Principles of Plant Genetics and Breeding. George Acquaah

in breeding based on biometrical genetics

      Biometrical genetics is concerned with the inheritance of quantitative traits. As previously stated, most of the genes of interest to plant breeders are controlled by many genes. In order to effectively manipulate quantitative traits, the breeder needs to understand the nature and extent of their genetic and environmental control. M.J. Kearsey summarized the salient questions that need to be answered by a breeder who is focusing on improving quantitative (and also qualitative) traits:

      1 Is the character inherited?

      2 How much variation in the germplasm is genetic?

      3 What is the nature of the genetic variation?

      4 How is the genetic variation organized?

      By having answers to these basic genetic questions, the breeder will be in a position to apply the knowledge to address certain fundamental questions in plant breeding.

       What is the best cultivar to breed?

      As will be discussed later in the book, there are several distinct types of cultivars that plant breeders develop – pure lines, hybrids, synthetics, multilines, composites, etc. The type of cultivar is closely related to the breeding system of the species (self‐ or cross‐pollinated), but more importantly on the genetic control of the traits targeted for manipulation. As breeders have more understanding of and control over plant reproduction, the traditional grouping between types of cultivars to breed 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