Crosses between varieties germplasm introductions and breeding lines are made to create new gene combination. In the generations to follow superior genotypes that is having presumably superior genes and gene combinations are selected and fixed in the homozygous state by means of self fertilization and selection. These selections are testes extensively with the goal of releasing the few best of them for cultivation.
With the method of selection the offspring from a crossing are planted at planting densities equal to commercial planting densities. During this period which may include a number of generations the label of homozygosity in the bulk population increases. This method is simple and cheap and involves less work than pedigree selection. It is necessary to plant large populations to ensure that the best segregation are selected when selection starts. Segregating generations are subjected to another single plant selection step. Few are records during earlier generations than with pedigree selection. This type of selection is especially carried out with crops which are usually planted at high planting densities that are small grain crops.
Parental lines are crossed and selection of plants with new gene combinations already takes place in the F2 generation. The offspring of selected population in the generations to follow are repeatedly subjected to selection and always single plants are taken as source of new offspring lines until genetic uniformity is reached. Only then seed yield of several plants per line is combined and mixed to have more seed for testing. Records are kept of the origin of the selected individuals or lines. The amount of generations of single plant and line selections as well as selection intensities can be varied in tactics according to the crop availability and facilities.
It is usually traits with high heritability that are quite easy to measure that are concentrated as long as single plant selection is carried out. In the later phase when plots are the units of assessment traits like yield with lower heritability are assessed and taken as basis for selection. One of the main objections against this methods is that the genetic variation, availability for selection of quantitative traits are drastically decreased in later generations due to the single plant selection carried out in early generations.
Seed purification and multiplication is usually incorporated in one of the final generations of pedigree selection. This method is very labor intensive. Breeding efficiency is one of the goals with early generations testing. This is done by early identification of superior heterogeneous populations. The early elimination of inferior populations and subsequent concentration of selection affords with in superior populations is assumed to result in increased efficiency. Accurate evolution of heterogeneous population is essential to the success of this method and assumes that trangressive segregates from inferior populations will not exceed selection from superior populations in performance.
This method of selection depends mainly on selection of plants according to their phenotypic performance. The seed from selected plants are bulked for the next generations. This method is used to improve the overall population by positive or negative mass selection.
Mass selection is only applied to a limited degree in self fertilizing plants and is an effective method for the improvement of land recess. This method of selection will only be effective for highly heritable traits. One demerit of mass selection is the large influence that the environment has on the development, phenotype and performance of single plants. It is often unclear whether the phenotypically superior plants are also genotypicaly superior and strong environmental differences may lead to low selection efficiency.
Pure Line Selection
A variety developed by this method will be more uniform than those developed by mass selections because all of the plants in such a variety will have the same genotype. The seed from selected plants are not added together but are kept apart and used to perform offspring tests. These are done to study the breeding behavior of the selected plants. The high uniformity instant and performance has been stressed in the past but the risk of highly specialized pathogens involving in is very high. More genetic variability would buffer the crop against such pathogens as well as stability of productions under varied environmental conditions.
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