Chapter Four - A practical guide to genetic gain
Introduction
For the majority of agricultural history, selective breeding of animal and plant species was done without a formal understanding of how selection leads to genetic improvement. The basic principles of inheritance were first described by Mendel (1866) and these principles could only fully explain the inheritance of traits that fall into discrete categories. Important characters like reproductive fitness in natural populations, as well as the yield of grain crops and carcass weight of livestock were known to exhibit a continuous range of variation. These characters are referred to as “quantitative traits.” It was not until 1918 when the model for the inheritance of quantitative traits based multiple Mendelian factors was comprehensively described by the statistician and geneticist Fisher (1918). Also around that time, the geneticist Sewall Wright described many quantitative genetic principles that became instrumental for breeding (Wright, 1920, Wright, 1921). Animal breeder, J.L. Lush was influenced by Wright's work, and was one of the first to apply quantitative genetics to breeding. The now widely recognized breeder's equation which describes how genetic improvement can be predicted from one generation of selection to the next was just one of Lush's many important contributions (Lush, 1937).
Although ideas about the application of quantitative genetics for breeding are over 80 years old, many plant breeding programs have not yet taken full advantage of this knowledge to increase rates of genetic improvement. This may be because there has been a great deal of emphasis on understanding what are the genes or genomic regions which affect traits, in hopes that this will enable a precise stacking of favorable genes. While such an approach may be useful for monogenic or even oligogenic traits, it is not a solution for the improvement for quantitative traits (Bernardo, 2008) which are known to be influenced by a large number of genes. For such traits, genetic improvement over cycles of selection is necessary so that favorable alleles can be brought together gradually. Such a population improvement approach is how natural selection works to drive adaptive evolution (Fisher, 1930), and it is required for achieving genetic gain in breeding programs. This chapter aims to help improve the understanding and appreciation of this concept for plant breeding.
Section snippets
Definition
Genetic gain from selection, or simply “genetic gain,” is defined as the improvement in average genetic value in a population or the improvement in average phenotypic value due to selection within a population over cycles of breeding (Hazel and Lush, 1942). Genetic gain may also be referred to as response to selection which may be a better term for describing changes that are not necessarily favorable. In this chapter, the symbol R is used to represent genetic gain.
Discussions on genetic gain
Utilizing multiple sources of information for selection
Previously, an example of selection for increased NLB resistance in a maize breeding population was given. In this example the selection units and the evaluation units were the same single plants, and only single phenotypic measurements were used as the criteria for selection. This method is called phenotypic mass selection or simply mass selection. Mass selection is the oldest form selection used in breeding. It requires no record keeping and it can be done by simply collecting seeds of
Expected genetic gain
Now that genetic gain and the process that generates it has been described, the theory behind why the breeding cycle leads to genetic gain and how rates of genetic gain can be increased will be shown. To do so, two basic genetic principles are explained and then used to show how to predict genetic gain per cycle and per unit time from simple phenotypic mass selection and selection based on selection indices or BLUPs.
Examples of genetic gain realized
Hundreds of selection experiments have been conducted since the early 1900s. More than 60 selection experiments conducted using animal species have been summarized in a 1988 review by Sheridan (1988), and more than 50 selection experiments conducted in self and cross-pollinated crop species have been reviewed by Hallauer and Darrah (1985). Outcomes of selection experiments have confirmed that the rate of genetic gain per cycle depends on the selection accuracy, selection intensity, and additive
How to improve rates of genetic gain
Although theory indicates that realizing genetic gain is possible as long as a population improvement strategy is followed and the traits of interest are heritable, there are also a series of components that should be in place in a breeding program to ensure that genetic gain can be realized. Once these components are well established, a breeding program can then incorporate additional components to help it achieve a higher rate of genetic gain. Following the example of Maslow's hierarchy of
Conclusion
The objective of this chapter was to increase awareness and knowledge about genetic gain so that breeding programs can begin to take actions that will enable them to be more successful. Genetic gain was defined as the improvement in average genetic value in a population or the improvement in average phenotypic value due to selection within a population over cycles of breeding assuming that the effect of environment remains constant. The importance achieving genetic gain over cycles of selection
Acknowledgment
This work was supported by the Bill and Melinda Gates Foundation project: Transforming Rice Breeding and Excellence in Breeding Platform (EiB) of the Consultative Group on International Agricultural Research (CGIAR).
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