Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 259

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.