Gene Interaction: Allelic or Non-epistatic Gene Interaction and Non-alleleic or Epistatic Gene Interaction

Several investigations have revealed that a combination of various genes controls, influences and governs the majority of the characteristics of living beings.

Gene interaction is the phenomenon whereby a single character is controlled by two or more genes and each gene affects the expression of the other genes involved.

Gene interactions can comprise two or more pairs of genes. The phenotypic expression of the same character is affected by all the gene interactions involving two pairs of non-allelic genes. Modified dihybrid ratios are produced by these interactions.

Gene Interaction:

Gene interaction is the process by which the expression of two or more genes influences one another in different ways as an organism develops a single characteristic. The majority of the traits that comprise living beings are coordinated by various genes.

Mendel and other researchers assumed that characters were controlled by a single gene, but it was later found that multiple characters were controlled by two or more genes. Such genes modify the conventional dihybrid (9:3:3:1) or trihybrid (27:9:9:9:3:3:3:1) ratios by influencing the development of the concerned traits in various ways.

The expression of one gene is influenced by the expression, presence or absence, of another gene in a gene interaction.

Types of Gene Interaction: Gene interactions are divided into two categories:

(a)  Allelic or Non-epistatic Gene Interaction: This gene interaction occurs between the alleles of a single gene.

(b)  Non-allelic or Epistatic Gene Interaction: This gene interaction involves interactions between genes on identical or distinct chromosomes.

(a)         Allelic or Non-epistatic Gene Interaction:

When phenotypic ratios diverge from Mendelian ratios, it is difficult for Mendelian genetics to explain some types of inheritance. This is because specific alleles can often be equally or partially dominant to each other or due to the lethal alleles. Allelic, non-epistatic or intra-allelic interactions are the terms used to describe genetic interactions between alleles of a single gene.

Incomplete Dominance (1:2:1):

A dominant allele could not entirely suppress the other allele. Thus, a heterozygote is phenotypically differentiated from either homozygote (intermediate phenotype).

In snapdragon and Mirabilis jalapa, the hybridization of pure-bred red-flowered and white-flowered plants results in pink-flowered F1 seedlings (deviated from parental phenotypes), which are intermediate between the two parents. When the F1 plant self-fertilizes, the F2 progeny displays three classes of plants in the ratio of 1 red:2 pink:1 white rather than 3:1.

Codominance:

Here, heterozygotes exhibit the expression of both alleles of a gene. Instead of the intermediate phenotype, the F1 hybrid displays the phenotypes of both parents. In humans, a single gene regulates the MN blood group.

There are just two alleles: M and N. Children born to a father with the N blood type (genotype NN) and a mother with the M blood type (genotype MM) would have the MN blood type (genotype MN). The hybrid exhibits both phenotypes. Thus, the blood groups that F2 segregates into are 1M:2MN:1N.

Over Dominance:

F1 heterozygotes occasionally have more extreme phenotypes than either of their parents. The heterozygous white eyes of Drosophila have more fluorescent eye pigment than either of its parents.

(b)         Non-allelic or Epistatic Gene Interaction:

When two or more genes affect each other’s expression in various ways, non-allelic or epistatic or inter-allelic interaction takes place, resulting in the development of a single character.

Epistasis:

Epistasis occurs when a gene or gene pair suppresses or hinders the expression of another non-allelic gene. The gene that causes the effect is referred to as an epistatic gene, while the gene whose expression is inhibited is referred to as a hypostatic gene. This phenomenon is called epistasis.

Classification of Epistasis Gene Interaction:

Epistatic gene interaction is categorized as follows based on how the involved genes affect one another’s expression:

·         Supplementary gene interaction

·         Complementary gene interaction

·         Inhibitory gene interaction

·         Duplicate gene interaction

·         Masking gene interaction

·         Polymeric gene interaction

Epistasis in Summer Squash Plants:

In many cases of epistasis, product of one gene may inhibit the expression of another gene. For example, it occurs in inheritance of fruit colour in summer squash plants.

• The plants that carry the dominant allele ‘C’ produce white fruits.
• The plants that are homozygous for the recessive allele ‘c’ produce coloured fruit.
• The squash plant may also be homozygous for recessive allele ‘g’. The ‘g’ is an independently assorting allele. In this case, the fruit will be green.
• However, if it carries dominant allele ‘G’ of this gene, then the fruit will be yellow.

These observations suggest that two genes control steps in the synthesis of green pigment.

Ø  The first step converts a colourless precursor into a yellow pigment.

Ø  The second step converts this yellow pigment into a green pigment.

If the first step is blocked (by presence of ‘C’ allele), neither of the pigment is produced. Thus fruit will be white.

If only second step is blocked (by presence of ‘G’ allele), yellow pigment cannot be converted into the green pigment. Thus the fruit will be yellow.

 

P                                              White                                       White

                                                          Cc Gg                   X                 Cc Gg

 

Male gametes

F1		CG	Cg	cG	cg
Female gametes	CG	CC GG white	CC Gg white	Cc GG white	Cc Gg white
	Cg	CC Gg white	CC gg white	Cc Gg white	Cc gg white
	cG	Cc GG white	Cc Gg white	cc GG yellow	cc Gg yellow
	cg	Cc Gg white	Cc gg white	cc Gg yellow	cc gg green

 

Phenotypic Ratio: 12 white: 3 yellow: 1 green

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