent products of
primary species. In oilseed brassicas, B. juncea (AABB), B.napus (AACC),
and B. carinata (BBCC) are digenomic species. It is interesting tonote
that these secondary species are basically self-pollinated compared to
theiroutbreeding diploid constituents. This is probably because genomic
divergence makesit unnecessary for continuous reshuffling of genes through
cross-pollination.
Digested RAMP: Markers
developed by digesting the RAMP products with a restriction enzyme.
Dihaploid. A haploid (n
= 2x) of tetraploid. It contains two basic sets of chromosomes.For example,
the haploid of S. tuberosum, which is a tetraploid species (2n = 4x),would
be called a dihaploid (n = 2x).
Dihaploid: A haploid cell or individual
containing two haploid chromosomesets - not to be confused with doubled-haploid
Dihybrid. Heterozygous
with respect to two loci.
Dihybrid: A cross between individuals that
differ with respect to two specifiedgene pairs
DIMBOA. A biochemical
(2,4-Dihydroxy-7-methoxy-1,4 benzoxazin-3-one). Itspresence in the leaves of
inbred lines of maize has been shown to confer resistance tofirst brood larvae
of stem borer. It has also been found as a primary chemical factor inthe
non-preference mechanism of resistant dent maize. As the feeding habits of
thesecond brood larvae are different, the above correlation does not hold true
in this case.
Dimorphism. The occurrence
of two forms (genotypes) within a population; theoccurrence of two types of
flowers on one plant.
Dioecious. A mating
system wherein staminate and pistillate flowers are borne ondifferent
individuals of the same species (papaya, for example).
Dioecious: Possessing male and female flowers or
other reproductive organs onseparate, unisexual, individual plants (eg. in
hemp, hops, asparagus, or spinach) diploid number
Dioecy. The state of being
dioecious. It is a condition in which male and female flowersoccur on different
plants of the same species (e.g., papaya, asparagus, spinach, hemp,hops, etc).
This mating system is genetically controlled, and prevents self-pollinationand
thus enforces cross-pollination. Although more frequent in animals, it is
onlysporadic in higher plants, probably because it is wasteful of gametes in
non-mobileorganisms.
Diploid
parthenogenesis: A type of
gametophyte apomixis by which a diploidembryo sac cell results in a diploid
embryo; the diploid condition is the result ofcytogenetic mechanisms occurring
in the egg stage
Diploid. An organism with two
chromosomes of each kind. Such individuals aredesignated by 2n.
Diploid: A cell with two chromosome sets or an
individual with twochromosome sets in each cell; a diploid state is written as
“2n” to distinguish itfrom the haploid state “n”. (A full set of genetic
material consisting of paired chromosomes, one chromosome from each parental
set)
Diploidisation: In polyploids, a natural or induced
mechanism in which thechromosomes pair completely or partially as bivalents,
although polyploid setsof chromosomes are present; it may be caused by a
structural differentiation ofhomologous chromosome sets or by genetic control;
for example, in bread wheatthree homoeologous genomes are available (AABBDD);
they do not pair ashexavalents but exclusively as bivalents
Diplotene. The stage of
meiosis which follows pachytene and during which chromatidsmove apart in pairs
but still united in the region of chiasmata.
Directed Mutagenesis. Altering some
specific part of a cloned gene and reintroducingthe modified gene back into the
organism.
Directional
dominance: A type of
dominance in which the majority of dominantalleles have positive effects in one
direction
Directional
mutation: A genetic change
that favours a certain genotype orpopulation
Directional Selection. Selection
favouring an extreme phenotype in a given direction. Itchanges the frequency of
an allele in a constant direction, either towards or away fromfixation for that
allele. It pushes a population towards homozygosity (fixation of theallele). If
practiced in the opposite direction, it may eliminate the allele from
the63population. Much of the selection practiced by plant breeders is of this
kind, sincethey select for extremes of yield, productivity, resistance to
diseases, etc. It may wellequally be important in evolution when the
environment of a population is changingand only extreme phenotypes happen to be
adapted for new conditions.
Directional
selection: Selection
resulting in a shift in the population mean inthe direction desired by the
breeder
Dirty crop
approach: Partially
resistant multilines where each component of themultiline carries a single gene
for resistance but none of the resistances are effectiveagainst all known races
of the pathogen (Marshall 1977); it has been advocated asa dynamic, natural
biological system for the control of crowd diseases in cropplants; multilines
could stabilise the race structure of the pathogen population
Dirty Crop. An approach in
which none of the component lines of a multiline variety isresistant to all the
prevalent races of the pathogen. Therefore, some sort of susceptiblereaction
for a few races of the pathogen in such a multiline variety always occurs;
thismakes the entire crop “dirty”. This approach of forming a multiline variety
has asignificant potential advantage over the clean crop approach. Since
moderatelysusceptible lines are also considered, the breeder is in the
advantageous position as hecan exercise selection for characters such as yield,
maturity, height, and the like. Itwill also indefinitely extend the useful life
of strong resistance genes.
Disassortative
mating: Occurs if the
plants mating resemble each other lessthan plants belonging to pairs of random
plants, with regard to some trait
Discontinuous Variation. Variation
having distinct phenotypic classes for a particularcharacter e.g., tall vs.
dwarf, resistant vs. susceptible).
Discovery array: A microarray
having all the fragments amplified following the complexity reduction
procedure.
Disease
avoidance: Avoiding the
disease by, for example, growing the cropsufficiently early so that the
vulnerable part of the plant’s growing cycle is overbefore the disease-causing
organism arrives in the area; this stops the disease fromstarting; it is
sometimes called “passive resistance”
Disease control: Several types of disease control can
be classified: (1) diseaseresistance, (2) protection, (3) avoidance, (4)
exclusion, (5) eradication, (6) therapy
Disease cycle: A cyclical sequence of host and
parasite development andinteraction that result in disease and in reproduction
of the pathogen
Disease escape: For a variety of reasons, some
individuals in a screeningpopulation may remain free of pests or disease; it
also known as chance escape;this phenomenon can be very misleading because it
is so easily confused withresistance
Disease Incidence. The proportion
of plant units infected, that is, percentage of diseasedplants or ears.
Disease
resistance: The ability to
resist disease or the agent of disease and toremain healthy
Disease Severity. The proportion
of the total area of plant tissue affected by a disease.For pathogens that
spread according to the compound interest law, disease severity isthe
cumulative result of infection frequency (proportion of spores that
result insporulating lesions), latent period (time from infection to
spore production), sporeproduction (mass of spores produced per lesion
or per unit area of tissue per unittime), and infection period (period
of sporulation).
Disease triangle:
The three
conditions required for a disease to occur, namely: a susceptible host, a
suitable pathogen, and an appropriate environment
Disease. A physiological
disorder or structural abnormality that is deleterious to plants,plant parts
and its products, reducing its economic value. It can be considered
amalfunctioning process caused by infectious agents resulting in sufferings of
plants.The optimum conditions for a disease to occur are a combination of three
factors: asusceptible host, an infective pathogen and a favourable environment.
A change inany of the three factors will cause a corresponding change in the
expression of diseaseby the host plant.
Disease: A condition in which the use or
structure of any part of a livingorganism is not normal; harmful deviation from
normal functioning ofphysiological processes; six types of causal agents can be
considered: (1) fungi, (2)bacteria, (3) viruses, (4) nematodes, (5) insects,
and (6) plant parasites
Disjunction. The separation
of daughter chromosomes at anaphase. (Also seechromosome disjunction).
Disomic. Individuals with two
chromosome sets, each chromosome in one set having ahomologue in the other; a
monoploid cell with one chromosome in duplicate (n+1).
Dispermy. The entrance
of two spermatozoa into an egg cell.
Dispersion. The pattern of
distribution of observations in a set of data. The measures ofdispersion
provide information about the nature of series. This is additionalinformation,
which measures of central tendency (mean, median or mode) cannotprovide.
Disruptive Selection (Thoday 1972). Simultaneous selection for both extremes (withinthe same generation).
Considerable genetic differences may be maintained betweengroups in such
populations in spite of crossbreeding between individuals fromdifferent
environments.
Disruptive
selection: A selection that
changes the frequency of alleles ina divergent manner, leading to the fixation
of alternative alleles in members ofthe population; the result after several
generations of selection should be twodivergent phenotypic extremes within the
population
Distal. Of a part of a
chromosome arm, which is farther from the centromere than another(proximal)
part.
Distant
hybridisation: The crossing
and/or hybridization of members ofdifferent genera
Distinctness: In PBR, the new
variety must be distinguishable from other varieties of the same crop for one
or more identifiable morphological, physiological, or other characteristics.
Distribution Function. A graph of
some precise quantitative measure of a characteragainst its frequency of
occurrence.
Diversifying
selection: Selection in
which two or more genotypes showoptimal adaptation under different environments
Diversity array technology: A
high-throughput, low-cost modification of AFLP procedure that uses
microarray-based nucleic acid hybridization for detection of polymorphism.
Diversity arrays
technology (DArT) marker: A DArT marker is a segmentof genomic DNA, the presence of which is
polymorphic in a defined genomicrepresentation; DArT markers are diallelic and
behave in a dominant (presentversus absent) or codominant (two doses versus one
dose versus absent) manner;to identify the polymorphic markers, a complexity
reduction method is appliedon the metagenome, a pool of genomes representing
the germplasm of interest;the genomic representation obtained from this pool is
then cloned and individualinserts are arrayed on a microarray resulting in a
“discovery array”; labelledgenomic representations prepared from the individual
genomes included in thepool are hybridised to the discovery array
Diversity panel: A sample
representing as much genetic diversity of the parent population as is
practically feasible.
Dizygotic: Two cells having been fertilised at
the same time (often resulting intwins.)
DNA (deoxyribonucleic acid): The molecule that encodes genetic
information.DNA is a double-stranded molecule held together by weak bonds
between basepairs of nucleotides. the four nucleotides in dna contain the bases
strandedmolecule held together by weak bonds between base pairs of nucleotides.
Thefour nucleotides in DNA contain the bases: adenine (A), guanine (G),
cytosine (C), and thymine (T). In nature, base pairs form only between A and T
and between Gand C; thus the base sequence of each single strand can be deduced
from that ofits partner.
DNA amplification fingerprinting:
A
dominant marker system; typically, a single 4–6 nt long single primer is used
for amplification of genomic DNAs.
DNA barcode: A standardized
genomic DNA sequence of over 400 bp length used for a reliable identification
of organisms.
DNA chip: A high density array of short DNA
moleculesbound to a solid surface for use in probing a biologicalsample to
determine gene expression, marker pattern, ornucleotide sequence of DNA/RNA.
DNA Chips. Also called microarray.
A flat surface about the size of a postage stampwith 10,000 to 100,000 distinct
spots, each containing a different immobilizednucleotide sequences. Gene chip
technology permits scientists to analyse thousands ofgenes at once.
DNA Clone. A clone of a
specific section of DNA. Such a segment is inserted into avector molecule (such
as a plasmid or a phage chromosome), and then replicated toform many copies.
DNA fingerprint(ing) (DFP): The unique pattern of DNA
fragments identifiedby SOUTHERN hybridisation (using a probe that binds to a
polymorphic regionof DNA) or by polymerase chain reaction (PCR) using primers
flanking thepolymorphic region
DNA Fingerprint. The
autoradiographic banding pattern of DNA. It is produced whenDNA is digested
with a restriction enzyme that cuts outside a family of VNTRs, and aSouthern
blot of the electrophoretic gel is probed with a VNTR-specific probe.
Unliketrue fingerprints, these patterns are not unique to each individual.
DNA Fingerprinting. Also called DNA
typing. It is a procedure in which unknown DNAsamples are matched with
their putative sources through the use of polymorphic DNAmarkers.
DNA fingerprinting: Multilocus/single
locus assays based on minisatellite/microsatellite DNAs for unequivocal
identification of individuals on the basis of gel patterns of the derived
fragments.
DNA hybridisation: Base pairing of DNA from two different
sources; inbiotechnology, a technique for selectively binding specific segments
of singlestranded(ss) DNA or RNA by base pairing to complementary sequences of
ssDNAmolecules that are trapped on a nitrocellulose membrane
DNA ligase (polynucleotide ligase): An
enzyme that creates a phosphodiesterbond between the 5′-PO4 end of one
polynucleotide and the 3′-OH end of another, thereby producing a single, larger
polynucleotide
DNA marker: A DNA sequence that exists in two or
more forms that can beused to genotype individual organisms.
DNA Markers. The specific
spots (nucleotide sequences) on DNA molecules in codingas well as non-coding
regions. These specific spots show sequences polymorphism indifferent
individuals. (Also see molecular markers).
DNA micro arrays: A powerful, versatile and economical
molecular techniquefor screening of genetic aberrations; high-density gene
sequences are printedonto glass slides; fluorephorelabeled genomic or
complimentary DNA (cDNA) ishybridized to slides with fixed signature patterns
and resolved using computerdriven fluorescent images
DNA Polymerase. An enzyme that
can synthesize new DNA strands from a DNAtemplate. Several classes such as DNA
polymerase I, II, III, etc. exist.
DNA polymerase: A group of enzymes mainly involved in
copying a singlestrandedDNA molecule to make its complementary strand
DNA probe: A single-stranded DNA molecule used
in laboratoryexperiments to detect the presence of a complementarysequence
among a mixture of other single-stranded DNAmolecules. Also called gene probe.
DNA profile: The distinctive pattern of DNA
restriction fragmentsor PCR products that can be used to identify, withgreat
certainty, any person, biological sample from a person,or organism from the
environment.
DNA replication: The use of existing DNA as a
template forthe synthesis of new DNA strands. In humans and othereukaryotes, replication
occurs in the cell nucleus.
DNA sequence: The relative order of base pairs,
whether in a fragment ofDNA, a gene, a chromosome, or an entire genome.
DNA sequencing: Determination of
base sequence of a DNA fragment.
DNA sequencing: The process of determining the order
of DNA bases along aDNA strand.
DNA. Deoxyribose Nucleic
Acid; a biochemical molecule of two chains ofpolynucleotides twisted into a
double helix and joined by hydrogen bonds between thecomplementary bases of the
two chains. The base sequence of the DNA molecule isspecific to each individual
and it determines individual hereditary characteristics. It isthe primary
genetic material except for a few plant viruses.
DNA-amplified fingerprinting: A technology based on amplification of
randomgenomic DNA sequences achieved by a single short (5–8 bases)
oligonucleotideprimer of arbitrary sequence; it produces a characteristic
spectrum of short DNApieces of varying complexity that are resolved on
polyacrylamide gel (PAGE)following silver staining; it is used to detect
genetic differences between genotypesas well as for detecting polymorphism even
between organisms that are closelyrelated, such as near isogenic lines
DNA-DNA hybridization: The annealing of two complementary DNA
strandsto produce hybrid
DNase. Also called deoxyribonuclease.
An enzyme that degrades DNA to nucleotides.
Doak Method (Doak, 1954). A method of hybrid seed production in cotton. It involvesmanual
emasculation of flower-buds of female parent a day before anthesis anddusting
pollen grains from desired male parent on to the stigma of emasculated budsthe
next day. Parental varieties are grown in contiguous fields in a ratio of four
haunder female to one ha of male parent.
Domain of a protein: A discrete continuous part of the
amino acid sequencethat can be equated with a particular function.
Domain: A discrete portion of a protein with
its own function. The combinationof domains in a single protein determine its
overall function.
Domesticates. Species/individuals
under domestication. Initially they differ little fromtheir wild ancestors in
amenability to agricultural circumstances, and selection leadsto the
directional change as desired by human beings. Cultivars are also
domesticatesin one sense. However, some consider domesticates as the altered
type that can nolonger survive without human intervention.
Domestication. Bringing a
wild species under human management. It is the oldestmethod of plant
breeding. It is a method of plant improvement owing to its capacity toprovide
for types better than previously available ones. The present application
ofdomestication is to bring one or a few genes from wild relatives to our
cultivatedspecies.
Domestication: The process by which plants are
genetically modified by selection over time by humans for traits that are more
desirable or advantageousfor humans. (The process of bringing wild plants
undercultivation to produce crops under the supervision ofhumans)
Dominance Gene Action. A type of
intralocus interaction in which the heterozygoteapproaches towards one of the
homozygotes. The change is such that the substitutionof A for a in
the genotype aa is not the same as in Aa, that is, the effect of
substitutionis non-linear; the effect so-produced on the phenotypic scale of
measurement is calleddominance effect. Selection for dominance effect is less
effective. It is so because ifall genes act in the fashion as exemplified
above, the hereditary variance will be oftwo types, that is, additive and
dominance variances. However, selection will beeffective for additive component
only.1AABB (7), 2AABb (6), 1AAbb (4)1AABB (7), 2AaBB
(7), 1aaBB (3)
Dominance Hypothesis (Davenport 1908, Bruce 1910). A suggested explanation toaccount for heterosis or hybrid vigour. It
assumes that heterosis results fromaccumulation of favourable dominant alleles
at individual loci in the F1 hybrid. Saidin another way, heterosis is the
consequence of sheltering of deleterious effects ofrecessive alleles by their
adaptively superior counterparts (dominant alleles) atrespective loci. Although
widely accepted hythesis to explain hybrid vigour forquantitative traits, it
fails to account for heterosis for monogenic (oligogenic)characters.
Dominance hypothesis: Heterosis is the
result of favorable dominant alleles masking the deleterious effects of
concerned recessive alleles in the heterozygotes.
Dominance hypothesis: In hybrid breeding, dominant alleles
of genes thatshould have stimulating effects on heterosis, while recessive ones
should showinhibitory effects
Dominance Variance. A component of
genetic variance, which results from aninteraction between different alleles at
a given locus. Selection is ineffective for thiskind of variance.
Dominance variance: That portion of the genetic variance
attributable todominant gene effects
Dominance. A phenomenon
in which the heterozygote falls towards one of thehomozygotes on the phenotypic
scale of measurement. If the heterozygote resembles one of the homozygotes, the
phenomenon is called complete dominance. It is exemplified by the
heterozygotes for all the seven characters studied by Mendel [e.g.,AA (3),
Aa (3) & aa (1)]. It is a case of an intra-locus interaction.
In this case (alsocalled simple dominance), dominance is considered a
functional or physiologicaleffect.
Dominance: The expression of a trait in the
heterozygous condition.
Dominant Allele. The expressed
allele in a heterozygote; the suppressed allele is calledrecessive. Strictly
speaking, an allele is neither dominant nor recessive; rather itseffect may be
dominant or recessive. However, in practical usage, it has come to becalled as
such.
Dominant gene: A gene whose phenotype is expressed
when the allele ispresent as either one or two copies of the pair (Eg. AA or
Aa).
Dominant Phenotype. The phenotype
of a genotype containing the dominant allele;Also, the parental
phenotype that is expressed in a heterozygote.
Dominant. Of genetically
controlled characters expressed in F1 heterozygotes derivedfrom a cross between
two pure-breeding strains differing in respect to somecharacters. allele67
Dominant: A phenotype that is expressed in an
organismwhose genotype may be either homozygous or heterozygousfor the
corresponding allele.
Dominant-negative mutation: A (heterozygous) dominant mutation on
oneallele blocking the activity of wild-type protein still encoded by the
normal allelecausing a loss-of-function phenotype; the phenotype is
indistinguishable fromthat of homozygous dominant mutation
Domino Effect. The effect (or
the theory) whereby one event sets off a series of similarevents; the fall of
one domino standing on end causing the whole row to fall in turn.In genetics
and plant breeding, if a genotype becomes susceptible to a race of a
givenpathogen, the neighbouring genotypes may naturally tend to follow the same
path.
Donor DNA. Any DNA to be
used in cloning.
Donor Parent. The parent
from which one or a few genes (and consequently thecharacters) are transferred
to the recurrent parent in a back cross breeding programme.The character(s) in
question must be in highly intense form for some intensity isusually lost in a
different genetic background of recurrent parent owing to influence ofa
different set of modifiers.
Donor parent: A homozygous
line from which the gene of interest is transferred into another line.
Dormancy. The enability
of viable seeds to germinate under conditions conducive togermination. From
evolutionary point of view, it is an adaptive advantage for naturalsurvival
under unfavourable condition, and a mechanism to ensure distribution intime and
space, eventually leading to perpetuation of the species. If dormancydevelops
due to conditions within the seed when it matures on the mother plant, it
is68termed primary dormancy. However, if a germinable seed becomes
dormant underspecific unfavourable conditions during storage (excessive drying)
or germination(high temperature), it is referred to as secondary dormancy.
It is regulated by externalcoverings (testa and pericarp) and tissues
surrounding embryo. Sometimes,underdeveloped embryo may be the real cause of
dormancy. All these physiologicalvariables may be governed and/or influenced by
genetic factors. Dormancy may bereleased by heat and light treatments,
treatments by growth hormones (e.g., GA3@100-1000 ppm with or without kinetin @
50 ppm) or with chemicals such as KNO3(0.2%), thiourea (0.5 - 3%).
Dosage Compensation. The process in
organisms using a chromosomal sexdetermination mechanism that allows standard
structural genes on the sexchromosome to be expressed at the same levels in
females and males, regardless ofthe number of sex chromosomes. In mammals,
dosage compensation operates bymaintaining only a single active X-chromosome in
each cell.
Dosage compensation: A genetic process that compensates for
genes that existin two doses in the homozygous dominants, so that the
heterozygotes produce thesame amount of gene product as the homozygotes
Dosage Effect. The
quantitative effect on the phenotypic expression of a characterproduced by
varying number of particular alleles of a gene(s).
Dosage effect: The influence upon a phenotype of the
number of times a geneticelement is present
Dose Rate (Radiation). The amount of
radiation received per unit time. The time overwhich a given dose is received
is a very important consideration. Large, single dosesdelivered at short time
intervals (minutes/hours) are known as acute doses, in contrastto chronic
doses that might be experienced continuously over the whole life cycle
orover a large period.
Double Cross (Jones 1918). The F1 between two single crosses [(A x B) x (Cx D)].
Thenumber of possible double crosses using n inbreds would be [ n.(n-1).(n-2).(n-3)
/ 8 ].Double cross hybrids have now been replaced by single cross hybrids in
maize.
Double cross: A cross between two F1 hybrids; the
method used for producinghybrid seed; four different lines (A, B, C, D) are
used; A × B = AB hybrid and C× D = CD hybrid; the singlecross hybrids (AB and
CD) are then crossed and thedouble-cross hybrid (ABCD) seed is used for the
commercial crop
Double crossing-over: The situation in which two
crossing-overs take placewithin a tetrad
Double digest
restriction-site-associated DNA sequencing: Genomic DNA is
simultaneously digested with two restriction enzymes, and a precise size
selection procedure is used to select only such fragments of specified size
that are produced due to one cut by each of the two enzymes.
Double fertilisation: The union of one sperm nucleus with
the egg nucleus toform the diploid zygote and of the other sperm nucleus with
the two polar nucleito form a triploid endosperm nucleus
Double Helix (Watson and Crick 1953). The structure of DNA with two interlockinghelices joined by
hydrogen bonds between paired bases.
Double helix: The shape that two linear strands of
DNAassume when bonded together.
Double monoisosomic: The presence of two isochromosomes,
one for eacharm; it can derive from a double monotelosomic individual.
Double monotelosomic: The presence of two telocentrics, one
for each arm
Double reduction: The genetic outcome of chromatid
segregation, as opposedto chromosome segregation, whereby two sister-chromatid
segments are includedin the same meiotic product
Double-cross hybrids: Hybrids resulting from crossing two
single crosshybrids
Doubled Haploid. An otherwise
diploid plant produced by doubling the chromosomenumber of the haploid (2n =
2x, 4x, 6x, etc).
Doubled haploid: A diploid plant, which results from
spontaneous or inducedchromosome doubling of a haploid cell or plant, usually
after anther or microsporeculture by using different means (An individual that
is produced by doublingits gametic (n) chromosome number into 2n)
Down promoter mutations: Mutations that decrease the frequency
of initiationof transcription; they lead to the production of less mRNA than is
the case in thenonmutated state
Drift. The changes in the
gene and genotypic frequencies in small populations due torandom processes.
Drought Avoidance. A mechanism of
drought resistance. It is also called droughttolerance with high tissue water
potential. It consists of mechanisms to reduce waterloss from the aerial
portion of plants, and of mechanisms to maintain water uptake.The maintenance
of water uptake under drought condition is related to severalproperties
concerning roots of plants such as root size and efficiency, root density,size
of xylem vessel, and the like. Genotypic differences have been noted for all
theseattributes. The reduction of water loss from aerial portions of the plant
includes69mechanisms such as stomatal regulation, morphology and orientation of
leaves, waterretention capacity of leaves, leaf rolling and wilting (which
curtails transpiration rateby about 46-83% in grasses), cuticular waxiness
(2-50% reduction in transpiration),and leaf reflectance (up to 50% reduction in
light absorption). These morphologicaltraits are easy to study. Some of them
have very simple inheritance pattern. Forinstance, leaf rolling which has been
noticed in drought resistant germplasms ofmaize, sorghum, and rice appears to
be a monogenic recessive trait.
Drought hardening: Adapting plants to survive periods of
time with little or nowater by stepwise reducing water supply or germinating
and/or growing underinsufficient moisture conditions
Drought Resistance. A heritable
property that enables a plant to withstand moisturedeficit situation. According
to Levitt (1972), drought resistance is the sum total ofdrought avoidance and
drought tolerance. In essence, a plant can resist droughtcondition either
through reduced water loss from aerial portions, increased wateruptake from
deep layers of the soil or giving more yield at low water potentials.
Thereseems to be controversy as to which component of drought resistance, that
is,avoidance or tolerance, is most important in crop plants. In reality, a
mixture of bothis required. Even the best drought avoiding species requires
tolerance, since somereduction in plant water potential is unavoidable during
severe stress. Indeed, there isevidence to suggest that drought avoidance is
operative during the vegetative phase,while the tolerance comes into play
during the reproductive phase in crops such ascereals.
Drought resistance: the ability of a plant to withstand drought;
this propertycan be very valuable in areas of uncertain rainfall, Eg. sorghum
has greater droughtresistance than maize, and is grown in many semi-arid areas
for this reason
Drought Tolerance. The ability of
a plant to withstand low tissue water potentials.Mechanisms affecting drought
tolerance, as against avoidance, of water stress areeven less clear; thus, it
becomes difficult to propose specific selection criterion forscreening of
drought tolerance genotypes. Under condition of drought, germination,seedling
vigour, net photosynthesis, and ultimately seed yield/plant appear to
beaffected adversely. Therefore, most studies of drought tolerance have
focussed onthese aspects. There has been renewed interest in selecting
genotypes having capacityfor osmotic adjustment. Attempts have also been made
for screening of droughttolerant genotypes through search for readily measured
plant constituents such asproline, non-structural carbohydrates (NSC), and the
like. However, these selectioncriteria vary with the crop species. Probably,
the most difficult problem breeders are70facing in breeding for drought
tolerance is how to evaluate the trait. Drought tolerancehas a low heritability
caused by high genotype-environment interaction. Even whenconsistently selected,
it is difficult to demonstrate drought tolerance consistently. Thisprobably
reflects the fact that it has a complex inheritance and is governed
bypolygenes.
Drought. A condition of
moisture deficit. It refers to a situation wherein available soilmoisture is
not sufficient to meet the demands of potential evapo-transpiration. Thekind of
drought that is of our interest is the agricultural drought. It occurs
where soilmoisture and rainfall are inadequate during the growing season to
support healthycrop growth to maturity and cause extreme crop stress and wilt.
It should carefully bedistinguished with physiological drought, which
refers to non-availability of water tothe plant due to unfavourable
physiological conditions (such as soil salinity, lowtemperature, and the like).
Water is present in the real sense, but plants are unable touse it.
Duplex. A polyploid containing
recessive alleles in all chromosomes except two withrespect to a particular
locus or a polyploid having two dominant alleles at a givengenetic locus, e.g.,
AAa, AAaa, AAaaaa, etc.
Duplicate Base Collections. Duplicates of the base collections that are housed ingeographically
different locations for security purposes. The objectives and methodsof storage
are essentially the same as for the base collections.
Duplicate Genes. Two gene pairs
that produce identical effects, whether alone ortogether. Non-floating habit of
rice is governed by duplicate genes. (Floating habit ispresent only when the
two gene pairs are recessive). The two gene pairs are located indifferent
chromosomes. Duplicate genes are of frequent occurrence, and are probablydue to
secondary polyploidy. A heterozygote for duplicate genes under the situation
ofcomplete dominance gives a 15:1 F2 ratio. If, however, dominant genes act inadditive/cumulative
manner, the F2 segregation gets modified to a 9:6:1 ratio.
Duplicate Genes. Two identical
allele pairs in one diploid individual. Either pair performs the same function.
Duplication. The presence
of a segment twice in a chromosome. Adjacent duplicationmay occur in tandem
sequence with respect to each other – abcbcd – or in reverseorder – abccbd.
Obviously, the pairing patterns obtained in these two cases are71different. In
general, duplications are hard to detect and are rare. However, they
areimportant from evolutionary point of view because they supply additional
geneticmaterial potentially capable of assuming new functions.
Duplication: Duplication of a sequence of DNA or
section of chromosome.
DUS. A triple criteria of
distinctness (D), uniformity (U) and stability (S), on the basis ofwhich a new
genotype is considered for release in the new plant variety protectionregime.
It will ensure precise descriptions on comparative basis of varieties
incultivation. Said in another way, we should have accurate identification keys
fordifferent varieties in a crop.
Dyad. A pair of sister
chromatids joined at the centromere, as in the first division ofmeiosis.
Dynamic allele-specific
hybridization: Specific
probes are hybridized with the target PCR products, and melting temperatures of
the duplexes so produced are used to deduce the SNP allele.
Dynamic traits: Quantitative
traits, e.g., plant height, which show different patterns of development in
different genotypes that may, in the end, show comparable/different values for
the concerned trait.
Dysploidy. A condition
referring to differing basic chromosome numbers withinpopulations or species.
It is frequently encountered among seed plants where a seriesof basic number
usually differing by one chromosome is displayed by individualsunder
investigation (e.g., x = 5, 6, 7, 8, 9, etc).
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