Terminologies in Genetics and Plant Breeding Part - 7

H4 (Patel 1971). The first commercial hybrid of cotton in the world. It was developed atCotton Research Station, Surat (India) by crossing two upland cotton (G. hirsutum L.)varieties namely ‘Gujrat 67’ and ‘American Nectariless’. The commercial seedproduction was performed manually.

Habitat. That place inhabited by a population, species, or a group of species.

Haldane mapping function:  It corrects recombination frequencies for multiple crossing over events assuming lack of interference.

Half sibs: Progeny with only one common parent

Half-length of D0:  The physical/genetic distance at which the value of D0 between two loci declines to 0.5.

Haploid (Blakeslee et al. 1922). A cell or organism with gametic chromosome number. Ithas also been defined as a sporophyte with the gametophytic chromosome number.Haploids, when viable, are typically smaller and less vigorous than diploids. They arecharacteristically sterile. The reason that accounts for sterility in monoploids holdstrue in this case also. Haploids are produced spontaneously in natural populations orthey can be produced artificially through a number of techniques. Haploid embryoscan arise from an egg cell (gyanogenesis), from a gametophytic cell other than the eggcell (apogamy) or from a male gamete (androgenesis). However, no one method ofproducing haploids is effective with many species. They have special significance inthe plant breeding as they can provide complete homozygous line after doublingchromosome number in a single generation. They are also useful in basic geneticinvestigation as genetic segregation is less complex compared to diploids (which havetwo chromosomes of a particular kind).

Haploid parthenogenesis: The situation in which the unfertilised egg hatchesand develops normally to produce a viable male adult whose cells contain only thehaploid number of chromosomes

Haploid: A cell or organism with a single genome.

Haploid: A single set of chromosomes (half the full set of genetic material),present in the egg and sperm cells of animals and in the egg and pollen cells ofplants.

Haploidization. Production of a haploid from a diploid (or a di- or poly-haploid from apolyploid) by progressive chromosome loss.

Haploidization: The process whereby diploid somatic cells become haploidduring a parasexual cycle or by experimental means, for example, in barley bycrossing with wild (bulb) barley (Hordeum bulbosum), or in wheat by crossing withpearl millet (Pennisetum glaucum)

Haplo-insufficiency: Situation where one normal copy of a gene alone is notsufficient to maintain normal function; it is observed as a dominant mutationon one allele (or deletion of it) resulting in total loss-of-function in a diploid cellbecause of the insufficient amount of the wild-type protein encoded by the normalallele on the other haplotype

Haplontic. Of organisms (called haplonts) with a life cycle in which meiosis occurs inthe zygote giving rose to four haploid cells. The diploid stage is short-lived.

Haplontic: Organisms in which meiosis occurs in the zygote resulting in fourhaploid cells

Haplotype: A set of closely linked alleles (genes or DNA polymorphisms) inherited as a unit. A contraction of the phrase “haploid genotype.”

Haplotype: The group of alleles of different genes that are located in the same chromosome and tend to be inherited together as a block.

Hardy -Weinberg law (equilibrium): The stable frequency distribution ofgenotypes, AA, Aa, and aa, in the proportions p2, 2pq, and q2, respectively (wherep and q are the frequencies of the alleles, A and a); that is, a consequence of randommating in the absence of mutation, migration, natural selection, or random drift;it states that in an infinitely large, interbreeding population in which mating israndom and in which there is no selection, migration, or mutation, gene andgenotype frequencies will remain constant from generation to generation

Hardy-Weinberg Equilibrium. A state of population in which the same proportion ofgenotypes is carried forward to successive generations with respect to any given locusor combination of loci. It is also called genetic equilibrium or Hardy-Weinberg-Tschetverikov equilibrium after the name of its propounders. If a population is not inequilibrium, equilibrium at a locus is established after one generation of panmixis(random mating). If, the gene frequency p or q in males is not equal to that of females,106then it takes one generation to equalize the frequency between the sexes, and thesecond generation to reach the equilibrium. For the similar reason, the equilibrium isnot attained even after two generations for the sex-linked genes if males and femalesstart with unequal gene frequencies. One consequence of this equilibrium is that rarealleles are virtually never in homozygous condition. Most copies of such rare allelesare found in heterozygotes. The relative frequency in heterozygotes as opposed tohomozygotes is p/q, which for q = 0.001 is a ratio of 999:1. For two genes that areindependent in inheritance, if we start with a population of individuals of which allare AaBb so that p = q = r = s (that is, all gametes AB, Ab, aB, ab are equallypresent), equilibrium is reached in one generation of random mating as occurs for onegene.

Hardy-Weinberg Law. A law dealing with constancy of gene frequency acrossgenerations in a panmictic population in the absence of disturbing forces. In aninfinitely large random mating population, the frequencies of adaptively neutralalleles of a gene and the corresponding genotypic frequencies (at equilibrium) remainconstant from one generation to the other unless there are disturbances due toselection, non-random mating, differential migration and differential mutation.

Harlequin Chromosomes. Sister chromatids that stain differently, so one appears darkand the other light.

Harvest Index. The proportion of dry matter that is accumulated in economically usableparts. If the crop is grown primarily for grain purpose, then, harvest index (HI) is theproportion of the dry matter that is accumulated in grain. As HI is a ratio of economicyield and biological yield, a genotype having low grain yield could have high HI andvice-versa. Thus selection entirely on the basis of HI is not reliable.

Heat shock protein: When certain plants are exposed to high temperature, heatshock proteins are synthesized; they provide thermal protection to subsequentheat stress

Heaving. Lifting effect of the soil due to alternate freezing and thawing. It may result inthe lifting up of plants and may tear them loose from the soil, or sheer off roots.

Helicase. An enzyme that breaks hydrogen bonds in DNA and unwinds it duringmovement of the replication fork after initiation of process of DNA replication.

Helix. The natural conformation of biological polymers (DNA and proteins)characterized by a spiral structure with a repeating pattern described by rotation andtranslation.

Hemi-SNP: A SNP that detects variation at homoeologous/paralogous loci in the two or more genomes of a polyploid species.

Hemizygous. Of genes and chromosomes present in only one copy in a diploid organism(X-linked genes in a male mammal, for instance). Such genes and chromosomes donot occur in pairs of alleles and homologues, respectively.

Herbicide-tolerant crops: Crops that have been developed to surviveapplication(s) of particular herbicides by the incorporation of certain gene(s)either through genetic engineering or traditional breeding methods. The genesallow the herbicides to be applied to the crop to provide effective weed controlwithout damaging the crop itself.

Hereditary determinant: Any genetically acting unit of an organism that isreplicated and conserved, transferred from generation to generation

Hereditary: Transmissible from parent to offspring or progeny

Heredity. The biological similarity of offspring and parents; the property by virtue ofwhich offspring resemble their parents.

Heritability (broad sense, H2). Capability of being inherited; the proportion of totalvariability that is due to genetic causes (heredity). Quantitatively, it is expressed as theratio of the genetic variance to the phenotypic variance; H2 = [geneticvariance/phenotypic variance] x 100 (in percentage) = [σ2G /σ2P] x 100. Forinstance, if H2 is 80%, it implies that one-fifth of the total variance is environmentaland four-fifths genetic in origin; in other words, only 80% of the phenotypicsuperiority of selected lines can be realized in the next generation. It can be high evenwhen σ2G is low. It is called broad sense heritability because it takes the total geneticvariance into account. As such it has little significance in plant breeding, and itsestimate in one population in a given environment cannot be extrapolated either toanother population or the same population in a different environment. With lowerheritability, considerable overlapping is encountered between genotypes; this posestremendous difficulty in selection.

Heritability (narrow sense; h2). The proportion of total variance that is due to additivegene action. It is the proportion of additive genetic variance to the total variance. h2 =[additive variance/phenotypic variance] = [σ2A/σ2P]. Since additive variance betterindicates the degree to which the progeny of the segregating individuals will resembletheir parents, h2 is more reliable compared to the H2. High heritability means highregression of offspring on their parents.

Heritability: A measure of the degree to which a phenotype is geneticallyinfluenced and can be modified by selection; it is represented by the symbol h2; this equals Vgen/Vphe where Vgen is the variance due to genes with additiveeffects and Vphe is the phenotypic variance;

Heritability: The proportion of genetic variance for a trait to its phenotypic variance.

Herkogamy: Pollination by the neighbour individual, population, or species

Hermaphrodite: A plant having both female and male reproductive organs inthe same flower, eg. cotton, sugarbeet, alfalfa, rapeseed, rye, or sunflower

Hermaphroditism. A condition concerning presence of reproductive organs of bothsexes in the same individual or in the same flower in higher plants. (Hermaphroditeanimals and monoecious plants are analogous!).

Heterocaryon. A cell having genetically differentiated nuclei; the result ofheterocaryosis. Two different nuclear types are contained in a common cytoplasm. Itis apparent that a heterocaryon containing different types of nuclei behaves like agenetic heterozygote, and hence possesses heterocaryotic vigour analogous to theheterotic advantage of heterozygotes in the cross-fertilized sexual species.Furthermore, the system is clearly capable of providing a type of somatic segregationbased on exchange of entire nuclei during hyphal fusion, and thus creates adequatevariation in pathogenic asexual fungi.

Heterocaryosis. The coming together of genetically distinct nuclei in the same cell ofmycelium to form heterocaryon. This may change pathogenicity of fungal races. Itmay also lead further to segregation and recombination of different nuclei andeventually to creation of new pathotypes at least in asexual fungi and the fungiimperfecti.

Heterochromatin. Densely staining condensed chromosomal regions that are believed tobe genetically inert for the most part. Heterochromatin can be divided into twoclasses: (a) constitutive heterochromatin, which may be an entire chromosome or aspecific segment that stain densely in every preparation from a given species, and (b)facultative heterochromatin, which may also be an entire chromosome or a specificsegment that appear as euchromatin in some preparations and heterochromatin inothers. It may be responsible for a variety of meiotic alterations. It can be used for: (1)increasing crossing over (through B-chromosomes in maize), (2) changing theposition of crossing over (through B-chromosomes in maize), (3) reducing pairingbetween homeologous chromosomes (through B-chromosomes in common wheat andLolium spp.), and (4) increasing pairing between homologous chromosomes (5Bchromosome in modern wheat).

Heterochromatin: The darkly stained regions in chromosomes

Heterochromosomes. Sex chromosomes or allosomes.

Heterodimer. A protein consisting of two nonidentical polypeptide subunits.

Heteroduplex analysis: A method of detecting gene mutation by mixingPCR-amplified mutant and wild-type DNA followed by denaturation andreannealing; the resultant products are resolved by gel electrophoresis, with singlebase substitutions detectable under optimal electrophoretic conditions and gelformulations; large base pair mismatches may also be analysed by using electronmicroscopy to visualise heteroduplex regions

Heteroduplex DNA: A double-stranded DNA molecule formed by theannealing of strands from two different sources, as opposed to homoduplex,which has homologous strands; as a result, there are regions noncomplementaryand showing abnormalities in the form of extra loops

Heteroduplex. A DNA double helix formed by annealing single strands from differentsources. If there is a structural difference between the strands, the heteroduplex mayshow such abnormalities as loops or buckles.

Heterogametic Sex. The sex having heteromorphic sex chromosomes (XY, for example)and thus producing two different kinds of gametes with respect to the sexchromosomes.

Heterogeneity. A description of the state of a population containing a number of purelines or genetically different individuals (in case of a random mating population).Heterogeneity in such a population (especially in selfers) often leads to: (a)phenotypic interactions providing gains in performance, and (b) mutual buffering orhomeostasis resulting in steadier performance. Therefore, such a population isexpected to contain cooperative or synergistic neighbours (genotypes). In other words,heterogeneity in this kind of population is favoured for synergistic genotypes.

Heterogeneous Nuclear RNA. Assortment of RNA types found in the nucleus, includingmRNA precursors and other types of RNA. It is designated as HnRNA.

Heterogeneous stock:  In outcrossing species with a short-generation time, a population generated by crossing several inbred strains/lines and maintained by random mating or mating in pairs a suitably large number of individuals.

Heterokaryon Test. A test for cytoplasmic mutations, based on new associations ofphenotypes in cells derived from specifically marked heterokaryons.

Heterologous probes: Probes prepared from one species and used in another species. Syn., heterospecific probes.

Heteromorphic Chromosomes. A chromosome pair with some homology but differingin size, shape, or staining properties.

Heteromorphic Incompatibility. An incompatibility system which operates throughdifferences in the morphology of flowers on different plants coupled with sporophyticcontrol of incompatibility reaction of the pollen. The best-known case is found inPrimula species; however, it is practically unimportant in higher plants.

Heteromultimer. A protein consisting of at least two polypeptide subunits, with at leasttwo of the subunits being nonidentical (e.g., haemoglobin).

Heteroplasmon. A cell containing a mixture of genetically different cytoplasms andgenerally different mitochondria or different chloroplasts.

Heteroploidy. A condition of deviation from the normal chromosome number by a fewchromosomes (2n ± a few chromosomes).

Heterosis (Shull 1908). A phenomenon in which an F1 derived by crossing geneticallydissimilar parents may fall outside the range of their parents with respect to acharacter (s). Improved biological fitness of such an F1 hybrid is often called hybridvigour. In strict sense, it describes a different genetic make-up of the F1 compared toits parents. Thus heterosis is the process and hybrid vigour is the consequence of thatprocess. However, for all practical purposes, these two terms are used synonymously.Historically, it was attributed to heterozygosity (dominance and/ or overdominance,that is, intra-locus interactions at individual loci) in F1 hybrids. However, the highproportion of homozygous loci (79% of the 23 loci studied) of the 4 most widelygrown single cross hybrids of maize (in USA) casts doubt on the single locusheterozygosity and associated overdominance in promoting high performance of suchhybrids. More recently, multi-locus epistasis has been shown to be the majormechanism of heterosis in a leading hybrid variety of rice (Shanyou 63) in China.Thus epistasis (especially multi-locus epistasis) plays a major role in the genetic basisof heterosis.

Heterosis QTLs: QTLs involved in the expression of heterosis.

Heterosis: Superiority of an F1 hybrid over both its parents in terms of yield or some other trait.

Heterosis: The increased vigor of growth, survival, and fertility of hybrids, ascomposed with the two homozygotes; it usually results from crosses between twogenetically different, highly inbred lines

Heterostyly. A mechanism to promote outbreeding in certain angiosperms. It implies thepresence in a species of two (distyly) or three (tristyly) different types of individuals,distinguished by the relative positions of stigma and anthers in the flowers. Also seeheteromorphic incompatibility.

Heterostyly: A polymorphism among flowers that ensures cross-fertilisationthrough pollination by visiting insects; flowers have anthers and styles of differentlength

Heterothallic Fungus. A fungus species in which two different mating types must uniteto complete the sexual life cycle. “Heterothallic” literally means, “different bodied”.Mating types are physically identical but physiologically different.

Heterotic group: Crosses between members of the same heterotic group show little or no heterosis, but those between members of different heterotic groups show moderate to high heterosis.

Heterotic pattern: The pattern of variation in the extent of heterosis observed in crosses between members of different heterotic groups.

Heterozygosity. A measure of the genetic variation in a population. It refers to thefrequency of heterozygotes (that carry dissimilar alleles of a gene) with respect to agiven locus.

Heterozygosity: The presence of different alleles at one or more loci onhomologous chromosomes.

Heterozygosity: The presence of different alleles at one ormore loci on homologous chromosomes.

Heterozygotes. Organisms carrying dissimilar alleles (Aa) of a gene pair. They aresometimes called hybrids.

Heterozygous Balance (Mather 1943). A type of genetic organization adjusted toheterozygosity. This adjusted state confers high adaptive value upon associated111individuals. Heterosis is supposed to be the outcome of such genetic organization.Autogamous species too show heterosis since it has also appreciable level ofheterozygous balance. This balance in self-fertilized species is supposed to beevolutionary remains of their outbreeding ancestors.

Heterozygous: Having two diffeent alleles (eg. Aa) governing a singlephenotypic trait. A hybrid corn plant, for example, is heterozygous at every locus(eg. Aa Bb Cc Dd...).

Heterozygous: Situation where the two alleles at a specificgenetic locus are not the same.

Hexaploid. A cell having six chromosome sets or an organism composed of such cells. Ifall the sets are identical, the individual is called an autohexaploid (e.g., sweet potato).If, however, genomes are only partially homologous, it is referred to as allohexaploid.

Hierarchical databases: The data are organized in a hierarchical (ordered tree) structure, and there are two or more levels of data organization.

Hierarchy. An arrangement into a graded series.

High Nutrient Rice. Genetically engineered rice with enhanced iron and cysteinecontent. It has a ferritin gene from P. vulgaris, and thermo tolerant phytase gene fromA. fumigates.

High-Input Agriculture. Agriculture augmented with a variety of inputs, such asirrigation and improved mechanization, and/ or with such inputs as fertilizers,pesticides, herbicides, and fungicides. Breeding programmes, in general, have beenvery successful and efficient in high-input agriculture (resource rich areas), perhapsbecause agricultural conditions in farmers’ fields and agricultural research stations inwhich most new varieties are bred and developed are closely similar. Furthermore,most of the variables of the production environments are predictable and manageable.

High-throughput DNA sequencing: The industrialised process of determiningthe exact order of the four letter code (bases, GAT or C) that composes all geneticmaterial; because a typical higher organism contains > 20,000 genes, each withseveral thousand bases of DNA, data storage and analysis is one of the mostimportant parts of this process

High-throughput genotyping: Simultaneous genotyping for few to several hundreds or thousands of markers in hundreds to thousands of individuals.

Histocompatibility Genes. The genes that encode the histocompatibility antigens that inturn determine the acceptance or rejection of a tissue graft.

Histogenesis. The process of formation and development of tissues; also calledhistogeny.

Histone. A type of basic protein that forms the unit around which DNA is coiled in thenucleosomes of eukaryotic chromosomes. Histones are a group of five basic proteinsthat associate with DNA in the cell nucleus to form chromatin.

Histone: One of a group of globular, simple proteins that have a high content ofthe amino acids arginine and lysine; it forms part of the chromosomal material ofeukaryotic cells and appears to play an important role in gene regulation; highlyconserved basic proteins that are involved in the packing of DNA; histone proteinsand the nucleosomes form, with DNA, the fundamental building blocks ofeukaryotic chromatin; they bind to the phosphate groups of DNA by their aminotermini; there are five major types of histone proteins; two copies of H2A, H2B, H3,and H4 bind to about 200 base pairs of DNA to form the repeating structure ofchromatin (nucleosome), with H1 binding to the linker sequence

Hitchhiking: An increase in the frequencies of alleles at essentially neutral loci located on either side of a locus subjected to selection.

Holandric Inheritance. A type of inheritance governed by genes completely linked tothe Y-chromosome. Such genes (often called holandric or Y-linked genes) aretransmitted exclusively from father to the sons and thus manifestation is limited to themale sex only.

Holliday Junctions. The structures formed during chiasma formation whenchromosomes exchange information during meiosis. Holliday junctions form fourdouble helical arms (branched DNA) flanking a branch point. The branch point canrelocate throughout the molecule by virtue of the homologous sequences.

Holocentric Chromosomes. Chromosomes having diffused kinetochores (centromeres)with microtubules attached along the length of the chromosomes. These may be foundin certain species or strains of a species (as in some strains of maize, for instance).

Holocentric: Applied to chromosomes with diffuse centromeres such that theproperties of the centromere are distributed over the entire chromosome

Homeologous Chromosomes. Partly homologous chromosomes originating fromdifferent but related genomes, between which there does not occur point-to-pointpairing at meiosis. The partial homology between them usually indicates someoriginal ancestral homology.

Homeosis. The replacement of one body part by another. It can be caused byenvironmental factors leading to developmental anomalies or by mutation.

Homeostasis. The tendency of a physiological system to resist to an external disturbancesuch that the system is not displaced from its normal values.

Homeotic Genes. Genes that control the fate of segments along the anterior-posterioraxis of higher animals.Homeotic Mutations. Mutations that can change the fate of an imaginal disk.

Homeotic mutation: A mutation that causes one body structure to be replacedby a different body structure during development

Homodimer. A protein consisting of two (more than two in case of homomultimer)identical polypeptide subunits.

Homoeologous: Partially homologous; chromosomes or genomes that arebelieved to have originated from ancestral homologous chromosomes

Homogametic Sex. The sex producing the same type of gametes with respect to the sexchromosomes. Such sex contains homologous sex chromosomes (XX, for example).

Homogamy. The preference of a mating individual for another with similar phenotype orgenotype.

Homogamy: The preference of individuals to mate with others of a similargenotype or phenotype; in botany, the condition in which male and female partsof the flower mature simultaneously

Homogeneous group: A group of individuals at Hardy–Weinberg equilibrium for all of several random markers/loci.

Homogeneous reaction: All the steps of such a reaction are carried out in a single vessel.

Homolog. A member of a pair of homologous chromosomes.

Homologies: Similarities in DNA or protein sequences between individuals ofthe same species or among different species.

Homologous Chromosomes. The chromosomes (or parts of chromosome) that synapseor pair during meiotic prophase; chromosomes with point-to-point pairing betweenthem at meiosis. They are also defined as the chromosomes in different but relatedspecies that have retained most of the same genes during their evolution from acommon ancestor.

Homologous chromosomes: A pair of chromosomes containingthe same linear gene sequences, each derived fromone parent.

Homologous chromosomes: Chromosomes that pair with each other at meiosis.

Homologous: Stretches of DNA that are very similar insequence, so similar that they tend to stick together inhybridization experiments. Homologous can also be usedto indicate related genes in separate organisms controllingsimilar phenotypes.

Homomorphic Incompatibility. A type of incompatibility identified only by breedingtests, and not by differences in floral morphology. The biochemical reaction of theincompatibility may be determined either by genetic constitution of the pollen itself(gametophytic) or by the genotype of the parent plant (sporophytic).

Homoplasy: The situation of two SSR alleles of identical size being different by descent.

Homopolymer-tailing: Attachment of identical nucleotides to the 3’ end of a DNA molecule that can be achieved with terminal deoxynucleotide transferase

Homothallic Fungus. A fungus species in which a single sexual spore can complete theentire sexual cycle (compare it with heterothallic fungus).

Homozygosity: The presence of identical alleles at one or more loci inhomologous chromosomal segments

Homozygote: An individual possessing a pair of identical alleles at a givenlocus on a pair of homologous chromosomes.

Homozygotes. Individuals containing identical alleles of a gene pair. They may be eitherhomozygous dominant (AA) or homozygous recessive (aa).

Homozygous Balance (Mather 1943). A condition in which genetic components ofindividuals are adapted to homozygosity. Inbreeders and certain cross-pollinatedspecies do not show inbreeding depression by virtue of this kind of geneticorganization.

Homozygous: Situation where the two alleles at a specificgenetic locus are identical to one another.

Honey Comb Selection (Fasoulas 1983). A design for selection. It was suggested as ameans to reduce the adverse effects of interplant competition on selection. As thename implies, plants are grown in a fashion similar to honeycomb. Each plant issurrounded by other six plants forming a hexagon. A plant that outperforms all the sixneighbouring plants is selected. It may improve selection during segregatinggenerations when individuals are grown in a non-replicated manner.

Honeycomb design: In a honeycomb design, the plant at the center of thehexagon is compared with every other plant within the hexagon; a plant is chosenonly if it is superior to every other plant in the hexagon; it was developed forselecting individual plants in a population; seeds or plants are usually spacedequidistantly from one another in a hexagon pattern; plants are spaced far enoughapart that they cannot compete with adjacent individuals; homogeneous checkscan be included; the size of the hexagon determines the selection intensity; it isused to minimise adverse effects of interplant competition

Hordeum bulbosum procedure: A method for producing zygotic haploids in barley by crossing Hordeum bulbosum with Hordeum vulgare genotypes; after formation of zygotes, the wild H. bulbosum chromosomes are subsequently eliminated during embryogenesis, which results in haploid H. vulgare plants

Hordicale (Qutuke 1940). A synthetic cereal obtained by crossing H. vulgare and S.cereale. Plants are more like paternal parent. However, seeds are devoid of embryoand endosperm because of their early abortion (14 days after pollination). Theincompatibility can be circumvented by spraying the spikes with growth regulators,combined with culturing of embryos in vitro.

Horizontal disease resistance: This type of resistance is governed by polygenes that reduce disease development and, particularly, the pathogen reproduction rate. It is effective against all the races/pathotypes of the pathogen and is ordinarily durable.

Horizontal Resistance (Van der Plank 1963). A kind of resistance in epidemiologicalterms. It is evenly effective against all genetic variants (races) of a particularpathogen. It can arise in two ways: (a) when the host genes do not operate in a genefor-gene way vis-à-vis pathogen genes, no differential interactions are possible, and(b) when several to many host genes with small effects (polygenes) operate on a genefor-gene basis with an equivalent number of genes in the pathogen populations,differential interactions are too small to be detectable, and the result appears to be114horizontal resistance. Owing to race non-specificity, the resistance is only partial,albeit durable and stable.

Horizontal resistance: Resistance conditioned by polygenes or quantitativegenes; it is race nonspecific in nature and does not reveal a gene-for-genehypothesis; this type of resistance is difficult to identify

Hormone. A molecule that is secreted by an endocrine organ into the circulatory systemand that acts as a long-range signalling molecule through activating receptors on orwithin target cells.

Host Evasion. A mechanism of functional resistance in which phenologies of the hostand insect do not synchronize. It takes place when plant growth pattern is modified soas to bring in asynchrony of insect-host phenologies. For example, early planting ofrice saves the crop from gall-midge attack.

Host plant resistance: A method of pest control in which resistant, tolerant, orunattractive host organisms are used; the inherited qualities of resistance influencethe extent of pest damage

Host Range. The spectrum of strains of a given plant species that a given strain of pestcan infect.Hot Spot. A place where an endemic disease(s) or pest(s) occurs regularly duringcropping season. In genetics, it is a part of a gene that shows a very high tendency tobecome a mutant site, either spontaneously or under the action of a particularmutagen.

Host. A cell or organism whose metabolism is used for the growth and reproduction of avirus, bacteria, pathogen, or any other parasite.

Housekeeping gene: Gene that is expressed in virtually all cells since it isfundamental to the any cell’s functions.

Human Genome Initiative: Collective name for several projects begun in 1986by DOE to (1) create an ordered set of DNA segments from known chromosomallocations, (2) develop new computational methods for analysing genetic mapand DNA sequence data, and (3) develop new techniques and instruments fordetecting and analysing DNA. This DOE initiative is now known as the HumanGenome Program. The national effort, led by DOE and NIH, is known as theHuman Genome Project.

Hybrid Breakdown. Distorted segregation in the F1 hybrid leading to the recovery ofparental types (cultivated and wild relatives) in the F2 or later segregating orbackcross generations. It may be ascribed to centromeric affinity, cryptic structuralhybridity, gene substitution or unfavourable nuclear-cytoplasmic interactions. Incertain interspecific hybrids of rice, hybrid breakdown has been reported due toduplicate recessive genes in the F2 and later segregating generations.

Hybrid breeding (syn heterosis breeding): The discovery of heterosis has beenrecognised as one of the major landmarks of plant breeding; in comparison to inbredlines and homozygous material, the phenotypic superiority of heterozygotes is thebasis of hybrid breeding; it is exploited for production of hybrid, synthetic, andcomposite varieties; hybrid breeding can be performed using traditional breedingtechniques, or the process can be hastened using gene marker technology to rapidlyidentify parents with desired genes for certain attributes; numerous commercialcrops are hybrids with increasing tendency; seeds from a hybrid variety, if planted,will not deliver the same benefits as the original seeds and after several offspringwill have lost the desired qualities from the original hybridisation

Hybrid Corn Makers. An acronym used in recognition of pioneer works of early maizebreeders like Shull, East, Jones, Hays, Rickey and Jenkins.

Hybrid Cotton. The cross product of genetically different parents of cotton. The term isalso used to designate transgenic cotton, the lint of which would have the propertiesof original cotton as well as polyester. Clothes made from such cotton would behavelike present cotton-clothes besides zero shrinkage. This way farmers may putchallenge to polyester industry.

Hybrid Dysgenesis. A syndrome of effects including sterility, mutation, chromosomebreakage, male recombination in the hybrid progeny of crosses between certainlaboratory and natural isolates of Drosophila.

Hybrid Equilibrium. The equilibrium state of hybridity in a stable breeding population.

Hybrid Incapacity. A collective term that includes sterility and inviability of usuallyinterspecific or intergeneric hybrids. It is the result of genetic imbalance which mayaffect either metabolism during early or later vegetative stages of development orgametogenesis leading to failure in gamete formation.

Hybrid Inviability. Inability of a hybrid (usually derived from wide crosses) to survivedue to abnormal developmental processes. Death of hybrids can occur from thezygotic stage up to the final differentiation of reproductive organs and formation ofgametes. It may be due to activation of lethal genes or disharmonious interactionsbetween cytoplasm of one species and nucleus of the other. It can be overcome byembryo rescue, in vitro/vivo embryo culture, ovule culture, ovary culture, graftinghybrids, reciprocal crosses or altering genomic ratio.

Hybrid inviability: Reduced vigour of hybrid plants compared to their crossingparents

Hybrid seed production: The production of hybrid seeds by combinationof more or less defined parental forms; usually those hybrid seeds are moreproductive or more suitable than pure lines; they are used for subsequent growingand commercial production of a crop

Hybrid selection: The process of choosing plants possessing desired traitsamong a hybrid population

Hybrid Sterility. Partial or complete reproductive incapacity of an F1 or later generationhybrids between genetically different populations (usually belonging to two differentspecies). It is clearly an isolating mechanism, which prevents flow of geneticinformation between populations. The basis of hybrid sterility may be genomicdisharmony, nuclear instability, hybrid break down and reversion to parental types,lack of recombination and presence of deleterious genes or undesirable linkages. Itmay appear as embryo sac sterility, pollen sterility, anther dehiscence, and asynchronyin the development of male and female gametes in the same spikelet. It may beovercome by chromosome doubling or backcrossing.

Hybrid Swarm. A collection of hybrids produced by complete or local breakdown ofisolating barriers between two sympatric species. Hybrid swarm replaces the parentalspecies and serves as a continuous bridge between two parental extremes.

Hybrid varieties: F1 generations from crosses between two and more purelines, inbreds, clones, or other genetically dissimilar populations/lines used for commercial cultivation.

Hybrid Variety. An F1 population obtained by crossing genetically dissimilar parentsand used for commercial plantings. It exploits heterosis to the greatest extent. Ahybrid variety must display hybrid vigour to the extent, which makes it profitable tothe farmers. Seeds of hybrid variety must be purchased afresh every year because thesuperior gene combination of the F1 generation breaks down in the F2 generationowing to Mendelian segregation and recombination. As a consequence, a largenumber of genetically different F2 individuals are produced; none of which is nearly116as productive as the F1 itself. The yield reduction in the F2 generation is nearly alwaysat least 10%, and often more than 30%, compared to the parental hybrid variety.

Hybrid variety: A variety produced from the cross-fertilisation of inbredlines with favourable combining ability; the progeny is homogeneous and highlyheterozygous; it can be produced by

Hybrid Vigour. See heterosis.

Hybrid. The product of a cross between genetically dissimilar parents; an individualcarrying dissimilar alleles at one, two, three,….., etc. loci is called mono-, di-, tri-,….,polyhybrids, respectively; a heterozygote. It is a progeny individual from any crossbetween parents with differing genotypes.

Hybrid: The progeny of a cross between two different species, races, cultivars, or breeding lines.

Hybridisation In Situ. Finding the location of a gene by adding specific radioactiveprobes for the gene and detecting the location of the radioactivity on the chromosomeafter hybridisation.

Hybridisation. A process of making a hybrid by crossing genetically unlike parents ofthe same species or of a different species. Its application in self-pollinated crops relateto the creation of enormous amounts of variability through segregation andrecombination of genes in the hybrid. Its main object in breeding self-pollinated cropsis to obtain a superior genotype (recombinant) in segregating generation, whichresults from recombination of desirable genes, which are found in two or moregenotypes. In cross-pollinated crops, this is an inseparable operation in populationimprovements and in the development of hybrid varieties.

Hybridisation: A method of breeding new varieties that applies crossing toobtain genetic recombination; in genetics, the fusion of unlike genetic material, such as sexual organs or DNA; in molecular biology, pairing of complementaryDNA and/or RNA

Hybridity. The union of genetically dissimilar gametes; the state of being heterozygousor hybrid.

Hybridization (or crossing): The process of pollen transferfrom the anther of the flower of one plant to the stigma ofthe flower of a different plant for the purpose of gene transferto produce an offspring (hybrid) with a mixed parentalgenotype.

Hybridization: Bringing complementary single strands ofnucleic acids together so that they stick and form a doublestrand. Hybridization is used in conjunction with DNA andRNA probes to detect the presence or absence of specificcomplementary nucleic acid sequences.

Hybridize. Forming a hybrid by performing a cross. Also annealing nucleic acid strandsfrom different sources.

Hydrogen Bond. A weak bond in which an atom shares an electron with a hydrogenatom. For example, it is formed after a hydrogen atom is shared between two atomshaving negative charge such as between oxygen and nitrogen or between two nitrogen117atoms. Hydrogen bonds are important in the specificity of base pairing in nucleicacids, and in the determination of protein shape.

Hydrogen bond: A weak bond involving the sharing of an electron with ahydrogen atom; hydrogen bonds are important in the specificity of base pairing innucleic acids and in the determination of protein shape.

Hydrophobic Interactions. Exclusion of water by non-polar groups of a macromolecule,causing compaction with hydrophobic groups inside and hydrophilic groups outside.

Hyperploid. An aneuploid containing a small number of extra chromosomes (2n + a fewchromosomes).

Hypersensitivity. The increased sensitivity of the host cells in the vicinity of infectedsite, which leads to the death of infected cells ultimately restricting the spread ofobligate pathogens through separating them from the living host (e.g., resistance tolate blight of potato). Hypersensitivity is now thought to involve several differentmechanisms such as antibiosis, presence of mechanical barriers (the presence of cellwalls, which cannot be easily degraded by the pathogens’ enzymes), or the absence orinsufficiency of nutrients or other essential compounds.

Hypersensitivity: The response to attack by a pathogen of certain host plantsin which the invaded cells die promptly and prevent further spread of infection

Hyperspectral reflectance data: Data collected at narrow (1–2 nm) bandwidths between 270 and 1,100 nm.

Hypoploid. An aneuploid with a small number of missing chromosomes (2n – a fewchromosomes).

Hypoploidy: Having missing chromosome complements compared to thestandard chromosome set