Principles of Plant Genetics and Breeding. George Acquaah

Figure 5.15 The test cross. Crossing a homozygous dominant genotype with a homozygous recessive genotype always produces all heterozygotes (a). However, crossing a heterozygote with a homozygous recessive produces both homozygotes and heterozygotes.

      Plant breeders use the progeny test for a number of purposes. In breeding methodologies in which selection is based on phenotype, a progeny test will allow a breeder to select superior plants from among a genetically mixed population. Following an environmental stress, biotic or abiotic, a breeder may use a progeny test to identify superior individuals and further ascertain if the phenotypic variation is due to genetic effects or just caused by environment factors.

5.10 Complex inheritance

Just how lucky was Mendel in his experiments that yielded his landmark results? This question has been widely discussed among scientists over the years. Mendel selected traits whose inheritance patterns enabled him to avoid certain complex inheritance patterns that would have made his results and interpretations more challenging. The inheritance of traits such as those studied by Mendel is described as simple (simply inherited) traits or having Mendelian inheritance. There are other numerous traits that have complex inheritance patterns that cannot be predicted by Mendelian ratios. Several factors are responsible for the observation of non‐Mendelian ratios as discussed next.

      5.10.1 Incomplete dominance and codominance

      Mendel worked with traits that exhibited complete dominance. Post‐Mendelian studies revealed that frequently, the masking of one trait by another is only partial (called incomplete dominance or partial dominance). A cross between a red‐flowered (RR) and white‐flowered (rr) snapdragon produces pink‐flowered plants (Rr). The genotypic ratio remains 1 : 2 : 1, but a lack of complete dominance also makes the phenotypic ratio 1 : 2 : 1 (instead of the 3 : 1 expected for complete dominance).

      Another situation in which there is no dominance occurs when both alleles of a heterozygote are expressed to equal degrees. The two alleles code for two equally functional and detectable gene products. Commonly observed and useful examples to plant breeding technology are allozymes, the production of different forms of the same enzyme by different alleles at the same locus. Allozymes catalyze the same reaction. This pattern of inheritance is called codominant inheritance and the gene action codominance. Some molecular markers are codominant. Whereas incomplete dominance produces blended phenotype, codominance produces distinct and separate phenotypes.

      5.10.2 Multiple alleles of the same gene

      The concept of multiple alleles can be studied only in a population. Any individual diploid organism can, as previously stated, have at most two homologous gene loci that can be occupied by different alleles of the same gene. However, in a population, members of a species can have many alternative forms of the same gene. A diploid by definition can have only two alleles at each locus (e.g. C¹C¹, C⁷C¹⁰, C⁴C⁶). However, mutations may cause additional alleles to be created in a population. Multiple alleles of allozymes are known to occur. The mode of inheritance by which individuals have access to three or more alleles in the population is called multiple allelism (the set of alleles is called an allelic series). A more common example of multiple allelism that may help the reader better understand the concept is the ABO blood group system in humans. An allelic series of importance in plant breeding is the S alleles that condition self‐incompatibility (inability of a flower to be fertilized by its own pollen). Self‐incompatibility is a constraint to sexual biology and can be used as a tool in plant breeding as previously discussed in detail in this chapter.

      5.10.3 Multiple genes

      Just as a single gene may have multiple alleles that produce different forms of one enzyme, there can be more than one gene for the same enzyme. The same enzymes produced by different genes are called isozymes. Isozymes are common in plants. For example, the enzyme phosphoglucomutase in Helianthus debilis is controlled by two nuclear genes and two chloroplast genes. Isozymes and allozymes were the first molecular markers developed for use in plant and animal genetic research.

      5.10.4 Polygenic inheritance

      Mendelian genes are also called major genes (or oligogenes). Their effects are easily categorized into several or many non‐overlapping groups. The variation is said to be discrete. Some traits are controlled by several or many genes that have effects too small to be individually distinguished. These traits are called polygenes or minor genes and are characterized by non‐discrete (or continuous) variation, because the effects of the environment on these genes make their otherwise discrete segregation to be readily observed. Scientists use statistical genetics to distinguish between genetic variation due to the segregation of polygenes and environmental variation. Many genes of interest to plant breeders exhibit polygenic inheritance.

      5.10.5 Concept of gene interaction and modified Mendelian ratios

      Mendel's results primarily described discrete (discontinuous) variation even though he observed continuous variation in flower color. Later studies established that the genetic influence on the phenotype is complex, involving the interactions of many genes and their products. It should be pointed out that genes do not necessarily interact directly to influence a phenotype, but rather, the cellular function of numerous gene products work together in concert to produce the phenotype.

Mendel's observation of dominance/recessivity is an example of an interaction between alleles of the same gene. However, interactions involving non‐allelic genes do occur, a phenomenon called epistasis. There are several kinds of epistatic interactions, each modifying the expected Mendelian ratio in a characteristic way. Instead of the 9 : 3 : 3 : 1 dihybrid ratio for dominance at two loci, modifications of the ratio include 9 : 7 (complementary genes), 9 : 6 : 1 (additive genes), 15 : 1 (duplicate genes), 13 : 3 (suppressor gene), 12 : 3 : 1 (dominant epistasis), and 9 : 3 : 4 (recessive epistasis) (Figure 5.16). Other possible ratios are 6 : 3 : 3 : 4 and 10 : 3 : 3. To arrive at these conclusions, researchers test data from a cross against various models, using the chi square statistical method. Genetic linkage, cytoplasmic inheritance, mutations, and transposable elements are considered the most common causes of non‐Mendelian inheritance.

Figure 5.16 Epistasis or non‐Mendelian inheritance is manifested in a variety of ways, according to the kinds of interaction. Some genes work together while other genes prevent the expression of others.

      5.10.6 Pleiotropy

      Sometimes, one gene can affect multiple traits, a condition called pleiotropy. It is not hard to accept this fact when one understands the complex process of development of an organism in which the event of one stage is linked to those before (i.e. correlated traits). That is, genes that are expressed early in development of a trait are likely to affect the outcome of the developmental process. In sorghum, the gene hl causes the high lysine content of seed storage proteins to increase as well as cause the endosperm to be shrunken. Declaring genes to be pleiotropic is often not clear cut, since closely associated or closely linked genes can behave this way. Conducting a large number of crosses may produce a recombinant, thereby establishing that linkage, rather than pleiotropy, exists.

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