Terminologies in Genetics and Plant Breeding Part -11

Nanotechnology. A technology to direct the assembly of highly structured materials withspecific nanoscale features (as in DNA) for material purposes. It presents a newchapter in the history of the molecule.

Narrow Genetic Base. Having little genetic variation as in an inbred line.

National Agricultural Policy (2001). A policy related to rejuvenation of Indianagricultural economy. The document is a result of a series of consultative processes atvarious levels that deal with technology, institutions and policy. For acceleratinggrowth in agricultural sector, three components such as technology, policy andinstitutions have been visualised indispensable.

National Agricultural Technology Project (1998). A project started jointly by the ICARand DAC to promote integration of research, technology generation, assessment andrefinement, and dissemination in area-specific and production system mode. It wasvisualised as an important component of the overall strategy for combining foodsecurity, poverty alleviation, rural development and conservation of natural resources.

Natural Crossing. Spontaneous crossing between genetically different plants/lines/strains being grown in proximity to each other. The extent of natural crossingvaries with time of flowering, wind direction, temperature, and interaction of thesefactors among themselves and with the particular genotypes of the plants.

Natural selection: A complex process in which the total environmentdetermines which members of a species survive to reproduce and so pass on theirgenes to the next generation

Naturalist. A student of field biology, in contrast to a laboratory worker.

Near-isogenic lines (NIL): Not fully isogenic; for example, in maize, twodistinct composites of F3 lines from a single cross, one consisting of lineshomozygous recessive and the other consisting of lines homozygous dominantfor a certain gene (i.e. there is same genetic background), however, differing onlyin being homozygous dominant versus recessive for the genes; in wheat, nearisogeniclines have been produced for different Rht (reduced height) genes causingdifferent straw length

Near-isogenic lines: Pairs of homozygous lines that are identical in genotype, except for a single gene/locus; in practice, they also differ for a variable length of the genomic region flanking this locus and some random genomic segments located elsewhere in the genome.

Nebulization: Mechanical shearing of DNA.

Negative Assortative Mating. Preferential mating between unlike parents or partners. Ifthe parents are genetically contrasting, it is called negative genetic assortative mating(AA × aa). If unlike parents are chosen on the basis of phenotypic appearance, it isreferred to as negative phenotypic assortative mating (tall × dwarf). Negativeassortative mating is also called disassortative mating.

Negative assortative mating: A type of nonrandom mating in whichindividuals of unlike phenotype mate more often than predicted under randommating conditions

Negative Control. Gene regulation mediated by factors that block or turn offtranscription. Repressor blocks the expression of structural gene(s).

Negative selectionThe use of molecular markers closely linked to undesirable alleles of known genes/QTLs to select against these alleles.

Neo-Darwinian Evolution. A modified Darwinian paradigm developed during 1930sand 1940s that incorporated concepts from modern population, ecological and evolutionary genetics into descent with modification; often referred to as the ModernEvolutionary Synthesis.

Neolithic Age. Cultural period beginning ~12,000 years ago; appearance of technicallyadvanced stone tools.

Nested association mapping (NAM): An integrated mapping strategy that allows genome-wide high-resolution mapping in a cost-effective way; it greatly facilitates complex trait dissection in many species; it is used as a large-scale maizemapping resource and as a genomics and bioinformatics tool; it requires largeamounts of SNPs and/or individuals, and it is sensitive to genetic heterogeneity

Nested parentsIn case of NAM, one or few parents crossed with all the founder parents.

Neurospora. A pink mould that is commonly found growing on old food.

Neutral Mutation. A mutation that has no effect on the biological fitness of its carriers.It is also defined as a kind of mutation that has no phenotypic effect. It may be sobecause the altered codon specifies functionally equivalent amino acid in thepolypeptide [e.g., AAA (lysine) to AGA (arginine); both are basic amino acids].

Neutral Polymorphism. A kind of polymorphism which is sustained owing to neutraleffects of alleles on survival and reproduction of genotypes.

Next-generation DNA sequencing methodsThese methods use PCR for in vitro cloning and sequence multiple copies of a very large number of relatively small DNA fragments.

NHI. Nitrogen Harvest Index. It refers to partitioning of nitrogen between grain and theremaining plant-parts. In semi-dwarf wheat, the new varieties have a greatly improvedNHI.

Niche. A group of multidimensional environmental factors of an ecosystem to which apopulation fits. According to Mayr (1963), it deals with the outward projection ofneeds and the specific way of utilizing the environmental factors by the individuals ofthe population.

Nicking. Nuclease action to sever the sugar-phosphate backbone in one DNA strand atone specific site.

nif Genes. A cluster of genes orchestrated to produce the enzyme nitrogenase. More than22 such genes are known. Out of these, only three (H, D, and K) are structural; someare regulatory (A, L, etc.), and the rest may assist in making the expressed productfully functional.

Nitrocellulose Filter. A type of filter used to hold DNA for hybridisation.

Nitrogen bases. Types of molecules that form important parts of nucleic acids. These arecomposed of nitrogen containing ring structures. The two strands of a DNA doubleare linked with each other through hydrogen bond established betweencomplementary bases.159

Nitrogenase. An enzyme that reduces nitrogen into ammonia. It has two sub-units: MoFebigger sub-unit (synthesized by nif K&D), and Fe smaller sub-unit (synthesized by nifH).

Nitrogenous base: A nitrogen-containing molecule havingthe chemical properties of a base.

No Dominance. A condition in which the heterozygote falls exactly the mid-waybetween the two homozygotes [for example, AA (3), Aa (2.5), and aa (2)].

No Selection. Relaxation in selection pressure such that no individuals are rejected, thatis, 100 per cent individuals are carried forward merely to maintain the population(selection intensity is lowest, i.e., 1.0 or 100%).

Nobilisation. A term coined by Dutch breeders in Java for the improvement of sugarcane. It refers to crossing and recrossing schedule to effect improvement of hardy anddisease resistant but inferior wild canes (S. spontaneum) by crossing it with moreattractive, sweeter noble canes (S. officinarum) as the female one. However, the termhas been extended to include crossing of any wild species of Saccharum or its relatedgenera to S. officinarum. When S. officinarum is used as the female, it uniquelytransfers 2n gametes to its progeny (in case of backcrosses, up to 2nd backcrossing);however, in reciprocal crosses, only n+n transmission has been observed.

Noble canes. The canes belonging to the species Saccharum officinarum. Canes are socalledbecause they are tall, handsome, large barrelled shaped and colourful.

Nodule. The rounded irregular mass of cells that appear as a small knot. It is a nitrogenfixing machinery on the roots of legumes. Root nodules are most common; however,stem nodules (Sesbania rostrata) and leaf nodules (Psychotria sp.) have also beenreported.

Noise reductionIn image analysis. morphological operations to correct the imperfections like holes present in the binary images.

Nonadditive Variance. The variance caused by non-additive gene action. It includesdominance, and dominance × dominance type of gene interaction.

Non-anchored ISSR primerIn case of ISSR, the primer consists solely of a microsatellite sequence.

Nondisjunction. The failure of homologs (at meiosis) or sister chromatids (at mitosis) toseparate from each other so that the two go to the same pole at anaphase.

Nondisjunction: The failure of separation of paired chromosomes at metaphase, resulting in one daughter cell receiving both and the other daughter cell none ofthe chromosomes in question; it can occur both in meiosis and mitosis

Nonhomologous chromosomes: The different chromosomes of a haploidchromosome set, which usually cannot pair with one another

Non-imaging fluorometerUse of portable handheld fluorometers to measure fluorescence from few square millimeter leaf area.

Non-linear Tetrad. A tetrad in which the products of meiosis are arranged in noparticular order.

Non-Mendelian Inheritance. Extrachromosomal inheritance.

Non-Mendelian inheritance: An unusual ratio of progeny phenotypes thatdoes not reflect the simple operation of Mendel ’s law, for example, mutant: wildtype ratios of 3:5, 5:3, 6:2, or 2:6, indicate that gene conversion has occurred; ingeneral, it refers to extrachromosomal and/or non-chromosomal inheritance

Non-Mendelian Ratio. An unusual ratio of progeny phenotypes that does not conform tothe simple operation of Mendel’s laws of inheritance. For example, mutant: wildratios of 3: 5, 5: 3, 6: 2 or 2: 6 in tetrads indicate that gene conversion has occurred.

Non-photochemical quenchingThe part of light energy lost as heat via the xanthophyll cycle.

Non-Preference. An insect response to characteristics of plants that make themundesirable for use by insects as a site for reproduction, food, shelter, or anycombination of these three. Two types of non-preferences have been reported : (a) onethat is manifested only in the presence of the preferred host, and (b) the second evenin the absence of the preferred host. Such a mechanism is strong enough to force theinsects to starve, particularly in early instar stages. Therefore, the extreme nonpreferencemay be mistaken for antibiosis and vice-versa. The plant characters thatinfluence non-preferences include colour, light reflection, type of pubescence, leafangle, odour, taste, etc. It has been observed that nectariless cultivars of cotton areresistant to cotton ball worm. Similarly, yellow-green varieties of peas are lessdesirable to the pea aphid than are blue-green cultivars. Solid stem has beenassociated with resistance to stem saw fly and stem borer in wheat and maize,respectively. However, solid stem is often associated with low yielding capacity, andprecisely for this reason, resistant cultivars are not grown where these insects areproblems in the respective crops. It seems likely that non-preference is polygenicallydetermined.

Nonpreference: A term used to describe a resistance mechanism whereparasites prefer to be on some host genotypes more than others; the less preferredgenotypes are resistant

Non-Random Mating. Deviations from random mating based either on mating of like tolike or on mating of unlikes. Both assortative and disassortative mating are nonrandommating systems.

Nonrandom mating: A mating system in which the frequencies of the variouskinds of matings with respect to some trait or traits are different from thoseexpected according to chance

Nonrecurrent apomixis: Refers to occasional apomixis, usually caused byhaploid parthenogenesis

Nonsense Codon. A codon for which no formal tRNA molecule exists, and the presenceof which causes termination of translation of genetic message in the form of ending ofthe polypeptide chain. Three such codons are UAG, UGA and UAA. Though thesebear the sense of termination of polypeptide chain, this misnomer still exists. (The useof the term such as stop codon instead of calling it nonsense one seems to be the mostappropriate).

Nonsense codon: (also called STOP codon) Any one of three triplets (UAG, UAA, UGA) that cause termination of protein sysnthesis.

Nonsense Mutation. A mutation that alters a gene in such a way that it produces anonsense codon in place of a “sense” one, for example, UGG (tryptophan) to UGA(stop codon).

Non-sense mutation: A mutation that alters a gene so that a non-sense codon isinserted; such a codon is one for which no normal tRNA molecule exists, thereforeit does not code for an amino acid; usually non-sense codons cause the terminationof translation; several non-sense codons are recognised (eg. amber, ochre, opal)

Nonsense Suppressor. A mutation producing an altered tRNA so that an amino acid isinserted in translation in response to a stop codon.

Nonsister Chromatids. Chromatids derived from separate homologous chromosomes orfrom non-homologous chromosomes of the chromosome complement. See sisterchromatids.

Norin 10. A Japanese semi dwarf variety of bread wheat discovered in 1946. It carriedtwo recessive genes for dwarfness, designated as Rht1 and Rht2 on 4A and 4Dchromosomes, respectively. These genes facilitate greater mobilization of dry mattertowards economic yield. They also do not produce negative correlation betweenbiological yield and harvest index. The exploitation of these genes has providedcorrelated response to a host of characters related to yield.

Norm of Reaction. The pattern of phenotypes produced by a given genotype underdifferent environmental conditions.

Normalising selection: The removal of genes and/or alleles that producedeviations from the normal phenotype of a population

Normalized difference vegetative indexThe ratio of difference between reflectance in the NIR (at 800 nm) and red (at 680 nm) regions to the total of the two.

North Carolina Designs (Comstock and Robinson 1948,’52). Mating designs developedto overcome problems of limited progeny relationships of biparental mating design,and to provide estimates for both additive and dominance variances. In all three suchdesigns (NCD I, II, and III), both full and half-sib progenies are produced byattempting biparental matings in F2 generation of a cross between two pure lines.Both NCD I and NCD II are influenced by the presence of maternal effects, anddepend on the assumption of gene frequency. NCD III is the most informative designas it also reflects some light about the effect of linkage on the average degree ofdominance. It is also economical, as it requires much less data.

Northern Blot. Transfer of electrophoretically separated RNA molecules from a gel ontoan absorbent sheet, which is then emersed in a labelled probe that will bind to theRNA of interest.

Northern blotting: A procedure analogous to Southern blotting, buttransferring RNA, instead of DNA, from a gel to a carrier (like nitrocellulose) andthen hybridizing to a radioactively labelled DNA probe complementary to thedesired sequence, and visualised by autoradiography; the technique can thereforebe used to locate and identify an RNA fragment containing a specific sequencegene

NoveltyA variety should not have been commercially exploited for more than one year before the grant of PBR protection.

Nucellar Embryony. A form of apomixis in which the embryo arises directly from thenucellus.

Nucellus. A parenchymatous tissue that extends from the chalaza at the base to themicropyle at the apex of the embryo sac.

Nuclear Division. The division of the cell nucleus that may follow different patterns suchas amitosis, mitosis, or meiosis.

Nuclear Instability. Any deviation from normal nuclear behaviour producing a nucleusof abnormal structure, karyotype or behaviour. It may be attributed to higher DNAcontent and large telomeric segments of heterochromatin. It affects seed fertility andendosperm development. It may lead to bridge formation at anaphase and shrivelledgrains.

Nuclear Membrane. The outer boundary of the nucleus. Also called nuclear envelope.

Nuclear Segregation. The segregation of genetically different nuclei contained in aheterokaryotic multikaryon. It is one of the important sources of variation in asexualpathogenic fungi.

Nuclease. An enzyme that can degrade DNA/RNA by breaking its phosphodiester bonds.Nucleases may attack a polynucleotide either from an end (exonucleases), or atrandom (endonucleases).

Nucleic Acid. A polymer of nucleotides (deoxyribonucleotides in the DNA orribonucleotides in the RNA). Nucleic acid (DNA, and in some cases RNA) acts as thegenetic material in living organisms. The presence of one free acid group in eachphosphoric acid molecule confers on the polynucleotide its acid properties andenables the molecule to form ionic bonds with basic proteins (histones). The free acidgroup also causes nucleic acids to be highly basophilic.

Nucleic acid: A large molecule composed of nucleotide subunits.

Nucleoid. A DNA mass within a chloroplast, mitochondrion, bacterium or a prokaryoticcell. It has variable shape but uniform texture. It lacks a membrane (nuclear envelope)and does not undergo processes comparable to mitosis and meiosis of eukaryotes.

Nucleolar Organiser. A region of the chromosome physically associated with thenucleolus and containing rRNA genes.

Nucleolus. An organelle found in the nucleus, containing rRNA and amplified multiplecopies of the genes encoding rRNA.163

Nucleoside. A nitrogen base bound to a sugar molecule. A phosphorylated nucleoside iscalled nucleotide.

Nucleosome. The basic unit of eukaryotic chromosome structure. It is a “ball” of eighthistone molecules wrapped about by two coils of DNA. It is an octamer of two unitseach of histones H2A, H2B, H3 and H4; the DNA is wrapped twice around theoctamer (of four histones). The additional histone, H1, (fifth one) is running down thecentre of the coil (of DNA), probably acting as a stabiliser. It is worth-mentioning thatH1 is not the part of the nucleosome.

Nucleotide Pair. A pair of nucleotides (one in each strand of DNA) that are joined byhydrogen bonds.

Nucleotide. A molecule composed of a nitrogen base, a sugar and a phosphate group; aphosphorylated nucleoside. It is the basic building block of nucleic acids.

Nucleotide: A subunit of DNA or RNA consisting of a nitrogenous base(adenine, guanine, thymine, or cytosine in DNA; adenine, guanine, uracil, orcytosine in RNA), a phosphate molecule, and a sugar molecule (deoxyribose inDNA and ribose in RNA). Thousands of nucleotides are linked to form a DNA orRNA molecule.

Nucleotide: A subunit of DNA or RNA consisting of anitrogenous base (adenine, guanine, thymine, or cytosine inDNA).

Nucleus: Membrane-bound structure in the cell that containsthe chromosomes (genetic material). The nucleus divideswhenever the cell divides.

Nucleus: The cellular organelle in eukaryotes that contains the genetic material.

Null Allele. An allele whose effect is either an absence of normal gene product at themolecular level or an absence of normal function at the phenotypic level.

Null allele: An allele that makes no gene product or whose product has noactivity of any kind; a deletion of a gene is necessarily a null allele.

Null alleleThe specific primers for an SSR locus consistently fail to amplify a detectable product.

Null Hypothesis. A hypothesis that there is no discrepancy between observation andexpectation based on some set of postulates.

Null hypothesis: A hypothesis that there is no discrepancy between observationand expectation

Null Mutation. A mutation that results in complete absence of function for the gene.

Null mutation: A mutation, which eliminates all enzymatic activity; usuallydeletion mutations

Nullihaploid: A cell or individual that possesses a haploid chromosome setplus a missing single

Nulliplex. The condition in which a polyploid is recessive in all chromosomes withrespect to a particular gene (e.g., aaaa is called a nulliplex tetraploid). Simplexdenotes recessiveness at all loci except one (Aaaa), duplex two (AAaa), triplex three(AAAa), quadriplex four (AAAA), etc.

Nullisome: A plant lacking both members of one specific pair of chromosomes

Nullisomic analysis: In nullisomic analysis, observations are made forphenotypic or other differences between the nullisomic for each chromosomeand the disomic condition within the same variety; the method is applied forlocalisation of genes within a given genome; the method can only be used inpolyploids, while diploids commonly do not tolerate the loss of both homologouschromosomes

Nullisomic. An otherwise 2n plant that lacks both members of one specific pair ofchromosomes, hence, with 2n–2 chromosomes. The missing ones are callednullisomes.

Nullisomy. A condition of having individuals with 2n-2 chromosomes. Such individualsare referred to as nullisomics. Nullisomy is normally a lethal condition except in a fewpolyploid species. In crops like modern wheat, nullisomics produce gametes with n-1chromosomes. Since male gametes with n-1 chromosomes frequently tend to be nonfunctional,the degree of sterility associated with nullisomics is very high.

Nullitetrasomic: A cell or individual, usually an allopolyploid, that lacks onepair of chromosomes, which is partially compensated for by a tetrasomic (fourfolddosage) of another, usually homoeologous, chromosome; a whole series ofnulli-tetrasomics has been produced in hexaploid wheat, and successfully used innumerous genetic and molecular studies

Numeric aberration: The variation of the number of genomes or chromosomes, for example, ploidy variation, aneuploids (nullisomics, monosomics, trisomics,tetrasomics, etc.), substitutions, or additions

Nutritional Value. The value of a food as judged by its taste, calorific value, quantityand quality of proteins, presence of vitamins and minerals, absence of antinutritionalfactors, etc.

Nutritious Diet. A diet that promotes health and prevents disease through supply of allthe essential nutrients, minerals and vitamins. Variations in nutritive content of fooddepends on soil nutrients and cultivars.