Genetics with the Principles of Breeding - M.P. Myhun - 2008

CHAPTER III. Inheritance of Chromosomal and Extrachromosomal Genes

3.2. Patterns of Inheritance in Gene Interaction

All patterns of Mendelian segregation in offspring with a specific set of corresponding ratios are possible under two essential requirements:

- if non-allelic genes are located on different pairs of homologous Chromosomes;

- if each Gene affects a separate trait or property of the Organism independently of other genes.

However, during the individual development of an organism, genes enter into complex interactions with one another. Many types of interaction between allelic and non-allelic genes are known, which cause certain deviations from the Mendelian ratio of phenotypic classes in F₂.

An organism is not a mosaic of individual and independent gene actions, but a complex system of sequential biochemical and morphophysiological processes determined by a system of genes.

A trait is a product of the individual development of an organism, determined by its genotype and environmental influences.

A gene, as a "unit of heredity" determining the traits and properties of an organism, has specific characteristics.

1. In its action, a gene is discrete, as it determines The Development of individual traits and Properties of the organism.

2. A gene can simultaneously affect the development of many traits and properties of an organism, acting plurally; this phenomenon is called the multiple or pleiotropic effect of a gene (pleiotropy).

3. A gene can lead to the enhancement or weakening of a trait's expression.

4. A gene interacts with other genes and, due to this effect, its expression can be modified.

5. Different genes are located on different pairs of chromosomes, but their action can simultaneously influence the Development of the same trait, enhancing or weakening it.

6. The expression of gene action depends on environmental factors.

Allelic gene interaction

1. Complete dominance: in heterozygotes Aa (F₁), only one dominant trait A is expressed. The Inheritance of Traits occurs in full accordance with the phenotypic segregation ratios.

2. Incomplete dominance: the expression of traits in heterozygotes Aa (F₁; F₂ ) is intermediate, with a greater or lesser deviation from the dominant and

recessive states. Our example with the four o'clock flower is the intermediate pink coloration of the petals.

3. Codominance of genes leads to the simultaneous expression of both alleles in heterozygotes, making them phenotypically distinct from homozygous forms and forming a separate phenotypic Class. (An example is the inheritance of Blood Groups in the ABO system).

Non-allelic gene interaction

1. Complementary gene interaction (complementarity): non-allelic genes do not express their effect individually, but when simultaneously present in the genotype, they determine the development of a new trait. Furthermore, the segregation ratio in F₂ changes; instead of 9:3:3:1, it can be 9:7; 9:6:1; or 9:3:4. This type of segregation was first identified in sweet peas. When crossing two white-flowered sweet pea varieties, the F₁ generation has red flowers, and the F₂ generation segregates into 9 red and 7 white. This result cannot be explained by standard Mendelian segregation. W. Bateson explained this gene interaction: the red flower color is determined by the mutual action in the genotype of two complementary dominant genes (A:B). Each gene individually produces only white color; therefore, in F₂, where only one gene, A or B, is present in the zygote, the flowers are white (7 such cases), and where both genes A and B meet in the zygote, the flowers are red (9 cases). In most cases, the combination of complementary (additional) genes produces traits that correspond to wild types, resulting in a reversion to wild-type traits.

2. Epistasis. The suppression of the action of one allelic pair of genes by a gene of another, non-allelic pair. AA>BB; BB>AA; Bb>AA. Genes that suppress the action of other non-allelic genes are called suppressors or inhibitors. They can be dominant or recessive. Segregation in F₂ can be 12:3:1; 13:3; or 9:3:4.

3. Polygeny (polymeria). Several non-allelic genes jointly influence the development of the same trait. Such genes are designated by the same letter, and different allelic pairs by numbers: A₁a₁ is the dominant and recessive allele of one gene, and А₂а₂ of the second gene. Genotypes containing two pairs of dominant polymeric genes are designated as А₁А₁А₂А₂ (double dominant); A₁a1_tA₂a₂ (double heterozygote); and а₁а₁а₂а₂ (double recessive). In polygeny, two or more Enzymes produced under the control of non-allelic genes act on the development of the same trait, enhancing its expression. An example of polygeny is the inheritance of wheat seed color: red is dominant, white is recessive. When crossing wheat varieties with red and white seeds, the F₁ generation is red, and the F₂ generation segregates into 15 red and 1 white. However, the intensity of the red seed color varies, ranging from dark red to pale red. There can be only one explanation: the intensity of the seed color depends on several dominant genes that affect this trait relatively equally. If the genotype is А₁А₁А₂А₂, the seeds are dark red; A₁A₁A₂a₂ is red; А₁А₁ is light red; А₁ is pale red; and а₁а₁а₂а₂ is white.

4. Modifier genes. Non-allelic genes that enhance or suppress the expression of the major gene. Modifiers include genes that do not directly determine the development of a trait (having no phenotypic expression of their own), but are capable of enhancing (intensifiers, enhancers) or suppressing (inhibitors, suppressors)—that is, modifying—the action of the major genes.

Deviations in the F₂ phenotypic segregation ratios from the classical Mendelian 9:3:3:1 ratio can be caused not only by gene interactions, but also by statistical factors, differential mortality of different genotypes, and the specific patterns of Gene Expression under particular conditions.