M. A general term for mutagenic treatment. M0
refers to the parent material likely to bemutagenized. M1, M2, M3,…are symbols
used to designate first, second, third, etc.generations after treatment with a
mutagenic agent. M2 is often assumed to beanalogous to F2 generation so far as
segregation events and relative degree of geneticvariability is concerned.
However, M1 cannot be compared to F1; while there is 100%heterozygosity in F1,
mutant loci in M1 are random and not so frequent, hence,heterozygosity is
variable.
Macro restriction map: Map depicting the order of and
distance between sitesat which restriction enzymes cleave chromosomes.
Macro-evolution. Evolution
above species level.
Macromolecule. A large
molecule with colloidal properties; a long polymer composed ofmonomers. The
molecular weight of such a molecule is greater than 1000. Polymerssuch as DNA,
RNA, a protein or a polysaccharide are macromolecules.
Macro-mutation. A mutation
with the recognizable effect on a single plant.
Macromutation: a mutation that results in a profound
change in an organism,such as a change in a regulatory gene that controls the
expression of manystructural genes, as opposed to micromutation
Macrophytes. Rooted or
large floating plants generally growing in shallow water only(as in a pond).Maisinte.
A maize-teosinte hybrid fodder crop. It has been produced by
backcrossingmaize-teosinte hybrid with maize. Its main features are quicker
growth, earliermaturity, higher fodder yield and high crude protein content
than parents. Owing toteosinte genes, it may give good response to stress
conditions.
Main effect
QTLA QTL that produces direct effect on expression of
the concerned trait.
Maintainer: Used for maintaining and
multiplication of a cytoplasmic malesterile line; usually genotypes containing
the normal cytoplasm and recessive atthe restorer locus
Major Genes. Genes with
large, easily recognizable and relatively stable effects. Thereare numerous
examples of well defined morphological or physiological characterswhich are
governed by single genes, and are little affected by either the genetic
orphysical environments, e.g., differences between field and sweet corn,
indeterminatevs. determinate growth habit in fenugreek, etc. It is, however,
interesting to mentionthat genes are neither major nor minor; nor
are they qualitative/quantitative; rather itis their effects which appear as
either major or minor. However, the term such asmajor genes is used frequently,
and it is accepted usage.
Major QTLA main effect QTL that explains 10 % or more of the phenotypic variance
for the concerned trait.
Male gametocide: Any substance that kills the male
reproductive cells of aplant (pollen or pollen mother cells), rendering it
male-sterile
Male Sterility. Absence or
non-functioning of pollens in plants. This provides a barrierto
self-fertilization. However, it is advantageous in hybrids seed production
since it138makes mechanical emasculation unnecessary. It occurs spontaneously
in almost allcrop plants irrespective of their pollination behaviour as a
consequence of mutation atany one of the several loci, which condition vital
steps in the formation of functionalpollen.
Male sterility: Producing no functional pollen
Malthusian. Relating to
Thomas Robert Malthus and his population theory (1798). Thetheory states that
population numbers tend to increase at a faster rate (geometricprogression)
than means of subsistence (arithmetic progression), leading tocompetition for
environmental resources in short supply.
Map distance: The distance between any two markers
on a genetic map, basedon the percentage of crossing-over; the minimum distance
between linked genesis 1% and maximum 50%
Map Unit. A unit of
distance in a linkage map. It is the “distance” between two linkedgene pairs,
recombination value for which is one per cent.
Map-based cloning: The isolation of important genes by
cloning the genein question on the basis of molecular maps, where the
biochemical function isunknown; it involves the identification of a mutant
phenotype for the trait ofinterest (obtained by mutagenesis or from natural
variation) and genetic finemapping using many progeny plants; this map is then used
for chromosomewalking or landing, with the help of large-insert DNA libraries
or physical mapsto isolate the gene
Mapping Function. The
relationship between distance in a linkage map andrecombination value (between
the two linked genes) or frequency. It is expressed by aformula: true map units
= -ln [(1-2 R.F.)/2] x 100, whereas, R.F. stands forrecombination frequency.
Mapping
functionsFormulas used for converting recombination
frequency into genetic distance.
Mapping
populationA population that is suitable for linkage mapping
of genetic markers, genes, and/or QTLs.
Mapping. The linear arrangement
of genes on a chromosome.
Mapping: The process of identifying the
location of a gene or DNA segmentalong a chromosome. In genetic mapping,
this is done by analysing patterns ofinheritance in segregating populations
(measured in recombinational units,commonly centimorgans). Inphysical mapping,
this describes the actual location ofa sequence in a particular genomic region
(measured in bp)
Marcotting Technique. A technique to
induce rooting of nodes by covering themselectively with suitable materials
such as mud, soil-coconut fibre mixture or peatmixtures. The rooted nodes are
then cut and replanted in the soil. This facilitatesreduction in the height of
the prospective parents to be crossed (as in sugar cane).
Marker Assisted Back Crossing (MABC). Introgression of specific trait(s) from adonor parent into the genetic
background of a recurrent parent (generally leadingvariety) using molecular
markers (Hospital 2005). The product of MABC is aline/cultivar containing only
the major gene/QTL from the donor parent, whileretaining the whole genome of
the recurrent parent. MABC approach generallyinvolves transfer of a limited
number of trait loci including transgenes from onegenetic background (donor
genotype) to the other genetic background (elite variety).MABC approach can
also be used to generate near-isogenic lines (NILs) orchromosome segment
substitution lines (CSSLs) for genomics research, which arepopulations that are
often used for genetic analysis of genes/QTLs. Gene pyramidingis an important
application of MABC in which a few different genes for the same trait139(e.g.,
resistance to different races) or for different traits are brought together in
onegenetic background using molecular markers.
Marker indexThe product of multiplex ratio and the average PIC score for a marker
system in a given population.
Marker-assisted
backcrossingA backcross program based on molecular markers.
Marker-assisted introgression: The use of DNA markers to increase the
speedand efficiency of introgression of a new gene or genes into a population;
themarkers will be closely linked to the genes in question
Marker-assisted
plant breedingThe use of molecular marker data for enhancing the
effectiveness of various breeding activities, including planning and execution
of breeding programs, and improving selection efficiency.
Marker-Assisted Recurrent Selection (MARS). Estimation of marker effects fromgenotyping F2 or F3 population and
phenotyping F2 derived F4 or F5 progenies,followed by two or three
recombination cycles based on presence of marker alleles forsmall effect QTLs
(Eathington et al. 2007). In the first step of MARS, de novo QTLidentification
is carried out initially, that is, QTLs are identified in the
breedingpopulation itself, generally derived from good × good crosses.
Subsequently, the linescarrying superior alleles for maximum QTLs are crossed
to pyramid superior allelesin one genetic background. Recombined lines are then
subjected to a final phenotypicscreening to select the best lines for
multi-location field traits to release them asvarieties. MARS is particularly
useful for capturing the several genomic regionsespecially to target more
number of minor as well as major QTLs. Therefore, geneticgain achieved by MARS
as compared to the MABC programme is higher (Bernardoand Charcosset 2006). The
recurrent-selection method is routinely used mainly incross-pollinated crops
like maize, and this process can be improved with the help ofmolecular markers;
therefore, the process is called marker-assisted recurrent selectionor MARS.
Marker-Assisted Selection (MAS). The selection of target gene(s) aided by tightlylinked markers. The
essential requirements for MAS in a breeding programme are:
(a)co-segregation or very close linkage (<1cM) of markers with the target
gene/ trait, (b)availability of an efficient means of screening large
populations for molecularmarkers, and (c) high reproducibility across
laboratories of the screening techniques.Besides, markers and screening
techniques should be economical and user-friendly.Of course, the population can
be screened at any growth stage and in anyenvironment. It is also independent
of interaction effects. Markers can also provideadditional information on the
breeding value of a genotype, and thus accelerateselection and breeding cycles.
It can be practiced especially for traits whosephenotypic selection is
difficult. It may be of special value in breeding species thathave large
stature or long generation time (arbour crops) where fewer individuals140might
save several hectares, and fewer generations may save several decades. MAS
isbeing used efficiently: (a) in gene pyramiding, (b) in marker-assisted alien
geneintrogression, and (c) for simultaneous identification and pyramiding of
QTLs fromprimitive cultivars and alien species.
Marker-assisted selection (MAS): Use of genetic/DNA markers to guide
inchoosing specific plants or lines with targeted traits for new variety
development.
Marker-assisted
selectionSelection for the desirable allele of a
gene/quantitative trait locus (QTL) on the basis of molecular marker(s) linked
to it in place of the phenotype generated by this allele.
Markers (Morphological). Traits or
genes (variant alleles), which are used to label abiological system throughout
the course of experimentation. These are usually mutantalleles, which may be
either dominant or recessive. These are limited in number,influenced by
environments, and developmental stage specific compared to molecularmarkers.
Further, they could appear as pleiotropic effect of other major genes.
Masking Action. Gene action
such that a gene hides the effect of another non-allelicgene (when both are
present). In oats, for instance, Y produces yellow seed coat and Bproduces
black seed coat. The gene Y will have no effect in presence of B, since
blackcolour masks yellow ones. It gives a ratio of 12 black (B-Y-, B-yy):
3 yellow (bbY-): 1white (bbyy).
Mass Pedigree Method. A system of
breeding in which a genetically diverse populationis propagated in mass until
conditions favourable for artificial selection occur, afterwhich pedigree
selection is practised. The bulking may end as early as F2 generationor may be
continued for many generations if the advent of conditions suitable
forselection is long-delayed.
Mass pedigree selection: A system of breeding in which a
population ispropagated in bulk until conditions favorable for selection occur;
usually, aftermass pedigree selection, pedigree selection follows
Mass Selection. A form of
selection in which individual plants are selected dependingupon their better
phenotypic values (usually without progeny test) and the nextgeneration is
propagated from aggregate of their seeds. In autogamous crop species, ithas
basically two uses: First, it can effect improvement in land varieties
very safelyand rapidly, and the second, it can be used to purify existing
varieties. In the crosspollinatedspecies, it is used to improve characters
having high heritability. It isvirtually powerless to effect any improvement in
a trait like yield (low heritability)obviously for three reasons: (a) selection
is exclusively based on phenotype, (b) thereis lack of pollen control on female
parent, and (c) there is no progeny test. Therefore,simplicity of mass
selection is the weakness in itself. However, some form of massselection is
almost always practiced in most breeding methods.
Mass selection: A form of breeding in which individual
plants are selectedon their individual advantages and the next generation
propagated from theaggregate of their seeds; the easiest method is to select
and multiply together thoseindividuals from a mixture of phenotypes, which
correspond to the breeding aim(positive mass selection); it is still applied in
cross-pollinating of vegetable species,such as carrots, radishes, or beetroots,
in order to improve the uniformity; whenall undesired off-types are rouged in
grown crop population, and the remainingindividuals are propagated further, the
method is termed negative mass selection;negative mass selection is no longer
an adequate breeding method for highlyadvanced varieties; it is usually applied
in multiplication of established varieties(i.e., for seed production in order
to remove diseased plants, casual hybrids, orother defects)
Maternal Effect. The
environmental influence of the mother’s tissues on the phenotypeof the
offspring. It may be result of nuclear genes as observed for cases like
shellcoilingexample in the water snail Limnaea.
Maternal effect: Any nonlasting environmental effect or
influence of thematernal genotype or phenotype on the immediate offspring
Maternal Inheritance. A type of
uniparental inheritance in which all progeny have thegenotype and phenotype of
the parent acting as the female. It was first discovered byCarl Correns (1909)
in his studies on four-o’clock plants (Mirabilis jalapa) for
leafvariegation (patches of green and white tissues in the leaf).
Maternal inheritance: Phenotypic differences found between
individuals ofidentical genotype due to an effect of maternal inheritance
Maternally Expressed Gene. A gene that
contributes to the phenotype of an offspring onthe basis of its expression in
the mother.
Mating System/ Mating. Any of a
number of schemes by which individuals are assortedin pairs leading to sexual
reproduction, and thus formation of zygotes.
Mating Types. The analogous
terms used in lower organisms for sexes in higherorganisms. Mating types differ
only physiologically, and not in the physical form.
Matriclinous Inheritance. A type of
inheritance in which all offspring have the nucleusdeterminedphenotype of the
mother.
Matriclinous. Having
predominantly maternal hereditary traits.
Matrix
substanceAn organic molecule that has the same energy absorption
spectrum as the selected laser wavelength and does not interact chemically with
the target biomolecule.
Maximum Likelihood. A statistical
procedure for estimating values of populationparameters from sample data. The
method identifies values that have maximumprobability of being the best fitting
for any given set of observations.
Mean Deviation. The sum total
of average deviations (d) of individual observationsfrom their arithmetic mean
without regard to the sign {d/N}. Though not
usedfrequently, it finds its greatest application in calculating combining
ability effect(s) ofa given parent of/or a cross in plant breeding experiments.
Mean. Also called arithmetic
mean. It is the arithmetic average and is obtained when thenumber of
individuals in the data divides the sum of the values of the observations
(inthat data). It is usually designated by μ and x in the population and
sample,respectively. Unlike median and mode, it is affected by the extreme
values. However,it is most commonly used and preferred as it provides more
information about thecentral tendency than other measures.
Mechanical Errors. Errors
occurring in the execution of an experiment. These can occurduring layout,
management and care of the experiment and data keeping andrecording. These are
primarily human errors, and thus cannot be effectively controlledby statistical
techniques. These can make a substantial difference in the experimentalresults.
Their prevention depends largely on the awareness and the skill of
theresearcher in relation to the various operations of the experiment.
Median. The middle value of a
series of observations arranged in ascending ordescending order of their
magnitudes. It divides the series into two equal halves, halfthe number of the
observations lying above it and half below. If the number ofobservations in a
series is odd, then (N+1)/2 is the middle item of the series (where Nis the
total number of observations). However, in case of an even series data,
twomiddle items [(N/2) and (N/2+1)] will represent the median.
Medium. Any material on which
experimental cultures are grown. Although compositionof a medium varies
according to the objectives of a programme, it usually consists ofsugar (as a
source of energy), certain inorganic salts, one or a few phytohormones
andvitamins, and agar (as a solidifying agent if a solid medium is to be
prepared).
Mega Base Pairs. One million
nucleotide pairs.
Megabase (Mb): Unit of length for DNA fragments equal
to 1 millionnucleotides and roughly equal to 1 cM.
Megaenvironment. Broad (not
necessarily continuous often transcontinental) area withsimilar biotic and
abiotic stresses, cropping systems and consumer preferences.Analysis of
megaenvironment is performed by AMMI model.
Megagametogenesis: The development of the female gametophyte
from afunctional megaspore
Megasporangium. A sporangium
producing megaspores.
Megaspore Mother Cell. The diploid
(2n) cell in the ovary that gives rise to four haploidmegaspores through
meiosis.Meiocyte. A cell embarking upon meiosis (or a cell in which
meiosis takes place).
Megaspore. One of the
four haploid spores originating from the meiotic division of thediploid
megaspore mother cell in the ovary, and which gives rise to the
megagametophyte.
Megasporogenesis: The development of the megaspore from
the archesporial cell
Meiocyte: The sporocyte giving rise to the
embryo sac and to pollen grains
Meiosis. A kind of nuclear
division occurring in reproductive cells, which results inproduction of gametes
(animals) or sexual spores (in plants) with haploid (n)143chromosome
number. Thus, it maintains chromosome number of sexuallyreproducing species.
Meiosis: A type of nuclear division that occurs
at some stage in the life cycleof sexually reproducing organisms; by a specific
mechanism, the number ofchromosomes is halved to prevent doubling in each
generation
Meiospores. Products of
meiosis in plants.
Meiotic Drive. Any mechanism
that affects the genetic composition of a populationthrough unequal
contribution of gene(s) [and thus chromosome(s)] to the gametesduring meiosis.
This preferential segregation makes a disproportionate contribution tothe gene
pool of the next generation. It is an exception to the rule of 1:1 recovery
ofsegregating alleles in heterozygous organism. This unequal or preferential segregationhas
also been termed segregation distortion.
Melting Pot Technique. A modified
polycross method as in sugar cane. It consists ofplacing arrows from a
number of prospective parents in small containers (added withpreservative
solution) and effecting random intermating by changing the position
ofcontainers (along with arrows) in relation to each other every day.
Seeds so obtainedinclude selfs and all possible crosses including reciprocals.
Melting temperature (Tm)The temperature
at which 50 % of the DNA duplexes would dissociate into separate strands.
Melting. Denaturation of DNA.
Melting: Denaturation of DNA.
Mendel ’s laws of inheritance: The inheritance of chromosomal genes
onthe basis of the chromosome theory of heredity; three laws are considered:
(1)law of dominance or of uniformity of hybrids, (2) law of segregation, (3)
law ofindependent assortment
Mendelian Inheritance. Inheritance of
chromosomal genes in contrast to extrachromosomalinheritance, which deals with
cytogenes.
Mendelian population: A group of (potentially) interbreeding
plants (crossfertilizingcrops), which may occur in a certain area or in an
experimental design
Mendelian Ratio. A ratio of
progeny phenotypes based on Mendel’s laws of inheritance.
Mendelian ratio: The segregation ratios according to
Mendel ’s laws ofinheritance
Mendelian Variation. The diverse
types resulting from gene mutations as well asrecombinations following
hybridization between types carrying these mutations. Thisis the major kind of
variation exploited by plant breeders.
Meristem Tip Culture. The in
vitro culturing of plant tissue from the meristem tipregion for the purpose
of regenerating pathogen-free plants.
Meristem. An area of
rapidly dividing plant cells. It may be a single cell (as in lowerplants) or it
may include many cells (as in higher plants).
Mesolithic Age. The cultural
period between the Paleolithic and Neolithic Ages;appearance of the bow and
cutting tools.Metabolism. The chemical reactions occurring in a living
cell.
Messenger DNA (mDNA): A single-stranded DNA that acts as a
messenger ofthe protein biosynthesis
Messenger RNA (mRNA): A single-stranded RNA molecule
responsible for the transmission to the ribosomes of the genetic information
contained in the nuclear DNA; it is synthesized during transcription and its
base sequencing exactly matches that of one of the strands of the
double-stranded DNA molecule
Meta-analysisCombining the results from many
different studies concerning a single research issue to identify common
patterns, sources of disagreements, and any other relationships among their
findings.
Metabolic QTLsThese QTLs
control the rates of various metabolic reactions and metabolite levels.
Metabolite. A product of
metabolism.
MetabolomeAll the metabolites, representing
the end products of cellular processes, present in a cell, tissue, organ, or
organism.
Metabolomics: An extended discipline of biochemistry
that involves the analysis (usually high throughput or broad scale) of
small-molecule metabolites and polymers, such as starch; it involves
descriptions of biological pathways andcurrent metabolomic databases, such as
the Kyoto Encyclopedia of Genes andGenomes (KEGG).
MetabolomicsA systematic study of the
characteristic small-molecule metabolite profiles generated by the various cellular
metabolic processes.
Metacentric Chromosome. A chromosome
with centromere placed in the middle so thatits both arms are equal, and take
V-shape at anaphase.
Metacentric: Applied to a chromosome that has its
centromere in the middle
MetadataThe details, usually in digital form, of
experimental conditions and the procedures followed for the phenomics studies
(or any other study).
Metaphase. The stage of
mitosis or meiosis at which chromosomes align along theequatorial plane of the
cell.
Metaphase: A stage in mitosis or meiosis during
which the chromosomes arealigned along the equatorial plane of the cell.
Meta-QTLsQTLs identified by QTL
meta-analysis. Also called true QTLs.
Metaxenia. The influence
of pollen on the maternal tissues of the fruit. It greatlyinfluences fruit
quality in date palm.
Methylation. Modification
of a molecule by the addition of a methyl group. Many amutagen acts through
methylation (e.g., MMS), causing mispairing between bases,and eventually
leading to GC AT transition.
Metroglyph Analysis (Anderson 1957). A semi-graphic method of studying variabilityin a large number of
germplasm lines taken at a time. For variability assessment, thefirst two
characters, which are highly variable, are depicted on X and Y-axes,separately.
The mean values of X (say, character-1) for each genotype is
plottedagainst the mean values of Y; thus, each line occupies a definite
position on the graph,called glyph. Variation for the remaining
characters of each genotype is displayed onthe respective glyph by distinctive
rays. Each character occupies a definite rayposition. The length of the ray for
a particular character on the glyph may be short,medium, or long depending on
the index value of a genotype. It is a very easytechnique to study the pattern
of morphological variation among genotypes, and canbe applied to both
replicated and unreplicated data. However, inclusion of a largenumber of
genotypes sometimes leads to overlapping of glyphs on the graph.
Microarray: A large set of cloned DNA molecules
spotted on to a solid matrix (such as a microscope slide) for use inprobing a
biological sample to determine the gene expression, marker pattern, or
nucleotide sequence of DNA/RNA.
MicroarrayA small plaque/wafer of silicon,
glass, or metal, onto which one end of multiple copies of each of a large
number of different single-stranded DNA molecules is covalently linked and the
different molecules are arranged in separate spots.
Microarrays: A tool to examine the expression
levels and intertwinedinteractions among genes and their products;
complementary DNA of interest isaffixed to a glass slide in an ordered array;
the expression level is determinedby the binding to cDNA; a small glass, or
filter, square may contain probes forthousands of gene products; in statistics,
a microarray is a block design with blocksof size 2; the concept arose in DNA
testing, where each array is probed with twoDNA samples
Microcentres (Harlan 1951). The small regions enriched with tremendous plantdiversity and rapid rate
of plant evolution. These microcentres may offer excellentopportunities to
collect valuable types, and to study evolution of cultivated
plantsexperimentally. (Also see ecological regions).
Micro-mutation. A mutation
with a small effect that can be detected only bymeasurement on a group of
plants.
Micron C7. The smallest
known gene, also designated by Mcc C7. It produces aheptapeptide
inhibiting protein synthesis in the Enterobacteriaceae. The
heptapeptideis synthesized from 21 bp open reading frame.
Micronaire. An instrument
that measures the rate of airflow through a standard volumeof cotton. From its
readings, fibre fineness and maturity of cotton samples may becompared.
Micropropagation. In vitro propagation of plants from very small tissues (parts). It isone of the
best and most successful examples of the commercial application of
tissueculture technology. Advantages of micropropagation include: (a) identical
progenytrue-to-the mother plant, (b) rapid and large-scale multiplication of
elite genotypes,and (c) rejuvenation of old clones/varieties. Besides, it is
season-neutral andfacilitates germplasm exchange by avoiding the risk of
spreading pathogens andinsect-pests.
Micropropagation: In vitro clonal propagation; the multiplication
of geneticallyidentical copies of a cultivar by asexual means is called clonal
propagation
Micropyle. The usual
point of entry for pollen tube in the embryo sac.
Microsatellite marker: Microsatellites or simple sequence
repeats are a typeof molecular marker; microsatellites consist of tandem
repeats of 1–6 nucleotidemotifs; the repeats usually are in units of ten or
more, although repeats as small assix units have been found
Microsatellite. A type of
repetitive DNA based on very short repeats such asdinucleotides.
Microsatellite: A repeated motif of nucleotides,
usually onlytwo or three bases in length, where the number of repeatsfrequently
differs between different members of a species.
Microsatellite-primed PCR (MP-PCR): A technique resulting in RAPD-like
patterns after agarose gel electrophoresis and ethidium bromide staining;
theMP-PCR technique is more reproducible than RAPD analysis because of
higherstringency
MicrosatellitesUsually, <100
bp long sequences comprising tandem repeats of 2–7 bp.
Microspecies. A small
breeding population with limited variability. It may be a biotypeof the
species.
Microspore Mother Cell. The diploid
cell (2n) in the anther, which gives rise, throughmeiosis, to four haploid
microspores.
Microspore. One of the
four haploid spores originating from meiotic division ofmicrospore mother cell
in the anther and which gives rise to the pollen grain.
Microsporocyte. The microspore
mother cell (or pollen mother cell).
MicrosyntenySynteny based on DNA sequence.
Midparent Value. The mean value
of a quantitative phenotype obtained from twospecific parents; the arithmetic
mean of two parents with respect to a trait(s). Forinstance, if a parent is 20
unit on the phenotypic scale of measurement and the secondone 30 unit, the
midparent value will be 25 unit [MP=(20+30)/2].
Migration. Any form of
introduction of genes (individuals) from one population intoanother. It can be
responsible for introducing new genes or moving up or downwardsthe frequency of
genes already present in the breeders’ or natural populations. Thechange in
frequency of the gene already present is expressed as: p = m
(P-pt),whereas, m is the proportion of migrants, P is frequency in donor
population, and pt isfrequency in the recipient population. Clearly there will
be no change in the gene frequency, if there is no difference in gene frequency
between donor and recipientpopulation.
Mini Core. See core
collection.
Minichromosome: A very small chromosome, usually as a
result ofchromosome aberrations; engineered minichromosomes offer an opportunity
toimprove crop performance; unlike conventional gene transformation
technologies,minichromosomes can be used simultaneously to transfer and to
stablyexpress (multiple) sets of genes; because they segregate independently of
hostchromosomes, they provide a platform for accelerating plant breeding;
strategiesfor producing artificial chromosomes may consider engineering of
endogenouschromosomes or de novo assembly from chromosomal constituents
Minimal Medium. A medium
containing only a carbon source (sugar), inorganic saltsand water.
Minisatellite. A type of
repetitive DNA sequence based on short repeat sequences with aunique common
core. It is used for DNA fingerprinting.
Minisatellite: Highly polymorphic DNA markers
comprised of a variablenumber of tandem repeats that tend to cluster near the
telomeric ends ofchromosomes; the repeats often contain a repeat of 10
nucleotides; they are usefultools for genetic mapping
MinisatellitesSequences,
typically 0.2–2 kb long, made up of 11–60 bp long tandem repeat units having
identical or almost identical sequences.
Minor QTLA main effect QTL that explains
less than 10 % of the phenotypic variance for the concerned trait.
Mispairing. The presence
of a non-complementary nucleotide at a given position in oneof the polynucleotide
chains of a DNA double helix. It may lead to a gene mutation.
Missense Mutation. A mutation
that alters a codon in such away that it encodes adifferent amino acid, for
example, GAA (glutamic acid) to GUA (valine). Thischange, which
occurs in the B polypeptide chain at 6th amino acid position, causessickle cell
anaemia in human population.
Missense mutation: A single DNA base change which leads
to a codonspecifying a different amino acid.
Missing heritabilityThe part of
phenotypic variation in a quantitative trait that is not explained by the QTLs
identified by various AM studies.
Missing Plot Technique. A technique to
calculate the value of any missing treatment inany block (replication). The
calculation for any missing value (x) is done as: x = [r Bj+ t Ti – GT / (r-1)
(t-1)]; whereas, r = no. of replication, t = no. of treatments, Bj =known jth
block total where x is missing, Ti = known treatment total for which x
ismissing, and GT = grand total with missing observation. Missing plot technique
is anadvantage for a randomised complete block design.
Mitochondrion. A eukaryotic
cell organelle that is the site of ATP synthesis, and of thecitric acid cycle.
It is a small body usually with distinct shelf like internal layers. Itprovides
energy through respiration and oxidation.
Mitogen. Any substance that
stimulates cells to undergo mitosis.
Mitosis. A kind of cell
division in which the nucleus is divided into two daughter nucleiwith
equivalent chromosome complements. It is a conservative process that
maintainsthe same chromosome number in the daughter cells compared to the
parental one. Itoccurs in both somatic as well as reproductive cells.
Mitosis: The process of nuclear division in
cells which producesdaughter cells that are genetically identical to eachother
and to the parent cell.
Mitotic Crossover (Stern 1943). A crossover resulting from the pairing of homologsduring mitosis; also
called somatic crossover. Like meiosis, it takes place in
fourstrandstage of chromosomes. The outcomes of somatic crossing-over depend
upon:147(a) the site of the crossing-over, and (b) the mode of orientation and,
thus distributionof the centromeres of the chromosomes engaged in somatic
crossing-over. It alwaysoccurs in the diploid cells. It can be an important source
of variation in asexualpathogenic fungi. It is also thought to be important in
allowing recessive cancercausingmutations to become expressed.
Mitotic recombination: The recombination of genetic material
during mitosisand the process of asexual reproduction; the mechanism for the
production ofvariation in heterokaryons
Mixed linear modelThe markers and
the population structure (Q) are treated as fixed linear effects and the
additive effects of the multiple background QTLs are considered as linear
random effects.
Mixoploidy. A term that
covers all types of chimeras in which the heterogeneity betweendifferent
elements involves differences in chromosome number (2n = x, 2x,
3x, 4x, 2x-1, 2x-2, etc)
Mode. The value of the
variate, which occurs most frequently in a data set. In a frequencytable, the
modal class is the class that has the greatest frequency.
Model organisms: Creatures used in genomic analysis
because they have many genetic mechanisms in common with each other and with
humans. Theseorganisms lend themselves well to classical breeding experiments
and directmanipulation of the genome.
Model-based clustering methodsCluster
membership is based on some parametric evolutionary model.
Modified Convergent Cross (Mackey, 1954). A convergent cross that involves a verygood variety as persistent
crossing partner at every stage of the crossing procedure.This results in 50%
of the genes from the persistent good variety. For example:A × B A × C A × D A
× E↓ ↓ ↓ ↓F1 × F1 F1 × F1↓ ↓F1 × F1↓F1(50% genes
from the parent A)The procedure can be combined further with different degrees
of back crossing(convergent back cross) of F1’s with the desired parent (A) to
ensure even higherpercentage of genes from the very good variety (A).
Modified Doak Method (Mehta and Patel 1983). A method of producing hybrid seed incotton without emasculation. The top
portion of the corolla is impressed deeply toexpose only the tip of the stigma.
The anthesis is prevented through placing lintdipped in mud over the remaining
portion of the corolla. The stigma is pollinated inthe next morning. The boll
setting in crosses has been found greater (specially in thediploid cotton)
than with Doak Method.
Modified Single Cross. The progeny of
a cross between a single cross, derived from tworelated inbred lines, and an
unrelated inbred line, (A1 × A2) × B.
Modifier. A gene(s) that
affects the expression of other gene(s) primarily in a quantitativemanner. It
can act as enhancer or inhibitor depending upon the situation. Its effectmay be
dominant or recessive. The degree of dominance or recessiveness of a genemay
also be influenced by modifying genes. Modifiers may have a large or
smalleffect (but the mode of action is almost always quantitative). Some of
them may havetheir own primary function and modifying actions appear as
secondary effects, whileothers have solely enhancing or inhibitory effects on
the expression of other genes.The well-documented example is that of spotting
in mice. Similar cases in plants doexist. Actually further improvement in
any character (wherein major genes havealready been exploited) will
depend upon manipulation of modifiers.
Molecular beaconsSpecially
designed oligonucleotide hybridization probes used for identification of SNP
alleles.
Molecular Biology. A branch of
modern biology in which biological phenomena arestudied by physical, chemical
and biochemical investigations at the molecular level.
Molecular biology: The study of the structure and
function of proteins andnucleic acids in biological systems.
Molecular biomarkersThose
dynamically expressed molecules that can be measured and used as indicators of
specific phenotypic features.
Molecular breeding: Plant breeding assisted by using DNA
markers or proteinmarkers.
Molecular Cytogenetics. A correlated
study of cytology and genetics at molecular level.It may provide clues
about the stability and expression of trans genes.
Molecular farming: The application of biotechnology for
the selected productionof pharmaceutical compounds or other health or
industrial compounds within aliving organism (eg. microbe or agricultural crop)
Molecular Genetics. The study of
genetic systems that can be described at the molecularlevel. It is, therefore,
the study of molecular processes underlying gene structure andfunction.
Molecular genetics: The study of genes, the base
components of heredity. Genesare made up of DNA, whose bases are placed in a
unique order that determinesthe hereditary traits in living organisms.
Molecular inversion probeA single 120
nucleotide (nt) long oligonucleotide that hybridizes to a specific sequence of
the genome and forms a circle that has a single base pair gap at the SNP site.
The assay involves primer extension and ligation producing a closed circular
molecule.
Molecular marker: A gene or DNA sequence that identifies
a particular locuson the chromosome (whether the actual location is known or
not) and whose inheritance can be followed. An identifiable physical location
on achromosome (e.g., restriction enzyme cutting site, gene) whose inheritance
can be monitored.
Molecular Markers. Agents that
mark genetic variation at DNA level. There arenumerous molecular markers, for
example, RFLP, RAPDs, AFLP, STS, EST, SSRs(also called microsatellites), SCARs,
and the like. Except RFLP (that is southernhybridisation based molecular
marker), all are PCR-based markers. AFLP combinesthe properties of RFLP
(restriction fragment length polymorphism) and PCR(polymerase chain reaction).
However, STS (sequence tagged sites) is more desirablebecause of high degree of
reproducibility and large-scale automation, which isessential for handling a
large number of samples. These are numerous, permanent anduninfluenced by
environments/developmental stage of plants. Molecular markers canbe reliably
used in the selection of superior genotypes. However, these are
expensivecompared to morphological or biochemical ones. These can be used to:
(a) develop149saturated maps, (b) fingerprint DNA for varietal
characterization, (c) studyphylogenetic and evolutionary relationship among
related species, (d) characterize A,B, and R lines and find mechanisms of
heterosis, (e) tag gene/ genes of interest, (f)practice marker-assisted
selection, and (g) map orthologous gene(s). Markers may beof two types: screenable
and selectable. The former allows screening of transformedcells
through expression of specific enzyme to produce phenotype enabling us
toidentify transformed cells. But selectable markers are genes that confer
resistance tosome compounds like herbicides and antibiotics usually toxic to
normal plants; thusonly cells having these markers survive under selective
conditions. It is emphasizedthat the importance of marker loci is solely that
they identify “short chromosomesegments” that carry predominantly favourable
alleles. However, such chromosomesegments must be small so that it may remain
intact through many cycles of crossingover and segregation. This is true for
marker loci marking even QTLs.
Monad. An individual cell
produced by a meiocyte (instead of tetrad) as a result ofmeiotic abnormality.
Monocentric chromosome: A chromosome with only one centromere
Monocistronic mRNA. An rnRNA that
encodes only one protein.
Monoclinous. Having male
and female germ cells in one and the same flower.
Monoecy. A condition/system
wherein staminate (male) and pistillate (female) flowersare borne separately on
the same plant (for example, corn).
Monogenomic Species. Species which
are basically diploid containing only a single kindof genome. B. campestris
(AA), B. nigra (BB), and B. oleracea (CC) are
some of theexamples. Such species are also called primary species.
Monogeny. The production
of only male or female offspring.
Monogerm. A sugar beet
seed with a single germ, in contrast to a multigerm seed.
Monohaploid: A haploid cell or individual
possessing only one chromosomeset in the nucleus
Monohybrid Cross. A cross
between two individuals identically heterozygous at onelocus, Aa × Aa,
for example.
Monohybrid. A hybrid that
is heterozygous with respect to one gene only, Aa, forexample. The
hybrid at two loci (AaBb) is called a dihybrid, and so on.
Monomer. A polypeptide.
Monophyletic Evolution. Evolution of
individuals from a single interbreedingpopulation.
Monoploid. An organism
having a single complement of a basic chromosome set of thespecies; an
individual having a single genome of the species. For instance, monoploidof
bread wheat may have either A, B, or D genome. It (monoploid) may
arisespontaneously in nature. The surviving monoploids look lean and thin.
Sincechromosomes do not have pairing-partners, random segregation occurs
duringmeiosis. Gametes are nearly always deficient in one or more chromosomes,
and thusmonoploids are usually sterile. For instance, a monoploid of barley has
sevenchromosomes. A functional gamete may be formed if it has all the sevenchromosomes.
However, the probability of going all the seven chromosomes to one ofthe poles
during meiosis I is (½)x-1 = (½)7-1 = (½)6. This amounts to the
probabilityof a functional gamete. Thus the probability that a monoploid of
barley sets one seedis (½)6× (½)6 = (½)12. This explains why a monoploid plant
is almost sterile.
Monosome. A chromosome
with no homolog to pair with; a single chromosome in anotherwise diploid
individual (2n-1).
Monosomic. An individual
lacking in one chromosome of the diploid complement,henece, having 2n-1
chromosomes. The missing chromosome is called a monosome.In monosomics,
the unpaired chromosome passes at random to either pole duringmeiosis; however,
it frequently lags at anaphase, and is not included in either daughternucleus.
For this reason, gametes with n-1 chromosomes are frequent compared
tothose with n chromosomes. However, this bias is not reflected
strikingly in thezygotic chromosome numbers because gametes with n-1
chromosomes often do notfunction (especially gametes). Furthermore, zygotes with 2n-2
chromosomes areinviable except in a few polyploid species. Thus most of the
progeny of monosomicsare either normal diploids (2n) or monosomics (2n-1).
Monosomic: A genome that is basically diploid but
that has only one copyof one particular chromosome type, so that its chromosome
number is 2n–1; monosomic series were developed
Monosomy: The total loss of one of a pair of
chromosomes. This occurs, forexample, in Turner Syndrome where one X chromosome
is lost leaving a total of45 chromosomes.
Monotelocentric: A cell or individual lacking one
chromosome pair butshowing one telocentric chromosome for one arm of the two
missing homologues
Monotelodisomic: A cell or individual lacking one
chromosome pair butshowing two homologous telocentric chromosomes for one arm
of the two missinghomologues
Monotelomonoisosomic: A cell or individual lacking one
chromosome pairbut showing a telocentric chromosome for one arm of the missing
homologouspair and an isochromosome for
Monotelotrisomic: A cell or individual showing an
additional telocentricchromosome to a certain pair of chromosomes more abundant
parents but alsopossess some of the characters of the other parent species
Morgan: A measure of genetic distance; one
Morgan (M) equals 100 centimorgans.
Morph. Any one of the genetic
forms that account for polymorphism.
Morphogenesis. A
developmental process that leads to changes in gross form, cellularfine
structure or both; processes giving final shape to an adult organism byestablishment
of specific pattern of tissues and organs.151Mosaic. Also called a chimera
(more frequently in plant species). A tissue containing twoor more
genetically distinct cell types or an individual composed of such tissues.
Morphological markers: Simply inherited
and easily scored morphological traits; the earliest genetic markers.
Mosaicism: Where a genetic or chromosomal
abnormality does not occur in allbody cells. Often related to X chromosome
innactivation.
Moving Means Method (Knott 1972, Townley-Smith and Hurd 1973).
A method oftesting the genetic potential
of early generation lines (F2/F3/F4) in unreplicated plots.It involves taking
mean of a number of adjacent plots (excluding the plot in question)as an
environmental index. The performance of a test genotype is expressed as
thedifference between its own value and the moving mean, or as the percentage
ofmoving mean.12.00 13.00 15.0018.00 20.00 17.0014.00 19.00 15.00In the
above example, the value of moving mean is{(12+13+15+18+17+14+19+15)/8} =
15.38. Thus, performance of the test genotype(middle plot) = (20.00/15.38) ×
100 130%. Its control over experimental error iscomparable to the use of
intermittent check plots; however, it has an added advantagesince no plots are
wasted for accommodating the check entries.
mRNA. An RNA molecule
transcribed from the DNA of a gene under the influence of anenzyme transcriptase.
The genetic message is translated into a polypeptide by theaction of ribosomes.
During the process, only the genetic message flows from DNA toRNA (there is no
physical transfer of any material).
Multiline (Jensen 1952). A blend of compatible lines, each selected for similarity ofheight,
maturity, and other agronomic or horticultural characteristics, but carrying
adifferent gene(s) for resistance. However, Norman Borlaug (1954) has suggested
asimilar but somewhat more sophisticated approach based on lines developed
bystandard back cross breeding. In the present context, a multi-line cultivar
is a mixtureof isogenic lines. In this approach, genes for specific resistance
(that reduce the initialamount of inoculum) are considered in the component
lines. However, in theaftermath, the overall result appears as horizontal
resistance, since the rate ofpathogen increase is also considerably reduced.
Browning and Frey (1969) have152called it synthetic horizontal resistance or
population resistance. It is only a concept;now-a-days, it is not used
practically.
Multiline Variety. A variety
developed through a composite of isolines. The term“multilines” sometimes is
applied to mixtures of genetically diverse lines produced invarious ways to
buffer against environmental stresses. More accurately, thesepopulations should
be called composites instead. Such a variety is now rare incultivation.
Multiline: A mixture of isolines, each of which
is different fora single gene conditioning different forms of the same trait.
Multilocus mixed model: It includes
multiple loci as cofactors in the AM model and employs a simple stepwise
mixed-model regression analysis combined with forward inclusion and backward
elimination of loci in the model.
Multimer. A protein
consisting of two or more subunits (monomers or polypeptides).
Multimeric Structure. A structure composed
of several identical or different subunitsheld together by weak bonds.
Multiparent advanced generation intercross
populations: A
collection of RILs produced from a complex crossbred/outbred population
involving several parental lines.
Multiple Allele. A member of a
series of several known allelic forms of a gene. Theexistence of such a
situation is called multiple allelism. The presence of multiplealleles
has been observed for the self-incompatibility system in the Nicotiana spp.Please
notice that the presence of all the allelic forms of a gene in any
naturalpopulation is a rare event. As the no. of allelic forms increase, the
possible kinds ofgametes (and homozygotes) and total no. of genotypes increase
exponentially. Forexample, for a single gene pair with four alleles, the kinds
of gametes (homozygotes)and total no. of genotypes will be 41 (= 4) and (4
×5)/2 (= 10), respectively. For twogenes with same no. of alleles at the two
loci, the values will be 42 (= 16) and(16×17)/2 (=136), respectively.
Multiple Correlation Coefficient. A statistic measuring the joint association of all theindependent
variables with the dependent one. It thus tells how much of the variationin the
dependent (Y) could be accounted for by reference to these independent
ones.Numerically, it is the square root of the ratio of the regression sum of
squares to thetotal sum of squares, and is denoted by R.
Multiple Cross. A cross
involving many parents. Theoretically its objective is to obtainrecombination
from many a parent; the practical utility of multiple cross, however, islimited
in plant breeding for two reasons: (a) the size of the population
increasesexponentially with increasing no. of parents to ensure occurrence of
the desiredgenotype in the segregating generation, and (b) it is often
difficult to find many welladapted parents having different useful genes. (Also
called convergent cross).
Multiple interval mapping: An approach for
simultaneous QTL mapping in multiple marker intervals.
Multiple QTL mapping: It combines simple
interval mapping with multiple regression analysis and includes all the
significant QTLs in the genetic model used for mapping.
Multiple Resistance. A strategy in
resistance breeding that involves placement of two,three, or more new and still
effective resistance genes into a new cultivar to impose154barriers of several
resistance to the pathogen population simultaneously. It should bean effective
strategy because a new race, to overcome multiple resistance genes, musthave
two or more simultaneous changes towards the virulence, whereas a new raceneeds
only one change towards virulence to overcome a single gene for resistance.
Itcan be effective if the breeding is coordinated centrally, and the production
area isisolated from other areas where the system is not applied.
Multiple-Factor Hypothesis (Nilsson-Ehle 1908). A hypothesis to explain quantitativevariation. Many genes, each with
a small but equal effect, segregate to produce acontinuous variation in a
quantitative character. Conclusive evidences for thishypothesis proved
beyond doubt that all genetic traits (whether qualitative orquantitative)
follow Mendel’s laws of heredity.
Multiplex PCR: Two or more
primer pairs used for amplification of two or more loci in a single PCR
reaction tube.
Multiplex ratio: The average
number of markers scored per assay of a marker system in a given population.
Multiplexed shotgun genotypingSize-selected
(250–300 bp) restriction fragments from several individuals are separately
ligated to distinct barcodes, pooled, and sequenced using an NGS platform.
Multiplex-endonuclease genotyping approach AFLPA modification
of AFLP; four or more endonucleases are used for the digestion of the sample
DNA; only one pair of adapters used for amplification.
Multiplexing: A sequencing approach that uses
several pooled samplessimultaneously, greatly increasing sequencing speed.
MultiplexingCarrying out two or more
different reactions, e.g., PCR amplification, in a single tube or separating
the products of two or more PCR reactions in a single gel lane.
Multispectral reflectance data Reflectance
data acquired at few selected wavelengths.
Multitrait introgressionIntrogression of
genes governing two or more different traits into a single RP.
Multitrait mixed model: It extends the
linear mixed-model approach of AM to the analysis of pairs of correlated
traits.
Multivalent. A pairing
configuration of three or more completely or partiallyhomologous chromosomes
observed during meiosis-I. Multivalent formation is acharacteristic feature of
autopolyploids.
Multivariate linear mixed modelIt allows
testing of associations between markers and multiple correlated phenotypes and
is able to control population structure.
Multivariate methodsMethods for the
analysis of data on multiple traits for each entity.
Mutagen Effectiveness. The number of
induced mutations per unit dose of a mutagen.
Mutagen Efficiency. A ratio of
specific desirable mutagenic change(s) to the undesiredeffects such as plant
damage, sterility, lethality, and the like.
Mutagen. An agent (physical,
chemical or even biological) that is capable of increasingthe mutation rate.
Mutagen: Any agent that is capable of inducing
and increasing the mutation rate.
Mutagenesis. Treatment of
plants or plant parts with a mutagen to increase mutationrates.
Mutant Allele. An allele
differing from the allele found in the standard or wild type.
Mutant allele: An allele differing from the allele
found in the standard, or wildtype.
Mutant Hunt. The process of
collecting different mutants showing abnormalities in acertain structure or in
a certain function, as a preparation for mutational dissection ofthat function.
Mutant Site. The damaged or
altered area within a mutated gene.
Mutant. An organism or cell
carrying a detectable mutation. The term is an adjective;thus it must precede a
noun (a mutant individual, for example).
Mutant: A gene having undergone genetic change
or mutation.
Mutation Breeding. A system of
breeding in which plants/plant parts/seeds or any kindsof propagules are
treated with any kind of mutagen accompanied by selection fordesired types in
succeeding generations.
Mutation Event. The actual
occurrence of a mutation in time and space.
Mutation Frequency. The proportion
of mutants in a population.
Mutation Pressure. The continued
recurrent mutation tending to increase the frequencyof the mutated allele in
the gene pool of a population.
Mutation Rate. The number of
mutation events per gene copy in a population per unit oftime (for instance,
per cell generation).
Mutation. In broadest
sense, any heritable variation in a gene or in chromosome structureand/or
number. However, the use of the term has now been restricted to include
onlygene mutation, which refers to a process by which new allele(s) is produced
throughheritable structural changes in the gene(s). It may be the process or
even the result ofthe process. It may occur in nature spontaneously, but
the frequency is very low(about one per million). The rate of mutation can be
increased in breeders’populations through chemical or physical agents called mutagens.
The molecularbasis of mutation relates to mispairing between nucleotides of the
DNA moleculeirrespective of whether the mutation is natural or artificial. In
populations of a species,it the ultimate source of all variation. If an
allele A mutates to a (ignoring backmutation) with the mutation
rate for a given number of generations n,
thenfrequency of the allele A after nth generation, pn = p0e-n (whereas, e
is base of thenatural logarithms). Thus the frequency of the A allele decreases
with time but therate will be very slow. Therefore, the process of mutation
cannot drive the process ofevolution unless supplemented with recombination or
migration. The statement that amutation, if not favoured, is lost from a
population is not true absolutely. Theprobability that it could be lost is
(2N-1)/2N (N = size of the population); if not, thenthe probability that it is
fixed is 1/2N. Thus a mutation can become established in apopulation even
though it is not favoured by natural selection simply by a process ofrandom
genetic drift.
Mutation: A permanent change in the genetic
material involving either a physical alteration in the chromosome or a
biochemical change in the underlying DNA molecule.
Mutational Dissection. The study of
the components of a biological function through astudy of mutations affecting
that function.
Mutational Equilibrium. The product of
frequency of mutations per individual pergeneration and average number of
generations during which mutated gene ismaintained in the population before it
is eliminated.
Mutator Gene. Any gene that
increases mutation rate of other genes.156
Mutein. A protein with one or
more mutationally altered amino acids in thepolypeptide(s). it is analogous to
normal or wild type protein. It may or may not lackenzymological,
immunological, or physio-chemical activity.
MutMap schemeA quick, reliable, and
cost-effective method for mapping of causal SNPs in induced mutations. It uses
a single bulk of the mutant plants from the F2 generation of mutant parent
cross and a reference genome for alignment of the NGS sequence data from this
bulk.
MutMap-Gap scheme It identifies
causal mutations located in the genomic regions missing from the
parental/reference genome.
Muton. The smallest part of a
gene that is involved in a mutation event. It is now knownto be a nucleotide
pair.
Mycoplasma. Small bacteria
that produce infectitious diseases in plants and animals.Among living
organisms, they have the smallest mass. Mycoplasma can be cultured invitro like
any bacteria. Mycoplasma genitalium with a genome size of 580 kb DNA
isthe first complete life.
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