Glossary of Terms from Allard, “Principles of Plant Breeding”
Adaptation: The process by which individuals (or parts of individuals), populations, or species change form or function in such a way to better survive under given environmental conditions. Also the result of this process.
Allele or Allelomorph: One of a pair or series of forms of a gene which are alternative in inheritance because they are situated at the same locus in homologous chromosomes.
Asynapsis: Failure of pairing of homologous chromosomes during meiosis.
Avirulent: Inability of a pathogen to produce a disease on its host.
Backcross: a cross of a hybrid to either of its parents. In genetics, a cross of a heterozygote to a homozygous recessive. (See test cross)
Backcross Breeding: A system of breeding whereby recurrent backcrosses are made to one of the parents of a hybrid, accompanied by selection for a specific character or characters.
Balance: The condition in which genetic components are adjusted in proportions that give satisfactory development. Balance applies to individuals and populations.
Basic Number: The number of chromosomes in ancestral diploid ancestors of polyploids, represented by x.
Biotype: A group of individuals with the same genotype. Biotypes may be homozygous or heterozygous.
Bivalent: A pair of homologous chromosomes united in the first meiotic division.
Breeder Seed: Seed produced by the agency sponsoring a variety and used to produce foundation seed.
Breeding: The art and science of changing plants or animals genetically.
Bulk Breeding: The growing of genetically diverse populations of self-pollinated crops in a bulk plot with or without mass selection, followed by single-plant selection.
Certified Seed: Seed used for commercial crop production produced from foundation, registered, or certified seed under regulation of a legally constituted agency.
Centromere: (See kinetochore)
Character: An attribute of an organism resulting from the interaction of a gene or genes with the environment.
Chiasma: An exchange of partners between paired chromatids in the first division of meiosis.
Chromatid: One of two threadlike structures formed by the longitudinal division of a chromosome during meiotic prophase and known as a daughter chromosome during anaphase.
Chromosomes: Structural units of the nucleus which carry the genes in linear order. Chromosomes undergo a typical cycle in which their morphology changes drastically in various phases of the life cycle of the organisms.
Clone: A group of organisms descended by mitosis from a common ancestor.
Combining Ability: General, average performance of a strain in a series of crosses. Specific deviation from performance predicted on the basis of the general combining ability.
Coupling: Linked recessive alleles occur in one homologous chromosome and their dominant alternatives occur in the other chromosome. Opposed to repulsion in which one dominant and one recessive occur in each member of the pair of homologous chromosomes.
Crossing Over: The exchange of corresponding segments between chromatids of homologous chromosomes during meiotic prophase. Its genetic consequence is the recombination of linked genes.
Diallel Cross, Complete: The crossing in all possible combinations of a series of genotypes.
Dihybrid: Heterozygous with respect to two genes.
Dioecious: Plants in which staminate and pistillate flowers occur on different individuals.
Diploid: An organism with two chromosomes of each kind.
Diplotene: The stage of meiosis which follows pachytene and during which the four chromatids of each bivalent move apart in two pairs but remain attached in the region of the chiasmata.
Disease: A departure from normal metabolism and a reduction of its normal potential for growth and reproduction.
Disjunction: The separation of chromosomes at anaphase.
Dominance: Intra-allelic interaction such that one allele manifests itself more or less, when heterozygous, than its alternative allele.
Donor Parent: The parent from which one or a few genes are transferred to the recurrent parent in backcross breeding.
Double Cross: A cross between two F1 hybrids.
Emasculation: Removal of the anthers from a flower.
Epistasis: Dominance of one gene over a non-allelic gene. The gene suppressed is said to be hypostatic. More generally, the term epistasis is used to describe all types of interallelic interaction whereby manifestation at any locus is affected by genetic phase at any or all loci.
Epiphytotic: An unarrested spread of a plant disease.
Expressivity: The degree of manifestation of a genetic character.
F1: The first generation of a cross.
F2: The second filial generation obtained by self-fertilization or crossing F1 individuals.
F3: Progeny obtained by self-fertilization of F2 individuals.
Factor: Same as gene.
Facultative: Parasites which can grow and live in environments other than living host tissue.
Family: A group of individuals directly related by descent from a common ancestor.
Fertility: Ability to produce viable offspring.
Fertilization: Fusion of the nuclei of male and female gametes.
Foundation Seed: Seed stock produced from breeder seed under the direct control of an agricultural experiment station. Foundation seed is the source of certified seed, either directly or through registered seed.
Gamete: Cell of meiotic origin specialized for fertilization.
Gene: The unit of inheritance. Genes are located at fixed loci in chromosomes and can exist in a series of alternative forms called alleles.
Gene Frequency: The proportion in which alternative alleles of a gene occur in a population.
Gene Interaction: Modification of gene action by a non-allelic gene or genes.
Germplasm: The sum total of the hereditary materials in a species.
Genome: A set of chromosomes corresponding to the haploid set of a species.
Genotype: The entire genetic constitution of an organism.
Haploid: A cell or organism with the gametic chromosome number (n).
Heritability: The proportion of observed variability which is due to heredity, the remainder
being due to environmental causes. More strictly, the proportion of observed variability due to the additive effects of genes.
Heterosis: Hybrid vigor such that an F1 hybrid falls outside the range of the parents with respect to some character or characters. Usually applied to size, rate of growth, or general thriftiness.
Heterozygous: Having unlike alleles at one or more corresponding loci (opposite of homozygous).
Homology of Chromosomes: Applied to whole chromosomes or parts of chromosomes which synapse or pair in meiotic prophase.
Host Resistance: The result of genetic manipulation of the host which renders it less susceptible to pathogens that would or do attack the host.
Hybrid: The product of a cross between genetically unlike parents.
I1, I2, I3… Symbols that are used to designate first, second, third, etc. inbred generations.
Inbred Line A line produced by continued inbreeding. In plant breeding, a nearly homozygous line usually originating by continued self-fertilization, accompanied by selection.
Inbreeding: The mating of individuals more closely related than individuals mating at random.
Independence: The relationship between variables when the variation of each is uninfluenced by that of others, that is, correlation of zero.
Isogenic Lines: Two or more lines differing from each other genetically at one locus only. Distinguished from clones, homozygous lines, identical twins, etc. which are identical at all loci.
Isolation: The separation of one group from another so that the mating between or among groups is prevented.
Kinetochore: Spindle attachment. A localized region in each chromosome to which the “spindle fiber” appears to be attached and which seems to determine movement of the chromosomes during mitosis and meiosis.
Line Breeding: A system of breeding in which a number of genotypes, which have been progeny tested in retrospect to some character or group of characters, are composited to form a variety.
Linkage: Association of characters in inheritance due to location of genes in proximity on the same chromosome.
Linkage Map: Map of position of genes in chromosomes determined by recombination relationships.
Linkage Value: Recombination fraction expressing the proportion of crossovers versus parental types in a progeny. The recombination fraction can vary from zero to one half.
Locus: The position occupied by a gene in a chromosome.
M1, M2, M3… Symbols used to designate first, second, third, etc. generations after treatment with a mutagenic agent.
Male Sterility: Absence or non-function of pollen in plants.
Mass-Pedigree Method: A system of breeding in which a population is propagated in mass until conditions favorable for selection to occur, after which pedigree selection is practiced.
Mass Selection: A form of a selection in which individual plants are selected and the next generation is propagated from the aggregate of their seeds.
Mating System: Any number of schemes by which individuals are assorted in pairs leading to sexual reproduction. Random; assortment of pairs is by chance. Genetic assortative mating; mating together of individuals more closely related than individuals mating at random. Genetic disassortative mating; mating together of individuals less closely related than individuals mating at random. Phenotypic assortative mating; mating individuals more alike in appearance than the average. Phenotypic disassortative mating; mating of individuals less alike in appearance than individuals mating at random.
Meiosis: A double mitosis occurring in sexual reproduction which results in production of gametes with haploid (n) chromosome number.
Metaphase: The stage of meiosis or mitosis at which the chromosomes lie on the spindle.
Mitosis: The process by which the nucleus is divided into two daughter nuclei with equivalent chromosome complements, usually accompanied by division of the cell containing the nucleus.
Modifying Genes: Genes that affect the expression of a non-allelic gene or genes.
Monoecious: Staminate and pistillate flowers born separately on the same plant.
Mutation: A sudden heritable variation in a gene or in a chromosome structure.
Obligate: Parasite that cannot multiply in nature without a host.
Oliogenic Resistance: Resistance determined by one or few genes whose effects are readily detectable.
Outcross: A cross, usually natural, to a plant of different genotype.
Pachytene: The double-thread or four strand stage of meiosis.
Parasite: Lives in or on another organism and obtains nutrients from it.
Parthenogenesis: Development of an organism from a sex cell in respect to some characteristic.
Parameter: A numerical quantity which specifies a population in respect to some characteristic.
Pathogen: A parasite which produces disease in its host.
Pedigree: A record of the ancestry of an individual, family, or strain.
Pedigree Breeding: A system of breeding in which individual plants are selected in the segregating generations from a cross on the basis of their desirability judged individually and on the basis of a pedigree record.
Penetrance: The frequency with which a gene produces a recognizable effect in the individuals which carry it.
Phenotype: Appearance of an individual as contrasted with its genetic make-up or genotype. Also, used to designate a group of individuals with similar appearance but not necessarily identical genotypes.
Phytolexins: Substances produced or formed by host plants in response to injury, physiological stimuli, infectious agents, or their products that accumulate to levels which inhibit the growth of microorganisms. Some include toxic substances produced to repel insects and nematodes.
Polycross: Open pollination of a group of genotypes (generally selected), in isolation from other compatible genotypes, in such a way as to promote random mating.
Polygenic: Determined by several genes whose effects are readily detectable.
Populations: In genetics, a community of individuals which share a common gene pool. In statistics, a hypothetical and infinitely large series of potential observations among which observations may actually constitute a sample.
Progeny Test: A test of the value of a genotype based on the performance of its offspring
produced in some definite system of mating.
Protandry: Maturation of anthers before pistils.
Protogyny: Maturation of pistils before anthers.
Pure Line: A strain homozygous at all loci, ordinarily obtained by successive self-fertilizations in plant breeding.
Qualitative Character: A character in which variation is discontinuous.
Quantitative Character: A character in which variation is continuous so that classification into discrete categories is not possible.
Random: Arrived at by chance without discrimination.
Randomization: Process of making assignments at random.
Recessive: The member of an allelic pair which is not expressed when the other (dominant) member occupies the homologous chromosome.
Reciprocal Crosses: Crosses in which the sources of the male and female gametes are reversed.
Recombination: Formation of new combinations of genes as a result of segregation in crosses between genetically different parents. Also, the rearrangement of linked genes due to crossing over.
Recurrent Parent: The parent to which successive backcrosses are made in backcross breeding.
Recurrent Selection: A method of breeding designed to concentrate favorable genes scattered among a number of individuals by selecting, each generation, among the progeny produced by matings of the selected individuals (or their selfed progeny) of the previous generation.
Registered Seed: The progeny of foundation seed normally grown to produce certified seed.
Rogue: A variation from the standard type of a variety or strain. Roguing; removal of undesirable individuals to purify a stock.
Resistance: The restriction of development of a pathenogenic agent or parasite. Can vary in degree from immunity (no development) to only slight retardation relative to a so called susceptible reaction.
S1, S2, S3… Symbols for designating first, second, third, etc. selfed generations from an ancestral plant (S0).
Segregation: Separation of paternal from maternal chromosomes at meiosis and consequent separation of genes leading to the possibility of recombination in the offspring.
Selection: In genetics, discrimination among individuals in the number of offspring contributed to the next generation. In statistics, discrimination in sampling leading to bias. Opposed to randomness.
Self-Fertilization: Fusion of male and female gametes from the same individual.
Self-Incompatibility: Genetically controlled physiological hindrance to self-fruitfulness.
Single Cross: A cross between two genotypes, usually two inbred lines, in plant breeding.
Species: The unit of taxonomic classification into which genera are subdivided. A group of similar individuals different from other similar arrays of individuals. In sexually reproducing organisms, the maximum interbred group isolated from other species by barriers of sterility or reproductive incapacity.
Strain: A group of similar individuals within a variety.
Synapsis: Conjugation at pachytene and zygotene of homologous chromosomes.
Synthetic Variety: A variety produced by crossing a number of genotypes selected for good combining ability in all possible hybrid combinations, with subsequent maintenance of the variety by open pollination.
Telophase: The last stage in cell division before the nucleus returns to a resting condition.
Tetraploid: An organism with four basic (x) sets of chromosomes.
Top Cross: A cross between a selection, line, clone, etc., and a common pollen parent which may be a variety, inbred line, single cross, etc. The common pollen parent is called the top cross or tester parent. In corn, a top cross is commonly an inbred-variety cross.
Transgressive Segregation: Appearance in segregating generations of individuals falling outside the parental range in respect to some character.
Translocation: Change in position of a segment of a chromosome to another location in the same or different chromosomes.
Variation: The occurrence of differences among individuals due to differences in their genetic composition and/or the environment in which they were raised.
Variety: A subdivision of a species. A group of individuals within a species which are distinct in form or function from other similar arrays of individuals.
Virulence: Capacity of a pathogen to incite a disease.
x: Basic number of chromosomes in a polyploid series.
X1, X2, X3… Symbols denoting first, second, third, etc. generations from and irradiated ancestral plants (X0).
Zygote: Cell formed by the union of two gametes and the individual developing from this cell.
Zygotene: A stage in meiotic prophase when the threadlike chromosomes pair.