George Acquaah - Principles of Plant Genetics and Breeding

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 inheritanceand the gene action codominance.Some molecular markers are codominant. Whereas incomplete dominance produces blended phenotype, codominance produces distinct and separate phenotypes.

The concept of multiple allelescan 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. C 1C 1, C 7C 10, C 4C 6). 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 ABOblood group system in humans. An allelic series of importance in plant breeding is the S allelesthat 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.

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.

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 polygenesor minor genesand 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.

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 5.16Epistasis 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.

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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