Principles of inheritance
Genotypic variation
Other forms of heritable variation
Quantitative variation and heritability
Novel sources of genetic variation
The practice of plant breeding
Breeding methods
Plant Breeders' rights
New technologies for plant breeding
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Glossary of Terms Commonly Encountered in Plant Breeding






Allele: alternate forms of a genetic locus. For example, at a locus determining eye colour, an individual might have the allele for blue eyes, brown, etc.




Breeding: the intentional development of new forms or varieties of plants or animals by crossing, hybridization, and selection of offspring for desirable characteristics




Chromosome: the structure in the eukaryotic nucleus and in the prokaryotic cell that carries most of the DNA


Cross-over: The point along the meiotic chromosome where the exchange of genetic material takes place. This structure can often be identified through a microscope


Crossing-over: The reciprocal exchange of material between homologous chromosomes during meiosis, which is responsible for genetic recombination. The process involves the natural breaking of chromosomes, the exchange of chromosome pieces, and the reuniting of DNA molecules




Domestication: the process by which plants are genetically modified by selection over time by humans for traits that are more desirable or advantageous for humans


DNA: an abbreviation for “deoxyribose nucleic acid”, the carrier molecule of inherited genetic information


Dwarfness: The genetically controlled reduction in plant height. For many crops, dwarfness, as long as it is not too extreme, is an advantage, because it means that less of the crop's energy is used for growing the stem. Instead, this energy is used for seed/fruit/tuber production. The Green Revolution wheat and rice varieties were based on dwarfing genes




Emasculation: The removal of anthers from a flower before the pollen is shed. To produce F1 hybrid seed in a species bearing monoecious flowers, emasculation is necessary to remove any possibility of self-pollination


Epigenetic: heritable variation caused by differences in the chemistry of either the DNA (methylation) or the proteins associated with the DNA (histone acetylation), rather than in the DNA sequence itself




Gamete: The haploid cell produced by meiosis. The male gamete is the pollen grain, while the female gamete is the egg cell


Gene: the unit of heredity, transmitted from generation to generation during reproduction. Each gene consists of a sequence of nucleotides, occupying a specific position along a chromosome. Most genes encodes a specific functional product


Genome: all the genetic material of an organism


Genotype: the inherited genetic constitution of an organism, see also phenotype


Genotyping: the process of identifying the genetic make-up of an organism, by using molecular markers, DNA sequencing, etc.


Germplasm: the collection of a set of genetic resources for an organism, which can consist of a seed collection, nursery, or other types


Genotype x Environment (GxE): the interaction of a plant’s genotype with the environment in which it is grown that contributes to its performance




Haploid: having a single set of chromosomes, for example as in a gamete


Haplotype: A combination of genetic variants (usually SNPs) which are inherited as a unit, because they are present along a stretch of DNA so short that the chance of a recombination event

occurring within it is effectively zero


Homozygous: contains identical alleles at a particular locus


Heterosis: the phenomena where the progeny of 2 inbred lines performs better than: either of the parents (best-parent heterosis) or the mean of the parents (mid-parental heterosis). The molecular basis of heterosis is still unknown


Heterozygous: contains different alleles at a particular locus


Hybridize: to cross-pollinate, to produce hybrids




Inbred lines: lines that have been selfed to the point of homozygosity


Inbreeding depression: the loss of vigor when some crops have reduced heterozygosity due to enforced self-pollination


Introgression: movement of a gene or locus from one species into another by hybridization




Landraces: a local variety that has been developed by adaptation to its local environment; usually heterogeneous varieties


Linkage, genetic linkage: when two chromosomal regions are located physically near each other such that there is a high likelihood they will be inherited together


Linkage drag: the negative association of non-targeted genes that are inherited with a gene of interest due to linkage


Linkage disequilibrium: the non-random association of alleles at two or more loci, i.e. they occur together more often than would be expected by chance




Mapping: the process of identifying the location of a gene or DNA segment along a chromosome. In genetic mapping, this is done by analyzing patterns of inheritance in segregating populations (measured in recombinational units, commonly centimorgans). In physical mapping, this describes the actual location of a sequence in a particular genomic region (measured in bp)


Molecular marker: a gene or DNA sequence that identifies a particular locus on the chromosome (whether the actual location is known or not) and whose inheritance can be followed


Mutation: an abrupt change in the genotype of an organism that is not the result of recombination




Pathogen: A simple organism which damages a crop plant. The most important crop pathogens are fungi, bacteria and viruses. Larger pathogens, such as nematodes, insects, rats, birds etc. are usually referred to as pests rather than as pathogens


Pedigree: line of descent, lineage, ancestry


Phenotype: the visible appearance of an organism. The phenotype reflects the combined action of the genotype and the environment where the individual exists


Plastid: Organelles within the plant cell cytoplasm which contain their own DNA, and replicate independently of the nucleus. Green tissue cells include both chloroplasts (which are largely resposible for photosynthesis) and mitochondria (which is the major site of energy production in the cell). Amyloplasts synthesize and store starch in non-green tissue such as the potato tuber


Polygenic: When applied to a trait, this term implies that many genes are involved in its determination. When only a few genes are involved, the trait is said to be under oligogenic control, and when only one gene is involved, the trait is under monogenic control. Most quantitatively inherited traits are under polygenic control


Polyploidy: a state in which multiple copies of a complete genome are present. Polyploidy is rare in animals, but common in plants. In animals (and also plants) some tissues within a diploid organism can be polyploid. The polyploid series is haploid (1 copy), diploid (2 copies), triploid (3 copies), tetraploid (4 copies), pentaploid (5 copies), hexaploid (6 copies) etc.


Pure line variety: A variety which has been multiplied by a succession of self-fertilizations from a single seed (or sometimes from the progeny of a single plant). Pure line varieties are expected to be essentially fully homozygous and so their phenotype should be highly homogeneous




Quantitative trait loci (QTL): regions of the chromosome associated with the inheritance of polygenic traits




Recombination: the formation among the offspring of a mating of genetic combinations not present in either parent, achieved via the physical exchange of genetic material during meiosis




Segregation: The process whereby alleles are separated from one another as a result of meiosis. Segregation can only occur with respect to genes which are in the heterozygous state in the parental plant


SNP: an abbreviation for “single nucleotide polymorphism”, pronounced "snip". A SNP which distinguishes two sequences can be used as a genetic marker



Trait: A recognizable, measurable character in a plant, which is under some genetic control


Transgenic: an organism containing genetic material from another organism transferred by genetic engineering




Zygote: The product of the fusion between one of the pollen sperm nuclei and the egg cell of the female gamete at fertilization. Following a number of mitotic divisions, the zygote differentiates into the embryo