The classical way of plant breeding is a long process. It is mainly done by crossing two plants that are different but related. Pollen from one plant is transferred in a natural way by wind or by human help to another plant. In that way during meiosis the genes of both plants are distributed randomly to the progeny. By repeated backcrossing, a specific method in plant breeding, specific traits, such as resistances can also be introduced from a wild type into a cultivar.

Normally the offspring of crossing two plants is produced through a random process, because during meiosis (the production of gametes) the genes from both parents are distributed randomly to the gametes. For that reason, the product of the crossing process cannot be predicted. The process to select for the desired trait takes a long time, because from each crossing process a high number of progenies, which contain different characteristics and have to be selected for the desired traits, arise. So it can take a long time to get a new cultivar that shows the wanted traits, which means that it is also quite expensive to produce a new cultivar by classical breeding. Further classical breeding is limited, because crossing can only take place with more or less related species. If a resistance gene from an unrelated species has to be introduced into another crop, this is not possible.

Furthermore, genes from completely different organisms, for example, the Bt- toxin from bacteria can only be introduced into plants by genetic engineering.