Traditional methods

The technology of plant breeding has developed from the science of genetics. But as crop improvement by breeding depends on recognition of particular traits needed for achieving high and stable yield, pest and disease resistance and quality, success in breeding also depends on an understanding of plant physiology, pathology and biochemistry.

The fundamental concept of genetics is the gene, the unit of inheritance. Each gene controls or influences some aspect of plant behaviour and the gene complement, or genome of a plant consists of 104–105 genes. A proportion of the genes can exist in more than one form, or allele, at any given locus, and so individuals of a species with the same loci, but with different allelic variants, will be different in their form or function. Many genes are common to most organisms. For a given species, an even greater proportion of the genes will be the same for all individuals, and only a relatively small proportion will display allelic variation. Traditional plant breeding is concerned mainly with the directed reassortment of the allelic variants to produce a combination, or genotype, which best approaches a supposed ideal combination. The allelic variants of many genes have very small effects (i.e. the difference between the alleles of a particular gene have only minor consequences for plant form or function), but some have larger easily recognisable effects. Loci which have allelic forms with very different effects are relatively easy to manipulate by crossing and selection.