George Acquaah - Principles of Plant Genetics and Breeding

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 actiondescribes 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 effectsare 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. Aa = AA ). The breeding implication is that the breeder cannot distinguish between the heterozygous and homozygous phenotypes. Consequently, both kinds of plants will be selected, the homozygotes breeding true while the heterozygotes will not breed true in the next generation (i.e. fixing superior genes will be less effective with dominance gene action).

Using the previous figure for additive effect, the extent of dominance (d A) is calculated as the deviation of the heterozygote, Aa , from the mean of the two homozygotes ( AA, aa ). Also, d A= 0 when there is no dominance while d is positive if A is dominant, and negative if a Ais dominant. Further, if dominance is complete d A= a A, whereas d A< a Afor incomplete (partial) dominance and d A> a Afor overdominance. For a single locus, m = ½ ( AA + aa ), a A= ½ ( AA − aa ), while d A= Aa − ½ ( AA + aa ).

Overdominance gene actionexists when each allele at a locus produces a separate effect on the phenotype, and their combined effect exceeds the independent effect of the alleles. From the breeding standpoint, the breeder can fix overdominance effects only in the first generation (i.e. F 1hybrid cultivars) through apomixes.

Epistasisis the interaction of alleles at different loci. It complicates gene action in that the value of a genotype or allele at one locus depends on the genotype at other epistatically interacting loci. In other words, the allelic effects at one locus depend on the genotype at a second locus. An effect of epistasis is that an allele may be deemed “favorable” at one locus and then deemed “unfavorable” under a different genetic background. In the absence of epistasis, the total genetic value of an individual is simply the sum total of the individual genotype values because the loci are independent. Epistasis is sometimes described as the masking effect of the expression of one gene by another at a different locus.

Estimation of gene action or genetic variance requires the use of large populations and a mating design. The effect of the environment on polygenes makes estimations more challenging. As N.W. Simmonds observed, at the end of the day, what qualitative genetic analysis allows the breeder to conclude from partitioning variance in an experiment is to say that a portion of the variance behaves as though it could be attributed to additive gene action or dominance effect, and so forth.

Understanding gene actions is critical to the success of plant breeding. It is used by breeders several ways:

In the selection of parents used in crosses to create segregating populations in which selection is practiced.

In the choice of the method of breeding used in crop improvement.

In research applications to gain understanding of the breeding material by estimating genetic parameters.

Breeding methods are discussed in detail in Chapters 17–20. The methods are grouped according to modes of pollination: self‐pollinated or cross‐pollinated.

When additive gene action predominates in a self‐pollinated species, breeders should consider using selections methods such as pure line selection, mass selection, progeny selection, and hybridization. However, when non‐additive gene action predominates, effective methods of breeding are the exploitation of heterosis in breeding hybrid cultivars.

When additive gene action predominates in a cross pollinated species, recurrent selection may be used to achieve general combining ability (GCA). Specific breeding products to pursue include synthetic varieties and composites. In case of non‐additive gene action, heterosis breeding, just like in self‐pollinated species, is recommended for breeding hybrid cultivars. Alternatively, breeders may consider recurrent selection for specific combining ability (SCA) for population improvement. Where both additive and non‐additive gene action occurs together, reciprocal recurrent selection may be used for population improvement.