and the methods used along the lines of the breeding system have diminished. The fact is that the breeding system can be artificially altered (i.e. self‐pollinated species can be forced to outbreed, and vice versa). However, the genetic control of the trait of interest cannot be changed. The action and interaction of polygenes are difficult to alter. As Kearsey notes, breeders should make decisions on the type of cultivar to breed based on the genetic architecture of the trait, especially the nature and extent of dominance and gene interaction (see Section 4.2.5 on gene action), more so than the breeding system of the species.
Generally, where additive variance and additive × additive interaction predominate, pure lines and inbred cultivars are appropriate to develop. However, where dominance variance and dominance × dominance interaction suggest overdominance predominates, hybrids would be successful cultivars. Open pollinated cultivars are suitable where a mixture of the above genetic architecture occurs.
What selection method would be most effective for improvement of the trait?
The kinds of selection methods used in plant breeding are discussed in Chapters 15–18. The genetic control of the trait of interest determines the most effective selection method to use. The breeder should pay attention to the relative contribution of the components of genetic variance (additive, dominance, epistasis) and environmental variance in choosing the best selection method. Additive genetic variance can be exploited for long‐term genetic gains by concentrating desirable genes in the homozygous state in a genotype. The breeder can make rapid progress where heritability is high by using selection methods that are dependent solely on phenotype (e.g. mass selection). However, where heritability is low, methods of selection based on families and progeny testing are more effective and efficient. When overdominance predominates, the breeder can exploit short‐term genetic gain very quickly by developing hybrid cultivars for the crop.
It should be pointed out that as self‐fertilizing species attain homozygosity following a cross, they become less responsive to selection. However, additive genetic variance can be exploited for a longer time in open‐pollinated populations because relatively more genetic variation is regularly being generated through the ongoing intermating.
Should selection be on single traits or multiple traits?
Plant breeders are often interested in more than one trait in a breeding program, which they seek to improve simultaneously. The breeder is not interested in achieving disease resistance only, but in addition, high yield and other agronomic traits. The problem with simultaneous trait selection is that the traits could be correlated such that modifying one affects the other. The concept of correlated traits is discussed next. Biometrical procedures have been developed to provide a statistical tool for the breeder to use. These tools are also discussed in this section.
4.2.5 Gene action
Additional information on gene action is found in Supplemental Material 1 at the end of the regular chapters of the book. There are four types of gene action: additive, dominance, epistasis, and overdominance. Because gene effects do not always fall into clear‐cut categories, and quantitative traits are governed by genes with small individual effects, they are often described by their gene action rather than by the number of genes by which they are encoded. It should be pointed out that gene action is conceptually the same for major genes as well as minor genes, the essential difference being that the action of a minor gene is small and significantly influenced by the environment. A general way of distinguishing between these types of gene action based on interaction among alleles is as follows:
| No allelic interaction | Allelic interaction | |
| Within locus interaction | Additive action | Dominance action |
| Between loci interaction | Additive action | Epistasis |
Additive gene action
The effect of a gene is said to be additive when each additional gene enhances the expression of the trait by equal increments. Consequently, if one gene adds one unit to a trait, the effect of aabb = 0, Aabb = 1, AABb = 3, and AABB = 4. For a single locus (A, a) the heterozygote would be exactly intermediate between the parents (i.e. AA = 2, Aa = 1, aa = 0). That is, the performance of an allele is the same irrespective of other alleles at the same locus. This means that the phenotype reflects the genotype in additive action, assuming the absence of environmental effect. Additive effects apply to the allelic relationship at the same locus. Furthermore, a superior phenotype will breed true in the next generation, making selection for the trait more effective to conduct. Selection is most effective for additive variance; it can be fixed in plant breeding (i.e. develop a cultivar that is homozygous).
Additive effectConsider a gene with two alleles (A, a). Whenever A replaces a, it adds a constant value to the genotype:Replacing a by A in the genotype aa causes a change of a units. When both aa are replaced, the genotype is 2a units away from aa. The midparent value (the average score) between the two homozygous parents is given by m (representing a combined effect of both genes for which the parents have similar alleles and environmental factors). This also serves as the reference point for measuring deviations of genotypes. Consequently, AA = m + aA, aa = m − a, and Aa = m + dA, where aA is the additive effect of allele A. This effect remains the same regardless of the allele with which it is combined.
Average effectIn a random mating population, the term average effect of alleles is used because there are no homozygous lines. Instead, alleles of one plant combine with alleles from pollen from a random mating source in the population through hybridization to generate progenies. In effect the allele of interest replaces its alternative form in a number of randomly selected individuals in the population. The change in the population as a result of this replacement constitutes the average effect of the allele. In other words, the average effect of a gene is the mean deviation from the population mean of individuals that received a gene from one parent, the gene from the other parent having come at random from the population.
Breeding valueThe average effects of genes of the parents determine the mean genotypic value of the progeny. Further, the value of an individual judged by the mean value of its progeny is called the breeding value of the individual. This is the value that is transferred from an individual to its progeny. This is a measurable effect, unlike the average effect of a gene. However, the breeding value must always be with reference to the population to which an individual is to be mated. From a practical breeding point of view, the additive gene effect is of most interest to breeders because its exploitation is predictable, producing improvements that increase linearly with the number of favorable alleles in the population.
Dominance gene action
Dominance action describes the relationship of alleles at the same locus. Dominance variance has two components – variance due to homozygous alleles (which is additive) and variance due to heterozygous genotypic values. Dominance effects are deviations from additivity that make the heterozygote resemble one parent more than the other. When dominance is complete, the heterozygote is equal to the homozygote in effects (i.e.
