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PLANT BREEDING AND GENETICS




  • PLANT BREEDING AND

    GENETICS

    DEFINITIONS

    Acrocentric chromosome: A chromosome whose Centromere lies very near one end.

    Active site: The part of an enzyme where actual enzymatic function is performed.

    Allele: Alternative form of a gene.

    Allelic exclusion: A process whereby only one immunoglobulin light chain and one heavy chain gene are transcribed in any one cell; other genes are repressed.

     

    Allo polyploidy: Polyploidy produced by the hybridization of two different species.

     

    PLANT BREEDING AND

    GENETICS

    DEFINITIONS

    Acrocentric chromosome: A chromosome whose Centromere lies very near one end.

    Active site: The part of an enzyme where actual enzymatic function is performed.

    Allele: Alternative form of a gene.

    Allelic exclusion: A process whereby only one immunoglobulin light chain and one heavy chain gene are transcribed in any one cell; other genes are repressed.

    Allo polyploidy: Polyploidy produced by the hybridization of two different species.

    Anaphase: The stage of mitosis and meiosis in which sister chromatids or homologous chromosomes are separated by spindle fibers from centromeres.

    Aneuploids: Individuals or cells exhibiting aneuploidy.

    Aneuploidy: The condition of the cell or of an organism that has additions or deletions of whole chromosomes from the expected, balanced number of chromosomes.

    Angiosperms: Plants whose seeds are enclosed in an ovary. Flowering plants.

    Antibody: A foreign substance capable of triggering an immune response in an organism.

    Ascospores: Haploid spores found in the asci of Ascomycete fungi.

    Autogamy: Nuclear reorganization in a single Paramecium cell similar to the changes that occur during conjugation.

    Auto polyploidy: Polyploidy in which all chromosomes come from same species.

    Autotrophs: Organisms that can utilize carbon dioxide as a carbon source.

    Back mutation: The process that causes reversion of mutation. A change in a nucleotide pair in a mutant gene that restores original sequence and hence the original phenotype.

    Backcross: The cross of an individual with one of its parents or an organism with the same genotype as a parent.

    Binary fission: Simple cell division in single-celled organisms.

    Blastopore: The embryonic opening of the future gut.

    Blastula: The first developmental stage of a developing embryo.

    Catalyst: A substance that increases the rate of a chemical reaction without itself being permanently changed.

    Centrioles: Cylindrical organelles, found in eukaryotes (except in higher plants), that organize the formation of the spindle.

    Centromeres: Primary constrictions in eukaryotic chromosomes in which the kinetochores lie.

    Chiasmata: X-shaped configurations seen in tetrads during the latter stages of prophase I of meiosis. They represent physical crossovers. (Singular; chiasma.)

    Chi-square distribution: The sampling distribution of the chi-square statistic. A family of curves whose shapes depend on degree of freedom.

    Chloroplast: The organelle that carries out photosynthesis.

    Chromatids: The subunits of a chromosome prior to anaphase of mitosis. At anaphase of meiosis II or mitosis, when the centromeres divide and the sister chromatids separate, each chromatid is then a chromosome.

    Chromatin: The nucleoprotein material of the eukaryotic chromosome.

    Chromomeres: Dark regions of chromatin condensation in eukaryotic chromosomes at meiosis or mitosis.

    Chromosome: The form of the genetic material in viruses and cells. A circle of DNA in prokaryotes; a DNA or an RNA molecule in viruses; a linear nucleoprotein complex in eukaryotes.

    Clone: A group of cells arising from a single ancestor.

    Coding strand: The DNA strand with the same sequence as the transcribed mRNA (given U in RNA and T in DNA).

    Complementary DNA (cDNA): DNA synthesized by reverse transcriptase using RNA as a template.

    Coupling: Allele arrangement in which mutants are on the same chromosome and wild-type alleles on the homologue.

    Covariance: A statistical value measuring the simultancous deviations of  variables from their means.

    Criss-cross pattern of inheritance: The phenotypic pattern of inheritance controlled by {-linked recessive alleles.

    Crossbreed: Fertilization between separate individuals..

    Crossing over: A process in which homologous chromosomes exchange parts by a breakage- and-reunion process.

    Cytokinesis: The division of the cytoplasm of a cell into two daughter cells.

    Deletion chromosome: A chromosome with part deleted.

    Denatured: Loss of natural configuration (of a molecule) through heat or other treatment. Denatured DNA is single-stranded.

    Diakinesis: The final stage of prophase I of meiosis when chiasmata terminalize.

    Dihybrid: An organism heterozygous at two loci or for two pairs of allelsss.

    Diploid: The state of having each chromosome in two copies per nucleus or cell.

    Discontinuous variation: Variation that falls into discrete categories (e.g., the color of garden peas).

    DNA-RNA hybridization: When a mixture of DNA and RNA is heated and then cooled, RNA can hybridize (form a double helix) with DNA that has a complementary nucleotide sequence.

    Dominant: An allele that expresses itself even when heterozygous. Also the trait controlled by that allele.

    Double helix: The normal structural configuration of DNA consisting of two helices rotating about the same axis.

    Dyad: Two sister chromatids attached to the same centromere.

    Enhancer: A eukaryotic DNA sequence that increases transcription of a region even if the enhancer is distant from the region being transcribed.

    Euploidy: The condition of a cell or organism that has one or more complete sets of chromosomes.

    Evolution: In Darwinian terms, a gradual change in phenotypic frequencies in a population that results in individuals with improved reproductive success.

    Exon: A region of a gene that has intervening sequences (introns) that is actually translated or expressed.

    Frequency-dependent selection: A selection whereby a genotype is at an advantage when rare and at a disadvantage when common.

    Functional alleles: Mutants that fail to complement each other in a cis-trans complementation test.

    Gametophyte: The haploid stage of a plant life cycle that produces gametes (by meiosis). It alternates with a diploid, sporophyte generation.

    Gene cloning: Production of large number of a piece of DNA after that piece of DNA is inserted into a vector and taken up by a cell. Cloning occurs as the vector replicates.

    Gene family: A group of genes that has arisen by duplication of an ancestral gene. The genes in the family may or may not have diverged from each other.

    Gene pool: All of the alleles available among the reproductive members of a population from which gametes can be drawn.

    Genetic code: The linear sequences of nucleotides that specify the amino acids during the process of translation at the ribosome.

    Genetic engineering: Popular term for recombinant DNA technology..

    Genome: The entire genetic complement of a prokaryote or virus or the haploid genetic complement of a eukaryotic species.

    Genotype: The genes that an organism possesses.

    Germ-line theory: A theory to account for the high degree of antibody variability found. The germ-line theory suggests that every B lymphocyte has all genes for every type of immunoglobin but transcribes only one.

    Guide RNA (gRNA) RNA that guides the insertion of uridines (RNA editing) into mRNAs in trypanosomes. found in transcripts from minicircles and maxicircles of DNA in kinetoplasts.

    Haplodiploidy: The sex-determining mechanism found in some insect groups among which males are haploid and females are diploid.

    Heritability: A measure of the degree to which the variance in the distribution of a phenotype is due to genetic causes. In the broad sense it is measured by the total genetic variance divided by the total phenotypic variance. In the narrow sense it is measured by the genetic variance due to additive genes divided by the total phenotypic variance.

    Hermaphrodite: An individual with both male and female genitalia.

    Heterogametic: The sex with heteromorphic sex chromosomes: during meiosis, it produces different kinds of gametes in regard to these sex chromosomes.

    Heteromorphic chromosomes: Chromosomes of which during meiosis, it produces different kinds of gametes in regard to these sex chromosomes.

    Heteromorphic chromosomes: Chromosomes of which the members of a homologous pair are not morphologically identical (e.g., the sex chromosomes).

    Heterozygote: A diploid or polyploid with different alleles at a particular locus.

    Homogametic: The sex with homomorphic sex chromosomes; it produces only one kind of gamete in regard to the sex chromosomes.

    Homothallic: A botanical term used for groups whose individuals are not of different sexes.

    Hormones: Chemicals that are secreted by one type of cell and act on a second type of cell.

    Hybrid DNA DNA whose two strands have different origins.

    Hybrid: Offspring of unlike parents.

    Immunity: The ability of an organism to resist infection. .

    In vitro: Biological or chemical work done in the test tube (literally. “in glass”) rather than in living systems.

    Incomplete dominance: The situation in which both alleles of the heterozygote influence the phenotype. The phenotype is usually intermediate between the two homozygous forms.

    Induction: Regarding temperate phages, the process of causing a prophage to become virulent.

    Interkinesis: The abbreviated interphase that occurs between meiosis I and II. No. DNA replication occurs here.

    Intragenic suppression: A second change within a mutant gene that results in an apparent restoration of the original phenotype.

    Inversion: The replacement of a section of chromosome in reverse orientation.

    Isochromosome: A chromosome with two genetically and morphologically identical arms.

    Isozymes: Different electrophoretic forms of the same enzyme. Unlike allozymes, isozymes are due to differing subunit configurations rather than allelic differences.

    Leptonema (leptotene stage): The first stage of prophase I of meiosis in which chromosomes become distinct.

    Level of significance: The probability value in statistics used to reject the null hypothesis.

    Locus: The position of a gene on a chromosome. (Plural: loci.)

    Lysis: The breaking open of a cell by the destruction of its wall or membrane.

    Mapping of gene: The study of the position of genes on chromosomes.

    Marker: A locus or allele whose phenotype provides information about a chromosome or chromosomal segment during genetic analysis.

    Meiosis: The nuclear process in diploid eukaryotes that results in gametes or spores with only one member of each original homologous pair of chromosomes per nucleus.

    Metacentric chromosome: chromosome whose centromere is located in middle.

    Metaphase: The stage of mitosis-meiosis in which spindle fibers are attached to kinetochores and the chromosomes are positioned in the equatorial plane of cell.

    Metastasis: The migration of cancerous cells to other parts of the body.

    Missense mutation: Mutations that change a codon for one amino acid into a codon for a different amino acid.

    Mitochondrion: The eukaryotic cellular organelle in which the Krebs cycle and electron transport reactions take place.

    Mitosis: The nuclear division producing two daughter nuclei identical to the original nucleus.

    Molecular imprinting: The phenomenon in which there is differential expression of a gene depending on whether it was maternally or paternally inherited.

    Monohybrids: Offspring of parents that differ in only one genetic characteristic. Usually implies heterozygosity at a single locus under study.

    Monosomic: A diploid cell missing a single chromosome.

    Mosaics: Individuals made up of two or more cell lines.

    mRNA: Messenger RNA; the basic function of the nucleotide sequence of mRNA is to determine the amino acids sequence in proteins.

    Multihybrid: An organism heterozygous at  numerous loci.

    Mutability: The ability to change.

    Mutants: The end result of mutation.

    Mutation rate: The proportion of mutants per cell division in bacteria or single-celled organisms or the proportion of mutations per gamete in higher organisms.

    Neoplasm: New growth of abnormal tissue.

    Neutral gene hypothesis: The hypothesis that most genetic variation in natural populations is not maintained by selection.

    Nondisjunction: The failure of a pair of homologous chromosomes to separate properly during meiosis.

    Nonhistone proteins: The proteins remaining chromatin after the histones are removed. The scaffold structure is made of nonhistone proteins.

    Nonrecormbinants: In mapping studies, offspring that have alleles arranged as in the original parents.

    Nuclear transplantation: The technique of placing a nucleus from another source into an enucleated cell.

    Nuclease: One of the several classes of enzymes that degrade nucleic acid.

    nucleoprotein: The globular, nuclear organelle formed at the nucleolar organizer. Site of ribosome construction.

    Nucleosomes: Arrangements of DNA and histones forming regular spherical structures in eukaryotic chromatin.

    Nullisomic: A diploid cell missing both copies of the same chromosome.

    One-gene-enzyme hypothesis: Hypothesis of Beadle and Tatum that one gene controls the production of one enzyme. Later modified to the concept that one ciston controls the production of one polypeptide.

    Oogenesis: The process of ovum formation in female animals.

    Oogonia: Cells in females that produce primary oocytes by mitosis.

    Outbreeding: The mating of genetically unrelated individuals.

    Outside markers: Loci on either side of another locus or specified region.

    Ovum Egg: The one functional product of each meiosis in female animals.

    Pachynema (pachytene stage): The stage of prophase I of meiosis in which chromatids are first distinctly visible.

    Parameters: Measurements of attributes of a population; denoted by Greek letters.

    Parthenogenesis: The development of an individual from an unfertilized egg that did not arise by meiotic chromosome reduction. .

    Passenger DNA: Foreign DNA incorporated into a plasmid.

    Pedigree: A representation of ancestry of an individual or family; a family tree.

    Phenocopy: A phenotype that is not genetically controlled but looks like a genetically controlled phenotype.

    Phenotype: The observable attributes of an organism.

    Pheromone: A chemical signal, analogous to a hormone that passes information between individuals.

    Plasmid: An autonomous, self-replicating genetic particle, usually of double-stranded DNA.

    Plastid: A chloroplast prior to the development of chlorophyll.

    Polar bodies: The small cells (which eventually disintegrate) that are the by-products of meiosis in female animals. One functional ovum and potentially three polar bodies result from meiosis of each primary oocyte.

    Polarity: Meaning “directionality” and referring either to an effect seen in only one direction from a point of origin or to the fact that linear entities (such as a single strand of DNA) have ends that differ from each other.

    Pollen grain: The male gametophyte in higher plants.

    Polynucleotide phosphorylase: An enzyme that can polymerize diphosphate nucleotides without the need for a primer. The function of this enzyme in vivo is probably in its reverse role as an RNA exonuclease.

    Polyploids: Organisms with greater than two chromosome sets.

    Polysome: The configuration of several ribosomes simultaneously translating the same mRNA.

    Population: A group of organisms of the same species relatively isolated from other groups of the same species..

    Primary structure: The sequence of polymerized amino in male animals.

    Primase: An enzyme that creates an mRNA primer for Okazaki fragment initiation.

    Probability: The expectation of the occurrence of a particular event.

    Progeny testing: Breeding of offspring to determine their genotypes and that of their parents.

    Prokaryotes: Organisms that lack true nuclei.

    Prophase: The initial stage of mitosis or meiosis in which chromosomes become visible and the spindle apparatus forms.

    Pseudoallels: Alleles that are functionally but not structurally allelic. Within gene families, pseudoalleles are alleles that are not expressed.

    Quantitative inheritance: The mechanism of genetic control of traits showing continuous variation.

    Quaternary structure: The association of polypeptide subunits to form the final structure of a protein.

    RAM mutants: Ribosomal ambiguity mutants that allow incorrect tRNAs to be incorporated into the translation process.

    Random mating: The mating of individuals in a population such that the union of individuals with the trait under study occurs according to the product rule of probability.

    Recessive: An allele that does not express itself in the heterozygous condition.

    Reciprocal cross: A cross with the phenotype of each sex reversed as compared with the original cross. Made to test the role of parental sex on inheritance pattern.

    Recombinant DNA technology: Techniques of gene cloning. Recombinant DNA refers to the hybrid of foreign and vector DNA..

    Recombination: The nonparental arrangement of alleles in progeny that can result from either independent assortment or crossing over.

    Regulator gene: A gene primarily involved in control of the production of another gene’s product.

    Relaxed mutant: A mutant that does not exhibit the stringent response under amino acid starvation.

    Repetitive DNA: DNA  made up of copies of the same nucleotide sequence.

    Replication: The process of copying.

    Repulsion: Allelic arrangement in which each homologous chromosome has mutant and wild-type alleles.

    Ribosomes: Organelles at which translation takes place. They are made up of two subunits consisting of RNA and proteins. .

    RNA editing: The insertion of uridines into mRNAs after transcription is completed; controlled by guide RNA. May also involve insertion of cytidines in some organisms, or possible deletions of bases.

    r RNA: Ribosomal RNA. RNA components of the subunits of the ribosomes.

    Satellite DNA: Highly repetitive eukaryotic DNA primarily located around centromeres. Satellite DNA usually has a different buoyant density than the rest of the cell’s DNA.

    Scaffold: The eukaryotic chromosome structure remaining when DNA and histones have been removed; made from nonhistone proteins.

    Secondary Oocytes: The cells formed by meiosis I in female animals.

    Self-fertilization: Fertilization in which the two gametes are from the same individual.

    Selfish DNA: A segment of the genome with no apparent function although it can control its own copy number.

    Semisterility: Non availability of a proportion of gametes or zygotes.

    Sex chromosomes: Heteromorphic chromosomes whose distribution in a zygote determines the sex of the organism.

    Sex-controlled traits: Traits that appear more often in one sex than in another.

    Sex-determining region Y (SRY): The sex switch, or testis-determining factor, in human beings, located on the Y chromosome. (Sry in mice.).

    Sex-limited traits: Traits expressed in only one sex. They may be controlled by sex-linked or autosomal loci.

    Species: A group of organisms belong to the same species if they are capable of interbreeding to produce fertile offspring.

    Spindle: The microtubule apparatus that controls chromosome movement during mitosis and meiosis.

    Sporophyte: The stage of a plant life cycle that produces spores by meiosis and alternates with the gametophyte stage.

    Structural alleles: Mutant alleles that are altered at identical base pairs..

    Suppressor gene: A gene that, when mutated, apparently restores the wild-type phenotype to a mutant of another locus.

    Svedberg unit; A unit of sedimentation during centrifugation. Abbreviation is S. as in 50S.

    Synapsis: The point-by-point pairing of homologous  chromosomes during zygotene or in certain dipteran tissues that undergo endomitosis.

    Telomerase: An enzyme that adds telomeric sequences to the ends of eukaryotic chromosomes. NO template is necessary.

    Telomere: The ends of linear chromosomes that are required for replication and stability.

    Telophase: The terminal stage of mitosis or meiosis in which chromosomes uncoil, the spindle breaks down, and cytokinesis usually occurs.

    Temperature-sensitive mutant: An organism with an allele that is normal at a permissive temperature but mutant at a restrictive temperature.

    Test cross: The cross of an organism with a homozygous recessive organism.

    Tetrads: The meiotic configuration of four chromatids first seen in pachytene. There is one tetrad (bivalent) per  homologous pair of chromosomes.

    Tetraploids: Organisms with four whole sets of chromosomes.

    Topoisomers: Forms of DNA with the same sequence but differing in their linkage number (coiling).

    Totipotent: The state of a cell that can give rise to any and all adult cell types, as compared with a differentiated cell whose fate is determined.

    Transcription: The process whereby RNA is synthesized from a DNA template.

    Transduction: A process whereby a cell can gain access to and incorporate foreign DNA brought in by a viral particle.

    Transformation: A process whereby prokaryotes take up DNA from the environment and incorporate it into their genomes, or the conversion of a eukaryotic cell into a cancerous one.

    Transgenic: Eukaryotic organisms that have taken up foreign DNA.

    Transition mutation: A mutation in which a purine/pyrimidine base pair is replaced with a base pair in the same purine/pyrimidine relationship.

    Translation: The process of protein synthesis wherein the primary structure of the protein is determined by the nucleotide sequence in mRNA.

    Translocation: A chromosomal configuration in which part of a chromosome becomes attached to a different chromosome. Also a part of the translation process in which the mRNA is shifted one codon in relation to the ribosome.

    Triplodis: Organism with three whole sets of chromosomes.

    Trisomic: A diploid cell with an extra chromosome.

    t RNA: Transfer RNA. Small RNA molecules that carry amino acids to the ribosome for polymerization.

    Tumor suppressor genes: Genes that normally control unlimited cellular growth. When both copies of the gene are mutated, cellular transformation follows. Examples are the p53 gene and the genes for retinoblastoma and Wilm’s tumor.

    Tumor: Abnormal growth of tissue.

    Unequal crossing over: Non reciprocal crossing over caused by mismatching of homologous chromosomes. Usually occurs in regions of tandem repeats.

    Unique DNA: A length of DNA with no repetitive nucleotide sequences.

    Variegation: Patchiness; a type of position effect that results when particular loci are contiguous with heterochromatin.

    Wild-type: the phenotype of a particular organism when first seen in nature.

    X-inactivation center (XIC): Locus on the X chromosome in mammals at which inactivation is initiated.

    Y-junction: The point of active DNA replication where the double helix opens up so that each strand can serve as a template.

    Z DNA: A left-handed form of DNA found under physiological conditions in short GC segments that are methylated. It may be important in regulating gene expression in eukaryotes.

    Plant breeding and genetics

    Earnst Haeckle (1870) first defined Ecology as” study of natural environment and of the relations of organism to each other and to their surrounding. In brief ecology is th3e scientific study of interactions between living organism and their environment.

    Population

    Group of individual of a species / sort in a given ecosystem is referred t as population.

    Population density

    Number of individuals of a species in a given area is referred to as population density.

    Biomass

    When the individual in a population have different sizes or distributions a better estimate would be’ total mass of the individuals present per unit area.”

    Ecosystem

    Biotic communities considered together with physical environment constitute what is called as ecosystem.

    Climate

    An average condition of solar radiation and precipitation etc of a region prevailing throughout the year. The average condition is usually based on information collected over a period of years.

    Biomes

    The natural ecological groupings of animals and plants on the basis of climate are called biomes e.g. desert, coniferrrous, and forests. Therefore, the entire ecosystem taken together in the given geographical area having the same type of climate is called a biome.

    Species

    Species is defined as a group of actually or potentially interbreeding population that is reproductively isolated from all other kinds of organism.

    Gene pool

    Each individual carries a certain genetic information carried by all individuals of an interbreeding population is the gene pool.

    Genetic recombination

    When members of a population interbreed, gametes combine to form new combination of genes(genetic recombination).

    Community

    The various populations or organism in an area taken together form a community or biotic community.

    Pollution

    Pollution in general term applies to the presence of undesirable substances in the environment.

    Crop plant

    The term crop plant in broadcast sense means any plant utilized by people for any purpose. However, the restricted but common sense of the term crop plant includes those useful plants that fit economically into scheme of our work and existence.

    Morphology

    The study of the arrangement of various tissues into systems and structures to produce the growing plant is called morphology.

    Pollination

    Pollination is the transfer of pollen grains from the anther to the stigma. There are two types of pollination. In self-pollination the transfer of pf pollens from the anther to the stigma of the same flower. in cross pollination the transfer of pollen from an anther to stigma of of a different flower.

    Fertilization

    Union of egg and a sperm to form zygote is known as fertilization. Self-fertilization is the union of an egg with a sperm of the same flower or the same flower of the same plant. In cross-fertilization the union of an egg wit a sperm from a plant of different clone.

    Clone

    A group of plants, which is propagated vegetatively from a single plant, is called clone.

    Apomixes

    Apomixes is a type of reproduction in which sexual organs or related structures take part but in which seed s are formed without union of gamete. In apodictic spps, sexual reproduction is either suppressed or absent.

    Parthenogenesis

    It means the development of an embryo directly from the egg cells.

    Inflorescence

    The way in which groups of flowers are arranged on the plant is termed as inflorescence.

    Double fertilization

    The pollen – tube pierces through the style of he gynaeceum and enters the ovule through the micropylar end. The pollen tune ruptures and two male gametes are released one fertilizes the egg cell to form 2 n embryo or the zygote the second male gamete fuses with the  endospermic nucleus

    Genetics

    Te term genetics was coined by Willam Bateson in 1905. it was derived from genesis meaning  to generate or creation of something. Genetics may define a “the science of heredity and variation which deals with resemblances and differences among individuals related by descent”.

    Heredity

    Heredity means similarity which progeny show to its parents and provides for the organic continuity between generations. Heredity therefore is the genetic continuity between the parents and offspring’s and thus preserves race by developing progeny in parental image.

    Variation

    The deviation from heredity is called variation.

    Mendel’s law of segregation

    It states that heredity characters are determined by some particulate factor, these factors occur in pairs and at the time of gamete formation, these factors segregate at random so that only one miner of a pair is transmitted to a particular gamete.

    Mendel’s law of independent assortment

    The production of these four type of gametes in equal number is possible only if the two heterozygous gene pairs, Tt and As assort independent of each other. This phenomenon is called “law of independent assortment”.

    Phenotype

    Outward expression of a character or physical appearance of an individual is called “phenotype”.

    Genotype

    Complex of genetic factors controlling this expression is called “genotype”.

    Allele

    The alternative form of a gene is called allele.

    Back cross

    When F1hybrid is crossed with one of its parents, it is called back cross.

    Test cross

    When F1 hybrid is crossed with the recessive parent, it is called a test     cross.

    Phenocopy

    Sometimes it is possible to bring about non-hereditary phenotypic changes     by changing the environment. These changes may apparently similar to      those caused by the genes as a result of mutation.

    Major genes

    Genes control the development of the major traits such as development of       brain or any other major human character are more important for a      normal function and are regarded as major genes.

    Minor genes

    In the other hand, which controls the less important traits, e.g. production of hair a second segment of fingers are called minor genes.

    Lethal genes

    The genes, which exert a drastic effect on the development of the organism as to cause4 any major abnormality or death of the individual, are “lethal genes”.

    Isoalleles

    Such alleles are called isolalleles because they are alive in their homozygous effect and differences appear only in special combinations.

    Pleiotropy

    Sometime a gene may have more than one affect i.e. it influences more than one character simultaneously. This is called pleiotrop. In man skin color is due to the presence of melanin.

    Linkage

    The tendency of genes present on t the same chromosome, to enter the gametes in parental combination.

    Crossing over

    The tendency of the genes to enter the gametes in combinations other than parental.

    Autosomes

    The chromosome other than sex chromosome is called as autosomes.

    Heredity variation

    Heredity variations are variations in size shape, color form or development in a genetically mixed population that result from heritable causes and are transmitted to the kprogeny. Heredity ratios may be simple and easy observe.

    Heteroploidy or polyplllloidy

    Change in chromosome number is known as heteroploidy. It is divided into two parts euploidy and aneuploidy.

    Translocation

    When tow non-homologous chromosomes exchange their corresponding portion they are said to have undergo translocation the translocation will be reciprocal.

    Mutation

    Mutation is derived from a Latin word mature i.e. to change. Thus mutation is defined as sudden heritable change in the organism other than mendelian recombination segregation or mendelian inheritance”.

    Mutangenesis

    The process of inducting mutation is mutagenesis.

    Plant breeding

    An art and science of changing and improving the genetic architecture of crop yield in relation to its economic yield.

    Variety

    A group of similar plants that differ by structural features and performance from other plants within the same species.

    Hybridization

    It means to combine desired characters from two different parents into a single variety or individual through crossing.

    Single cross

     A cross between two inbred lines to reduce single cross hybrid called single cross. A x B          AB.

    Double cross

     A cross between two single cross hybrid to produce double cross hybrid called double cross e.g. Ab x CD                ABCD

    Synthetic variety

    In maize it refers to the open pollinate increase from a multiple hybrid. It can also be defined as advanced generations of open pollinated seed mixtures of a group of strains, clones or inbred or of hybrids among them synthetic variety is formed from inbred line synthetic variety is formed from inbred line on the basis of general or specific combining ability.

    Composite variety

    A mixture of genotypes from several sources maintained by normal pollination.

    Flower

    The flower is highly modified shoot responsible for the reproduction of seed and fruit.

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