BIOLOGY Lecture Notes - Golden Pages 2003

6. FUNDAMENTALS OF HEREDITY

Main Types of Non-Allelic Gene Interaction

When several genes determine a single phenotypic trait, they interact with one another. In such cases, the offspring of a diheterozygote may exhibit unusual segregation ratios — 9 : 3 : 4; 9 : 7; 13 : 3; 12 : 3 : 1; 15 : 1, which are modifications of the standard Mendelian ratio (9 : 3 : 3 : 1). Interactions involving two, three, or more genes are well known. The primary types of non-allelic Gene interaction include complementarity, epistasis, and polygenic inheritance (polymerism).

Complementary interaction occurs when dominant genes interact to produce a novel trait that is absent in either parent.

Epistasis is a type of gene interaction in which one pair of alleles suppresses the expression of another. Genes that suppress the action of other genes are termed epistatic genes, inhibitors, or suppressors; they can be either dominant or recessive. Based on Changes in the number and ratio of dihybrid segregation classes in F₂, several types of epistatic interactions are distinguished: simple recessive epistasis (a > B; a > b or b > A; b > a), which results in a 9 : 3 : 4 segregation ratio; simple dominant epistasis (A > B; A > b or B > A; B > a) with a 12 : 3 : 1 ratio, and so forth.

Polymerism (polygenic inheritance) is a type of gene interaction in which multiple genes with identical phenotypic effects control the Inheritance of the same trait. Such genes are referred to as polymeric genes. The greater the number of such genes in an Organism's genotype, the stronger the expression of the trait; in other words, as the gene dosage increases (A1, A2, A3, etc.), their effects are summed up, or cumulated. This mode of gene action is known as cumulative polymerism. Many Quantitative Traits, such as human Skin color, are inherited via cumulative polymerism.