Gene Interaction Epistatic interactions. Notes

Dominance Relationships in Genetics

Genetics studies how traits pass from parents to offspring. Alleles of the same gene can interact in several different ways — these are called dominance relationships. Below are the main types with definitions, simple explanations, and clear examples.

IMPORTANT TERMS

Allele: A variant form of a gene.

Homozygote: An organism with two identical alleles at a locus (e.g., AA or aa).

Heterozygote: An organism with two different alleles at a locus (e.g., Aa).

Phenotype: The observable trait or appearance.

Genotype: The genetic makeup (allele combination).

INTRODUCTION

Alleles may show different relationships when paired in a heterozygote. The way alleles interact determines whether one allele masks another, whether they blend, or whether both appear equally. The major relationships are: Complete dominance, Incomplete dominance, Co-dominance, and Overdominance.

COMPLETE DOMINANCE

Definition

When one allele completely masks the expression of the other allele in a heterozygote, the relationship is called complete dominance.

Explanation

The dominant allele shows its phenotype even if present as a single copy (heterozygote). The recessive allele appears only in the homozygous recessive state.

Example (Mendel’s pea flower color)

Alleles: P = purple (dominant), p = white (recessive)

Genotype → Phenotype

• PP → Purple
• Pp → Purple
• pp → White

Explanation: In Pp the purple allele (P) completely masks white (p), so flowers are purple.

Punnett square: Cross Pp × Pp

	P	p
P	PP Purple	Pp Purple
p	Pp Purple	pp White

Phenotypic ratio: 3 Purple : 1 White

INCOMPLETE DOMINANCE

Definition

When the heterozygote displays an intermediate phenotype between the two homozygotes (a blend), it is called incomplete dominance.

Explanation

Neither allele fully dominates. The result is a mixture or intermediate appearance in the heterozygote.

Example (Snapdragon flower color)

Alleles: R = red, r = white

Genotype → Phenotype

• RR → Red
• Rr → Pink (intermediate)
• rr → White

Explanation: Rr does not show red or white alone; it shows pink — a blend of both parental colors.

Punnett square: Cross Rr × Rr

	R	r
R	RR Red	Rr Pink
r	Rr Pink	rr White

Phenotypic ratio: 1 Red : 2 Pink : 1 White

CO-DOMINANCE

Definition

When both alleles in a heterozygote are fully and simultaneously expressed, the relationship is called co-dominance.

Explanation

The heterozygote displays characteristics of both alleles at the same time — not a blend, but both features visible together.

Example (Human ABO blood groups)

Alleles: IA, IB, i

Genotype → Phenotype

• IAIA or IAi → Blood group A
• IBIB or IBi → Blood group B
• IAIB → Blood group AB (both A and B antigens expressed)
• ii → Blood group O

Explanation: IA and IB are co-dominant: in IAIB both antigens A and B are present and expressed equally, producing AB blood type.

OVERDOMINANCE (HETEROZYGOTE ADVANTAGE)

Definition

Overdominance occurs when the heterozygote has a phenotype (or fitness) that is superior to either homozygote.

Explanation

This often appears as improved survival, disease resistance, or productivity in the heterozygote compared with either homozygote. Overdominance is one genetic explanation for heterosis (hybrid vigor).

Example (Sickle-cell trait and malaria)

Alleles: HbA (normal hemoglobin), HbS (sickle hemoglobin)

Genotype outcomes:

• HbA HbA → Normal hemoglobin, but more susceptible to severe malaria.
• HbS HbS → Sickle-cell disease (severe health problems).
• HbA HbS → Carrier (sickle-cell trait): usually mild or no disease and increased resistance to malaria.

Explanation: The heterozygote (HbA HbS) has a survival advantage in malaria-endemic regions — a classic example of overdominance where heterozygote fitness > both homozygotes.

COMPARISON SUMMARY

Type	Heterozygote phenotype	Example
Complete dominance	Shows dominant phenotype	Mendel’s pea flower (Pp = purple)
Incomplete dominance	Intermediate (blend)	Snapdragon (Rr = pink)
Co-dominance	Both traits expressed equally	AB blood group (IAIB)
Overdominance	Heterozygote superior (greater fitness)	Sickle-cell trait (malaria resistance)

Gene Interaction (Intergenic Interaction)

INTRODUCTION

Mendel explained inheritance of traits controlled by single genes with clear dominant and recessive relationships. However, in many traits, a single character is not governed by just one gene but by the interaction of two or more non-allelic genes. This phenomenon is called gene interaction or intergenic interaction.

Gene interaction alters classical Mendelian ratios and results in new phenotypic expressions.

CHARACTERISTICS OF GENE INTERACTION

• Involves non-allelic genes located at different loci.
• Leads to modification of Mendelian ratios such as 9:7, 15:1, 12:3:1, etc.
• Often produces novel phenotypes.
• Different types include duplicate, complementary, supplementary, inhibitory, masking, polymeric, and additive.
• Has a biochemical or molecular basis involving multiple enzymes or regulators in a pathway.

GENE INTERACTION FOR COMB SHAPE IN POULTRY

Comb shape in chickens is controlled by two non-allelic genes: R and P.

• RRpp → Rose comb
• rrPP → Pea comb
• RRPP or RrPp → Walnut comb (novel phenotype due to interaction)
• rrpp → Single comb

Explanation: Walnut comb appears only when both dominant alleles (R and P) are present together. This shows complementary gene interaction.

TYPES OF GENE INTERACTION

1. Duplicate Gene Interaction

Two different non-allelic dominant genes produce the same phenotype independently. Either one is sufficient for expression.
Modified Ratio: 15:1
Example: Seed capsule shape in Shepherd’s purse (Capsella bursa-pastoris): Either gene A or B gives triangular capsule; only aabb produces ovoid capsule.

2. Complementary Gene Interaction

Two non-allelic dominant genes are required together for phenotype expression. Absence of either prevents full expression.
Modified Ratio: 9:7
Example: Flower color in Sweet pea (Lathyrus odoratus): Both C and P required for purple color. Any cc or pp results in white flowers.

3. Supplementary Gene Interaction

One gene produces a particular phenotype while another modifies or enhances it.
Modified Ratio: 9:3:4 or 9:3:1
Example: Coat color in mice: Gene A produces pigment (agouti), gene C modifies to black. Recessive cc produces albino.

4. Inhibitory Gene Interaction

A dominant allele of one gene suppresses the expression of another non-allelic gene.
Modified Ratio: 13:3
Example: Leaf color in rice: Gene I inhibits green produced by gene G. II or Ii = white leaves, only iiGG = green.

5. Masking (Epistasis)

One allele at a locus masks the expression of alleles at another locus.

• Dominant epistasis → 12:3:1 (e.g., fruit color in summer squash: W masks yellow/green).
• Recessive epistasis → 9:3:4 (e.g., coat color in mice: cc masks agouti and black → albino).

6. Polymeric Gene Interaction

Two or more non-allelic dominant genes act together for enhanced expression. Each gene alone produces the same phenotype, but combined they produce stronger effect.
Modified Ratio: 9:6:1
Example: Fruit shape in Cucurbita: A_B_ = disc, A_bb/aaB_ = spherical, aabb = long.

7. Additive Gene Interaction

Multiple non-allelic genes contribute additively to phenotype. Each gene has a small cumulative effect.
Phenotypic Result: Continuous variation (quantitative traits).
Example: Kernel color in wheat (Nilsson-Ehle): More dominant alleles = deeper red, all recessives = white.

8. General Interaction

Broad term covering any non-allelic interaction where one gene modifies the effect of another.
Example: Comb shape in poultry (Walnut) demonstrates general gene interaction.

MOLECULAR BASIS OF GENERAL INTERACTION

• Enzymatic pathways: Many traits result from metabolic pathways requiring multiple enzymes coded by different genes (e.g., flower color in sweet pea needs both C and P enzymes).
• Regulatory roles: Some genes act as regulators, turning on/off the expression of others (e.g., inhibitory gene I suppresses pigment gene G in rice).
• Structural proteins: Different structural proteins encoded by separate genes combine to form the phenotype.
• Additive effects: Polygenic traits (height, skin color, yield) arise due to cumulative contributions of many genes.

Thus, gene interaction is the genetic and molecular reality that most traits are controlled by networks of interacting genes rather than isolated single genes.