Heterozygous and heterogeneous cultivars

Other approaches of breeding produce heterozygous and homogeneous cultivars, for example, synthetic and composite breeding. These approaches allow the farmer to save seed for planting. Composite cultivars are suited to production in developing countries, while synthetic cultivars are common in forage production all over the world.

Homozygous and heterogeneous cultivars

An example of such a breeding product is the mixed landrace types that are developed by producers. The component genotypes are homozygous but there is

such a large amount of diverse genotypes included that the overall cultivar is not uniform.

Clonal cultivar

Clones, by definition, produce offspring that are not only identical to each other but also the parent. Clones may be very heterozygous but whatever advantage heterozygosity confers is locked in for as long as propagation is clonally conducted. The offspring of a clonal population is homogeneous. Once the genotype has been manipulated and altered in a desirable way, for example through sexual means

The changes are fixed for as long as clones are used for propagation. Flowering species such as cassava and sugarcane may be genetically improved through sexbased methods, and thereafter commercially clonally propagated.

Types of self-pollinated cultivars

In terms of genetic structure, there are two types of self-pollinated cultivars:

Those derived from a single plant.

Those derived from a mixture of plants.

     Single plant selection may or may not be preceded by a planned cross but often it is the case. Cultivars derived from single plants are homozygous and homogeneous. However, cultivars derived from plant mixtures may appear homogeneous but, because the individual plants have different genotypes, and because some outcrossing occurs in most selfing species, heterozygosity would arise later in the population. The methods of breeding selfpollinated species may be divided into two broad groups – those preceded by hybridization and those not proceeded by hybridization.

Common plant breeding notations

Plant breeders use shorthand to facilitate the documentation of their breeding programs. Some symbols are standard genetic notations, while others were developed by breeders. Unfortunately, there is no one comprehensive and universal system in use, making it necessary, especially with the breeding symbols, for the breeder to always provide some definitions to describe the specific steps in a breeding method employed in the breeding program.

Symbols for basic crosses

The symbol F denotes the progeny of a cross between two parents. The subscript represents the specific generation. If the parents are homozygous, the F1 generation will be homogeneous. Crossing of two F1 plants yields an F2 plants. Planting seed from the F2 plants will yield an F2 population, the most diverse generation following a cross, in which plant breeders often begin selection. Selfing F2 plants produce F3

plants, and so on. It should be noted that the seed is one generation ahead of the plant, that is, an F2 plant bears F3 seed.