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Population Genetics of Plant Pathogens
Glossary of Terms


Allele:  One of a pair, or series, of alternative forms of a gene that occurs at a given locus in a chromosome.

Allele frequency:  The proportion of one allele relative to all alleles at a locus in a population.  Also called gene frequency.

Assortative mating:  Non-random mating on the basis of phenotype; in positive assortative mating, the partners are chosen because they are phenotypically similar.

Balancing selection:  A general term for selection that results in the maintenance of different alleles.  Could result from heterozygote advantage (overdominance) or frequency-dependent selection.

Biogeography: The study of the geographical distributions of organisms.  Biogeography  involves a range of scientific disciplines that includes geography, geology and biology.

Bottleneck: A severe, temporary reduction in effective population size, sometimes leading to genetic drift.

Cladistics: A classification system that classifies organisms on the basis of historical sequences by which they have diverged from common ancestors.

Codominant alleles:  Alleles that produce independent effects when heterozygous.

Coevolution:  The coordinated evolutionary change in two or more species in response to the presence of other species.

Diploid:  An organism or cell with two sets of chromosomes (2N) or two genomes.

Directional selection:  Selection for a higher or lower value of a character than its current mean.

Disassortative mating:  Non-random mating on the basis of phenotype; in disassortative or negative assortative mating, the partners are chosen because they are phenotypically dissimilar.

Disruptive selection:  Selection in favour of two or more modal phenotypes and against those intermediate between them.  Also called diversifying selection.

Dominant:  A condition in which one member of an allele pair is manifested (i.e. determines the phenotype) to the exclusion of the other.

Effective population size:  The number of breeding individuals in a population that contribute offspring to the next generation.

Elicitor:  A molecule that binds to a receptor and initiates the plant defense response system.  Elicitors are often encoded by the avirulence allele of the pathogen in a gene-for-gene interaction.

Epidemiology:  The study of the development of disease in populations.

Equilibrium:  A state at which there is no change in the genetic constitution of a population.  An equilibrium may be stable, unstable, or neutral.

Evolution:  The change over time of the proportions of individual organisms differing genetically in one or more traits. These changes are brought about by the origin and subsequent alteration of the frequencies of alleles or genotypes from generation to generation within populations. 

Fitness:  The average contribution of one allele or genotype to the next generation or to succeeding generations compared with that of other alleles or genotypes. 

Fitness cost:  The reduction in fitness compared to the population mean fitness due to the presence of a particular allele or phenotype.

Founder effect:  The principle that the founders of a new colony carry only a fraction of the total genetic variation of the source population.

Gametic equilibrium: The random association of alleles at different loci into gametes.  At equilibrium, the frequency of gametes is equal to the product of the frequencies of the constituent alleles.

Gene diversity:  A measure of the total number and frequency of alleles within a population.

Gene flow:  The incorporation of genes into the gene pool of one population from one or more other populations.  Can be used synonymously with migration.

Gene-for-gene:  A model to explain the genetic basis for the interaction between plants and parasites introduced by H.H. Flor.  For each gene determining resistance in the host, there is a corresponding gene in the parasite with which it specifically interacts.

Genetic bottleneck:  A temporary reduction in population size from which future generations are derived.

Genetic drift:  Changes in allele frequency in small breeding populations due to chance fluctuations.

Genotype:  The set of genes possessed by an individual organism.

Haploid:  An organism or cell having only one complete set (N) of chromosomes or one genome.

Hardy-Weinberg equilibrium: After one generation of random mating, the single-locus genotypic frequencies in a population can be represented as a binomial (with two alleles) or multinomial (with multiple alleles) function of the allelic frequencies for that locus.

Hybridization:  Interbreeding of species or unlike forms (races) within a species.  A hybrid individual or population is formed by mating between unlike forms, usually genetically differentiated populations or species.

Hypersensitive response:  A specialized plant defense response, usually part of a gene-for-gene interaction, that leads to host cell death.

Inbreeding:  Matings between related individuals.

Inbreeding depression:  Reduction, as a consequence of inbreeding, of the mean value of a character, often used to refer to overall fitness of an individual.

Mating type:  A specialized form of individual that requires an opposite form to complete a cycle of sexual reproduction.  Mating type genes control the fusion of fungal individuals for sexual reproduction.

Metapopulation:  A set of local populations connected by migrating individuals.

Mutation:  A change in the DNA at a particular locus in an organism.  This can include point mutations as well as chromosomal changes.

Natural selection:  Differential survival and reproduction in nature that favors individuals that are better adapted to their environment; or elimination of less fit individuals.

Outcrossing:  Mating of unrelated individuals.

Overdominance:  The expression of two alleles in a heterozygous condition of a phenotypic value for some characteristic that lies outside the range of the two corresponding homozygotes; a possible basis for heterosis or heterozygote advantage.

Phenetics:  A type of classification based on phenotypic similarity.

Phylogenetics:  A branch of genetics that organizes species according to lines of descent from common ancestors.

Phylogeography:  A field of study concerned with the principles and processes governing the geographical distributions of genealogical lineages, especially those within and among closely related species.  A sub-discipline of biogeography.

Population:  A group of conspecific individuals that occupy a more or less well defined geographic region and exhibit reproductive continuity from generation to generation.

Population genetics:  The branch of genetics that deals with frequencies of alleles and genotypes in breeding populations.

Recessive:  A term applied to one member of an allele pair lacking the ability to manifest itself when the other or dominant member is present.

Receptor:  A molecule that binds to an elicitor and initiates the plant defense response system.  Receptors are often encoded by the resistance allele of the host plant in a gene-for-gene interaction.

Recombination:  Any process which gives rise to cells or individuals (called recombinants) that associate in new ways two or more genes by which their parents differed.

Resistance:  The ability of a plant to overcome, completely or in some degree, the effect of a pathogen.

RFLP: Restriction Fragment Length Polymorphism (RFLP) is a codominant type of molecular genetic marker based on differences in restriction enzyme digestion patterns between individuals.

Selection:  Differential and non-random reproduction of different genotypes within a population, often leading to a change in gene or genotype frequencies across generations.

Selection coefficient:  The difference between the mean relative fitness of individuals of a given genotype and those of a reference genotype.

Speciation:  The process of species formation.  It can occur either when new species are in the same area (sympatric), in different areas (allopatric), or in adjacent areas (parapatric).

Stabilizing selection:  Selection against phenotypes that deviate in either direction from the optimal value of a character.