Terminologies in Genetics and Plant Breeding Part -8

Identical by descent: Two genes that are identical in nucleotide sequencebecause they are both

Identical by descentThe copies of an allele of a gene present in two individuals/lines originated by replication of the same ancestral copy of the allele.

Identity preservation: The segregation of one crop type from another at everystage from production and processing to distribution. This process is usuallyperformed through audits and site visits and provides independent third-partyverification of the segregation.

Identity. Individual values of variables. It differs from correlation in that values can becorrelated without being equal. Thus correlation is not the same thing as identity. Thedifference between these two statistics arises when we consider the effect ofenvironment on heritable traits. Parents and offspring could be perfectly correlated insome trait such as height; yet, because of environmental difference betweengenerations, every child could be taller than its parents.

Ideotype (Donald 1968). A biological model expected to perform in a predictablemanner within a defined environment. Such an ideotype can be a plant with modelcharacteristics known to influence photosynthesis, growth and (in cereals) grainproduction. It can be seen as a specified model of a plant, which is to produce adesired product in a required amount and quality in a way that minimizes the effect ofenvironmental variability during growth and subsequent losses. In practice, such amodel is usually described in terms of phenotype, but theoretically, it could also bevisualized as a genotype. There is no doubt that when a plant breeder defines a newsuperior variety in terms of phenotype, whether defined by morphology or physiology,he is contriving a new ideotype. We should note that ideotype breeding is ahypothetical ideal concept that is unfortunately dogmatic and unrealistic. In practicalterms, the ideal may not be attainable or even desirable. However, it is a logical steptowards new levels of yield and stability. It provides a guideline to the breeder in theselection of parents to be crossed and in the selection pressure to be imposed in thelater generations.

Ideotype breeding: A method of breeding to enhance genetic yield potentialbased on modifying individual traits where the breeding goal for each trait isspecified

Ideotype: Crop plant with model characteristics known to influencephotosynthesis, growth, and grain production; “ideotype” was coined by Donald(1968); it means literally a form denoting an idea, and in the broadest sense it is abiological model which is expected to perform or behave in a predictable mannerwithin a defined environment; more specifically a crop ideotype is a plant modelwhich is expected to yield a greater quantity of grain, oil or other useful productwhen developed as a cultivar

Idiogram: A diagrammatic representation of the karyotype of a plant

Idiotype. The sum total of hereditary determinants of an individual. It includes bothgenotype (nuclear genes) and plasmatype (cytogenes) of the individual.

Idiotype: The sum of the hereditary determinants of a cell or plant consistingof the genotype and plasmotype; idiotype is an established term, first used bySiemens (1921); “idio” in idiotype is derived from the Greek word “idios”, meaningone’s own, personal, separate, distinct

IJI-Line. Inbred line.I1 , I2 , I3… symbols used to designate first, second, third, etc. inbred generations.

Image analysisProcessing of an image for deriving the desired information; it involves retrieval, preprocessing, segmentation, reduction, composition, description, and classification steps.

Image compositionIndividual fragments of an object are merged to create the object.

Image descriptionQuantification of such features as area, height, width, etc., of the object.

Image preprocessingThe use of filters to minimize noise or increase sharpness.

Image retrievalLoading of the images from the database/storage folder into the image analysis software.

Image segmentationThe image is divided into objects to be analyzed and those to be excluded from analysis.

Imaging fluorometerA fluorescence imaging system is used to acquire images of the fluorescing objects.

Imago. An adult insect.

Immortalized F2 populationThe population of single cross F1s produced by intercrossing a set of RILs in pairs or as per some other scheme.

Immune System. The animal cells and tissues that recognize and attack foreignsubstances within the body. Such a system is likely to exist for plant species also.

Immunity. An absolute term that refers to complete freedom of plants from disease(s). Itis the extreme case of disease reaction in which host-parasite interaction does notoccur. Thus there is no question of disease symptoms on the plants. For example,potato is immune to wheat rusts.

Immunogenetics. An area of genetics that deals with genes governing antigens,antibodies and their reaction.

Imprinting: Patterns of inheritance affected by whether the inheritance wasfrom the mother or father.

In silico association mappingAM based on phenotype and genotype data on inbreds and breeding lines routinely collected in the breeding programs.

In silico: Modern term used to characterise biological experiments carried outentirely in a computer

In situ conservation of germplasmEstablishment of gene sanctuaries or biosphere reserves in areas of high variability within the centers of origin of the concerned crop species by protecting the demarcated areas from human interference.

In situ conservation: Refers to the maintenance of plant populations inthe habitats where they naturally occur and have evolved; in crop species, conservation can be achieved in home gardens or on-farm, in wild species throughthe designation of protected areas, such as natural parks and nature reserves

In situ hybridisation: Use of a DNA or RNA probe to detect the presence ofthe complementary DNA sequence in cloned bacterial or cultured eukaryotic cells.

In Situ. “In place”.

In situ: in place; where naturally occurring

In vitro culture: The cell, organ, or tissue culture performed under artificialconditions in tubes, glasses, dishes, etc.

In vitro fertilisation: Pollination performed aseptically in vitro by directapplication of the pollen to the ovule; it is used to overcome prezygoticincompatibility

In Vitro Mutagenesis. The production of either random or specific mutation in a piece ofcloned DNA. Typically, the DNA will then be repackaged and introduced into a cellor an organism to assess the results of the mutagenesis.

In vitro mutagenesis: Methods for altering DNA outside the host cells mutagenesis can be random or specific for the site and base change depending onthe technique used

In vitro selection: Used to screen large numbers of plants or cells for a certaincharacteristic before growing them in the field or in glasshouses, eg. salt tolerance

In Vitro. In an experimental situation outside the organism (literally “in glass”).

In vitro: Literally, “in glass;” but applied more generally to studies andpropagation of living plant material that are performed under artificial conditionsin tubes, glasses, dishes, etc. Performed in a test tube or other laboratory apparatus.

In Vivo. In a living cell or organism.

In vivo: Literally, “in life”; applied to studies and propagation of whole, livingorganisms, on intact organ systems therein, or on populations of microorganisms. In the living organism.

Inbred Line. The product of inbreeding; in plant breeding, a nearly homozygous lineusually originating by continued self-fertilization accompanied by selection fordesired traits.

Inbred line: A line produced by continued inbreeding; usually a nearlyhomozygous line originating by continued self-fertilisation, accompanied byselection

Inbred pure lines: Involves inbreeding of annual seed-propagated material;homogeneous and homozygous lines are isolated by selection of desiredrecombinants or segregates in F₂ to F₇ generations of crosses between parentalpure lines (generally monogenotypic lines can be blended to form multilines, eg.in tomato, lettuce, soybean, pea, cowpea, snapbean, field bean, Arabian coffee,Capsicum pepper, eggplant, okra, lentil, and papaya)

Inbred. The result of inbreeding; same as the inbred line.

Inbred: A plant resulting from successive self-fertilisation of parents throughoutseveral generations

Inbred-Variety Cross. The F1 cross of an inbred line with a variety (usually an openpollinatedone). The purpose of such a cross is to know general combining ability ofinbred lines.

Inbred-variety cross: The F1 cross of an inbred line with a variety

Inbreeding Coefficient. A quantitative measure of the intensity of inbreeding. It is ameasure of the probability that any two alleles of a locus will be identical by virtue ofdescent from a common ancestor. It shows proportional reduction in heterozygosityunder various systems of genetic assortment. The rate of reduction in heterozygosityper generation due to inbreeding is 0.5 with self-fertilization, 0.25 with sib mating,0.125 with half sib mating and 0.0625 with cousin mating. Starting from aheterozygote Aa, 50% heterozygosity is lost in the first generation of selfing, thusinbreeding coefficient (F) is 0.5; in the second generation, 75% heterozygosity is lost,and thus F is 0.75. Said in another way, the amount of homozygosity at initial stage iszero and hence F = 0; after second generation of self-fertilization, amount ofhomozygosity is 75%, and thus F = 0.75. This is why I.C. is also defined as the120probability that the two alleles at a locus is homozygous by descent. F varies fromzero for random mating population to unity for a completely inbred system. It is avery useful parameter to describe the property of any other population relative toMendelian population:AA Aa aap2 + 2pq + q2 = 1.0 (Mendelian population)[p2 + F (p) (q)] + [2pq - F (2pq)] + [q2 + F (p) (q)] = 1.0 (Inbred population)

Inbreeding coefficient: The probability that the two genes at any locus in adiploid individual are identical by descent (i.e. they originated from the replicationof one gene in a previous generation)

Inbreeding Depression. A reduction in biological fitness and vigour followinginbreeding (in individuals, which are normally cross-pollinated). The degree of ID is afunction of number of deleterious recessive genes kept concealed in theheterozygotes. These harmful alleles are exposed in homozygotes upon inbreedingthrough segregation and recombination. Thus ID may vary among the inbred linesderived from the same population. Numerically, it is expressed: ID = [(F1 - F2) / F1] x100 (in percentage).

Inbreeding depression: Reduction in vigor and fertility as a consequence of inbreeding.

Inbreeding depression: The loss of vigour when some crops have reducedheterozygosity due to enforced self-pollination

Inbreeding Load. The extent to which inbreeding, which ordinarily increases geneticload, impairs an equilibrium population under random mating.

Inbreeding load: The extent to which a population is impaired by inbreeding.

Inbreeding Superiority. A condition wherein mean F2 performance is greater than theaverage performance of the respective F1 hybrid. Under stress conditions, even asmall difference in physiology of individuals can make a big difference. Contrary toF1 population, F2 population has better buffering capacity owing to genetic differencesamong individuals which may result in superior performance under stress conditions.

Inbreeding Tolerance. The ability of a genotype or a population or a species (belongingto outcrossing group) to endure with the stress imposed by inbreeding. Self-pollinatedspecies are highly tolerant to inbreeding. Among outcrossing species, cucurbits andsunflowers are most tolerant, whereas alfalfa is highly susceptible. Genotypicvariations for inbreeding tolerance have been noticed in maize.

Inbreeding. Mating between relatives; mating of individuals more closely related thanindividuals mating at random. The degree of inbreeding is a function of geneticcorrelation between the mating partners. The closest form of inbreeding is selffertilization;however, in self-incompatible species, sib mating is the closest possible121form of inbreeding. According to Allard (1999), the term inbreeding in plant breeding is usually applied when matings are made between closely related individuals, andparticularly when mating is by selfing (as compared to genetic assortative mating).There are two general causes of inbreeding: (a) restriction in population size, and (b)various mechanisms that affect the mating systems.

Inbreeding: Mating between individuals related by descent.

Inbreeding: The breeding of individuals that are related.

Inbreeding: The crossing of closely related plants; one important purposeof induced inbreeding is the development of genotypes that can be maintainedthrough multiple generations of seed production; self-pollinated cultivars arereproduced for many generations by inbreeding; inbreeding is also used to reducethe frequency of deleterious recessive alleles in genotypes that serve as parents ofa synthetic or a vegetatively propagated cultivar; inbreeding increases the geneticand phenotypic variability among individuals in a population; four matingsystems are used to increase the homozygosity in a breeding population

Inclusive composite interval mapping: It uses a modification of the CIM algorithm: the markers having significant regression coefficient estimates are selected as background markers or cofactors and the regression coefficients for the remaining markers are set at zero; this is done only once during the entire analysis.

Incompatibility (homomorphic or heteromorphic): A genetically determinedinability to obtain fertilisation and seed formation after self-pollination or cross pollination;there are several types of progamous or postgamous incompatibility;in contrast to heteromorphic incompatibility (eg. heterostyly in Primula sp.),homomorphic incompatibility is not associated with morphological differences

Incompatibility group: Plasmids that are incompatible with each other belongto the same incompatibility group

Incompatibility. A genetically controlled physiological hindrance to fruitfulness; failureof self- or cross-fertilization owing to antigenic differences that act betweenpollination and fertilization. Self-incompatibility is a mechanism that enforces crosspollinationthrough providing physiological barriers to self-fertilisation even thoughboth male and female gametes are functional.

Incomplete block design: A block design in which not every treatment occursin a block; it is preferable where large numbers of cultivars are compared in asingle yield trial; the entries in each replication are subdivided into smaller blocks,in a manner designed to reduce the error caused by soil variation; usually, it refersto a lattice design, considering the restriction that (1) the number of testers mustbe harvested, (2) inferior strains cannot be discarded prior to harvest to reduceharvest expenses, and (3) researchers still must analyze the experiment as a latticedesign

Incomplete Block Designs. Experimental designs in which each block (replication)contains only a fraction of the treatments to be tested. These designs are exemplifiedby lattice block design, group balanced block designs, and the like. These designs aresuited for experiments with a large number of treatments.

Incomplete Dominance. The situation in which a heterozygote deviates from midparentalvalue towards one of the homozygotes on the phenotypic scale ofmeasurement. (Exact intermediacy or mid-parental value implies no dominance orlack of dominance).

Independence. The relationship between two or more variables such that the variation ofeach one is uninfluenced by that of others, that is, correlation is zero.

Independent Assortment. Free matching or combinations of different alleles of two ormore genes following segregation at gametogenesis. However, linkage hindersindependent assortment of genes; the degree to which it affects is a function of themapping distance between the two genes in question.

Independent assortment: The random distribution in the gametes of separategenes; if an individual has one pair of alleles A and a, and another pair B and b thenit should produce equal numbers of four types of gametes: AB, Ab, aB, and ab; it isasserted in Mendel ’s second law—the law of independent assortment

Independent Culling. A method of selection for multiple resistance within the randommating population. It involves screening of the population independently for each ofdifferent pests/diseases, thus leaving plants only with multiple resistance in thepopulation. The next cycle of selection starts in the population derived by intermatingplants that carry resistance to all the multiple pests/diseases.

Indeterminate. Descriptive of an inflorescence in which the terminal flower is last toopen. The flowers arise from axillary buds, and the floral axis may be indefinitelyprolonged by a terminal bud. It is a dominant monogenic trait in fenugreek, clusterbean, broad bean, peas, etc.

Index Selection (Matzinger et al. 1976). Simultaneous selection for two or more traitsthat are antagonistically associated. The proponents found that simultaneous selectionfor both short height and more number of leaves in tobacco could be useful.

Indica Rice. Tropical ecotypes of rice, traditionally characterized by tall stature, weakstraw, photoperiod sensitivity, easy shattering, broad droopy leaves, and graindormancy.

Indirect embryogenesis: Embryoid formation on callus tissues derived fromzygotic or somatic embryos, seedling plants, or other tissues in culture

Individual. A unit of breeding population that is expected to contribute to the gene poolof that population.

Induced Mutation. Heritable genetic change resulting from treatment with a mutagen.

Induced mutation: A change in a gene caused by a treatment

Induced Resistance. A term that may be used for temporarily increased resistanceresulting from some condition of plant or environment. For example, aphids areparticularly negatively correlated with the level of potassium in the soil.

Induced resistance: A form of resistance activated in plants by an externalstimulus - for example, elicitor treatment or preinoculation with a microorganism ; it can be a local or systemic acquired resistance

Inducer pollinator strain: In maize, a strain that induces high-frequency haploid development when it is used a pollinator.

Inducer. An environmental agent that triggers transcription from an operon.

Inflorescence. A cluster of flowers; the arrangement and mode of development of theflowers on a floral axis.

Informatics: The study of the application of computer and statistical techniques to the management of information. In genome projects, informatics includes the development of methods to search databases quickly, to analyse DNA sequence information, and to predict protein sequence and structure from DNA sequence data.

Infrared imaging: Imaging using IR sensor or IR/thermal camera based on IR thermometers, and the IR radiation pattern is converted into pseudo-color images.

Inheritance of Acquired Character (Lamarck 1809). A disproved theory related toevolution. According to it, the changes that accrue in the phenotype of an organismduring its lifetime can be passed on to its offspring.

Inheritance. The transmission of genetic information from parents and ancestors to theiroffspring.

Inheritance: The passing of familial elements from one generation to the next.

Initial variety: A variety used for the development of a new variety.

Inoculate. Placing inoculum where it is likely to produce an infectious disease.

Inoculum. Spores, bacteria, or fragments of mycelium of pathogens that can infect plants,or soil.

Inosine. A rare base that is important at the wobble (third) position of some tRNAanticodons.

INSA. International Seed Testing Association founded in 1924 for ensuring uniformity inseed testing. ISTA rules for seed testing are used all over the world by its member andnon-member countries. It has a system of international and regional referee testingthrough which the participating stations can appreciate their shortcomings. It issuesdifferent kinds of certificates, which (especially orange international certificates) areused extensively, and are of great importance for the international trade in seeds.

Insecticide resistance: The development or selection of heritable traits (genes)in an insect population that allow individuals expressing the trait to survive inthe presence of levels of an insecticide (biological or chemical control agent) thatwould otherwise debilitate or kill this species of insect. The presence of suchresistant insects makes the insecticide less useful for managing pest populations.

Insect-resistance management: A strategy for delaying the development ofpesticide resistance by maintaining a portion of the pest population in a refugethat is free from contact with the insecticide. For Bt crops this allows the insectsfeeding on the Bt toxin to mate with insects not exposed to the toxin produced inthe plants.

Insect-resistant crops: Plants with the ability to withstand deter or repel insectsand thereby prevent them from feeding on the plant. The traits (genes) determiningresistance may be selected by plant breeders through cross-pollination with othervarieties of this crop or through the introduction of novel genes such as Bt genesthrough genetic engineering.

Insertion Sequence. A mobile piece of bacterial DNA that is capable of inactivating agene into which it inserts. It may be several hundred nucleotide pairs in length.

Insertional mutagenesis: Induction of mutations by insertion of either a transposable element or Agrobacterium T-DNA sequence within the concerned gene.

Insertional Translocation. The insertion of a segment from one chromosome intoanother nonhomologous one. The best-known classical example is transfer of a smallsegment (gene?) from Aegilops umbellulata into our cultivated bread wheat for leafrust resistance. The detailed analysis by Sears (1956) showed that the segmentinsertion occurred near to kinetochore of one of the chromosomes, and the genotypewas slightly later in maturity in addition.

Instability. Inconsistency in performance of a genotype(s) across years and locations. It isa type of variation that appears to be random in direction. It may be due todevelopmental noise. It has no genetic basis.

Integrated Pest Management. Exploitation of several pest suppression tactics to keepthe pest population below damaging threshold. It is a multidimensional approachinvolving a range of biological, cultural, physical, biochemical and chemicaltechniques to hold pest population below economically damaging levels withoutdisrupting the agro-ecosystem. In fact, the initial factor in IPM is pest resistance inplant cultivars. Its use as a basal component in IPM has several advantages: (a) itpermits lower level of resistance in plant varieties, (b) by doing so, we can increasethe number of cultivars available for commercial cultivation, and (c) it reduces theselection pressure for the development of pesticide resistant biotypes of insects, andother components of IPM reduce the selection pressure for development of biotypesthat would infest resistant cultivars.

Integrated Plant Nutrition. A system to maintain soil fertility, sustaining productivityand improving farmers’ profitability through judicious and efficient use of mineralfertilizers, organic manures and biofertilizers.

Intellectual property rights: Legal recognition of the right of an inventor or his assignee to derive exclusive economic benefits from his intellectual property.

Intellectual property rights: The legal protection for inventions, including newtechnologies or new organisms (such as new plant varieties). The owner of theserights can control their use and earn the rewards for their use. This encouragesfurther innovation and creativity for the benefit of us all. Intellectual propertyrights protection includes various types of patents, trademarks, and copyrights

Intellectual property: All patent applications, patents, plant variety protection(PVP) certificates, or trade secrets that make up proprietary information

Intellectual property: An idea, a design, an invention, a manuscript, etc., which can ultimately generate a useful product/application.

Interbreeding. Hybridisation between genetically dissimilar individuals capable ofexchanging genetic information (genes). It holds individuals together in a population,or populations together in a sub-species or species.

Intercalating Agent. A chemical that can insert itself between the stacked bases at thecentre of the double helix, possibly causing a frame-shift mutation.

Intercalating agent: A chemical that can insert itself between the stacked basesat the center of the DNA double helix, possibly causing a frameshift mutationInterchange trisomic: An additional chromosome to the diploid set, which iscomposed of two different chromosomes via translocationInterference: The effect of recombination in one interval on the probability ofrecombination in an adjacent interval gene; in patenting biological material, whentwo or more patent applications

Interchange Trisomic. A trisomic developed in such a way that two members of thetriplicate are interchanged chromosomes. It differs from a tertiary trisomic in havingtwo interchange chromosomes (instead of one in TT).

Interchromosomal Recombination. Recombination resulting from independentassortment of nonhomologous chromosomes.

Interconnected mapping populationsPopulations produced by crossing a set of homozygous parental lines in such a way that two or more crosses have at least one parent in common.

Intercross recombinant inbred linesRILs produced following few to several generations of random mating or intermating among the individual plants beginning in the F2 generation.

Interference. The effect of one crossover on the probability that another will occur in theimmediate vicinity. It is a measure of the independence of crossovers from each other,and is calculated by subtracting the coefficient of coincidence from 1.

InterferenceOccurrence of crossing over at a chromosomal site interferes with the occurrence of another crossing over in its surrounding regions.

Intergeneric cross: Spontaneous or experimental crosses of individuals ofdifferent genera, for example, wheat (Triticum aestivum) and rye (Secale cereale);this cross even resulted in a human-made new crop plant “triticale”

Interphase. The stage between nuclear divisions of the cell cycle, when chromosomesare extended and functionally active.

Interphase: The period in the cell cycle when DNA is replicated in the nucleus;followed by mitosis.

Interploid Crosses. Crosses involving parents having different ploidy levels [e.g., a crossbetween S. tuberosum (2n = 4x = 48) and S. phureja (2n = 2x = 24)]. Interploidcrosses are made sometimes to produce haploids, which may develop throughparthenogenesis or haploid apogamy.

Inter-simple sequence repeat marker: A marker based on a single primer composed of a microsatellite sequence. Syn., inter-SSR PCR marker.

Interspecific cross: A crossing between two species

Interspecific Hybridization. A method as well as design of evolution of crop plants. Itconsists of crossing distinct taxonomic species with the preservation of improvedtypes from the segregation products. As many of the segregants have lower biologicalfitness, its major application as a method of breeding has been mostly in vegetativelypropagated species.

Inter-SSR PCRMarkers generated by anchored ISSR primers.

Interstitial Region. The chromosomal region between the centromere and the site of arearrangement.

Intervarietal: Effects and phenomenons between varieties (cultivars)

Intervening Sequence. An Intron; a segment (comprising several nucleotide pairs) oflargely unknown function within a gene. The segment is initially transcribed, but itgets eliminated from the functional mRNA transcript.

Intrachromosomal Recombination. Recombination resulting from crossing-overbetween two gene pairs.

Introgression library: Increasing the genetic diversity of elite breedingmaterials with exotic germplasm requires techniques that minimize negative sideeffects attributable to genetic interactions between recipient and donor; this seemsachievable by an introgression library approach involving the systematic transferof a limited and/or restricted number of short donor chromosome segments froman agricultural unadapted source (donor) into an elite line (recipient or recurrentparent); established introgression libraries represent a dynamic resource that cansubstantially foster breeding programs and provide an opportunity to proceedtowards functional genomics

Introgression line libraryA NIL set; the sum total of DP genome segments present in these NILs, ideally, represents the entire DP genome.

Introgression. The transfer of desirable genes from the wild species into the cultivatedones.

Introgression: The incorporation of genes of one species into the gene poolof another; if the ranges of two species overlap and fertile hybrids are produced,the hybrids tend to backcross with the more abundant species; it results in apopulation in which most individuals resemble the

Introgressive Hybridisation (Anderson 1949). A kind of interspecific hybridisation. Itconsists of recrossing of interspecific hybrids with the parental species in such a waythat both the species are enriched with desired features of each other without loosingtheir taxonomic identity. It aims at broadening the genetic base of different species.Sometimes, introgression is difficult to detect because genetic contamination is veryslight.

Introgressive hybridisation: Crossbreeding of plants from different speciesthat results in introgression

Intron. A sequence of a gene that is transcribed into nuclear RNA but is removed fromfunctional transcript before translation of the message. After removal from mRNA, itis rapidly degraded.

Intron: A segment of DNA of unknown function within a gene; it may betranscribed in precursor RNA, but cannot be found in functional mRNA

Intron-targeting polymorphism markersEach ITPmarker is based on a pair of primers specific to the conserved regions of exons flanking an intron.

Intuitive Approach. An approach of choosing parents for crossing programme on thebasis of phenotypic assessment of agronomic, pathological and quality traits incontrast to predictive approach that is based on genetic analysis. Breeders mostfrequently use it.

Invader technologyIt exploits the ability of certain enzymes to specifically recognize the invasive nucleic acid structure and cleave at a specific site one of the strands forming this structure.

Invasive nucleic acid structureA replication forklike structure formed when the single-stranded 30 end of a DNA molecule invades a homologous DNA duplex and displaces the strand having the same sequence as the invading strand.

Inversion. A rearrangement of a chromosome segment so that its genes are in reversedlinear order. It is a kind of chromosome mutation consisting of the removal of asegment, its rotation through 180° and its reinsertion in the same location.

Inversion: Occurs where a chromosome breaks in two and becomes reattachedin reverse orientation. This may or may not affect gene function.

Inverted Repeat Sequence. A sequence found in identical but inverted form (forexample, at the opposite ends of a transposon).

Ion semiconductor sequencing DNA sequencing method uses a semiconductor-sensing device or ion chip for detection and quantification of the H+ ions liberated during DNA synthesis by DNA polymerase.

Ionizing Radiations. Very high-energy radiations that can remove electrons from atomsand attach them to other atoms, thereby producing positive and negative ion pairs (α,β, γ, χ-rays, for example).

IPRs. Intellectual Property Rights. It is a monopolistic right, granted under law toinventions for a fixed period to exclude others from imitating, manufacturing, using126or selling a protected matter or process. Various forms of IPRs are patents, copyrights, trademarks, trade secrets, PBRs, PVP, etc.

IR 8. A semi-dwarf variety of rice introduced in India from IRRI, Manila (Phillipines). Itsintroduction was a major event in the history of rice breeding in the country. It hadvery good plant type, and high productivity. It was developed from a cross between‘Peta’ and ‘Dee-geo-woo-gen’. It carries sd1 gene (for dwarfness) from Dee-geo-woogen.

Irradiation. Exposure of plants, plant parts, seeds, etc. to any kind of radiation toincrease mutation rates.

Isoaccepting tRNAs. The various types of tRNA molecules that carry a specific but thesame amino acid to the site of protein synthesis.

Isoalleles. Alleles indistinguishable except by special tests. Such alleles act within thesame phenotypic range of each other. Effects of isoalleles are ordinarily notidentifiable individually. Such alleles produce only slight differences in phenotypicexpression, making their identification bound to the use of special methods.

Isochromosome. A chromosome with equal and genetically identical arms. The two armsof the chromosome are mirror images of each other.

Isochromosome: A chromosome with two identical arms; it usually derivesfrom telocentric chromosomes

Isodicentric chromosome: A structurally abnormal chromosome containing a duplication of part of the chromosome including the centromere; the resultingstructure contains two centromeres and a point of symmetry that depends on theposition of the breakpoint

Isodirectional Dominance. Dominance occurring in the same direction at all loci. Itcould be one of the reasons of skewness observed in the distribution of genotypeswith respect to traits conditioned by four or five genes. However, its effect inproducing skewness decreases as the gene number increases as some may showdominance in opposing direction.

Isoelectric Point. The pH at which the sum of positive and negative charges is zero. Atisoelectric point, the net charge is zero, and the molecule will not migrate in anelectrical field. This is the basis for isoelectric focussing, a technique used to separateproteins.

Isoenzyme (isozyme): Different chemical forms of the sameenzyme that can generally be distinguished from one anotherby electrophoresis.

Isogenic Lines. Identical lines differing at only one locus. Such lines are producedusually through back cross method. True isolines (isogenic lines) are difficult, perhapsimpossible, to produce with back cross breeding owing to linkage between the gene of127interest and those influencing other traits. The transfer that occurs during backcrossing involves a block of closely linked genes instead of a single one.

Isogenic lines: Two or more lines differing from each other genetically at onelocus only

Isogenic linesLines having identical genotype, except for the alleles of a single gene.

Isolate. A group of individuals (population) or of populations, who because of theirisolation, choose mating partners from that group. Because of their isolation, oneisolate is completely separated from other such groups.

Isolation Distance. The minimum distance required to separate seed-producing fieldfrom other varieties of the same or related species in order to prevent outcrossing orundesired mating. This is an indispensable endeavour to maintain the genetic purity ofa variety during the process of its multiplication.

Isolation. The separation of one group from another so that mating between or amongstthem is prevented. In genetic term, it is the restriction to exchange of geneticinformation between distinct populations due to barriers to interbreeding. If the geneflow is prevented through differences in reproductive mechanisms, it is calledreproductive isolation.

Isomeric Genes. Genes which can each produce the same phenotype. If a number of suchgenes are present in one individual, they may be cumulative or non-cumulative intheir action.

IsoschizomersTwo restriction enzymes that recognize the same target sequence, but one of them is methylation sensitive and the other is methylation insensitive.

Isotope. One of several forms of an atom having the same atomic number but differingatomic masses (12C6 and 14C6, for example).

Isozymes. Multiple molecular forms of a single enzyme that differ by minor variations inamino acid composition and sometimes in regulation. Lactic dehydrogenase, whichcatalyses the conversion of pyruvate to lactate, is a tetramer, and have five isozymes(isoenzymes) derived from all the combinations. They differ in their electrophoreticmobility in starch gels. The relative proportions of the isoenzymes are characteristicfor each tissue and for each stage of development.

IsozymesDifferent forms of an enzyme present in the same individual and having the same catalytic function. In strict sense, each variant is encoded by a different gene and, in practice, includes allozymes.

Iteration. A procedure that makes use of repeated trials to find the best-fitting value of aparameter from observed data.