Gene Interaction

Gene interaction refers to the phenomenon where two or more genes influence a single trait or phenotype. Unlike simple Mendelian inheritance, where one gene controls one trait, gene interaction involves multiple genes working together to determine the expression of a particular trait.  This concept helps explain why many characteristics in living organisms do not follow simple Mendelian ratios, such as 3:1 or 9:3:3:1, but instead exhibit modified ratios due to the combined effects of multiple genes.

1.0Introduction to Gene Interaction

• Gregor Mendel’s experiments with pea plants laid the foundation of genetics. 
• He proposed that each trait is controlled by a single pair of factors (genes) and that they segregate and assort independently. However, later studies revealed that:
• Some traits are controlled by more than one gene.
• Genes can interact to modify the expected phenotypic ratios.
• This deviation from simple Mendelian ratios is known as gene interaction.
• The term was first introduced by William Bateson and R.C. Punnett in the early 20th century when they observed deviations in phenotypic ratios in their experiments with sweet peas (Lathyrus odoratus).

2.0Types of Gene Interaction

Gene interactions are broadly classified into two types:

1. Intragenic (Allelic) Interaction – between alleles of the same gene.
2. Intergenic (Non-allelic) Interaction – between alleles of different genes.

3.01. Intragenic (Allelic) Interaction

This occurs when different alleles of the same gene interact to produce a particular phenotype.

a) Complete Dominance

• One allele completely masks the expression of another.
• The dominant allele is expressed in both homozygous and heterozygous individuals.
• Example: Mendel’s tall (T) and dwarf (t) pea plants.
• • TT and Tt → Tall, tt → Dwarf.

b) Incomplete Dominance

• Neither allele is completely dominant over the other.
• The heterozygote shows an intermediate phenotype.
• Example: In Mirabilis jalapa (four o’clock plant), crossing red (RR) and white (rr) flowers produces pink (Rr) flowers.
• Phenotypic and genotypic ratios are the same: 1:2:1.

c) Codominance

• Both alleles express themselves equally and independently in the heterozygote.
• Example: ABO blood group in humans.
• • IA and IB alleles are codominant, producing blood group AB when both are present (IAIB).
• The ratio remains 1:2:1, but the phenotypes differ.

4.02. Intergenic (Non-Allelic) Interaction

This occurs when two or more genes at different loci interact to produce a single phenotype. Such interactions often modify the classic 9:3:3:1 dihybrid ratio seen in Mendel’s experiments.

a) Complementary Gene Interaction (9:7 Ratio)

In this type, two different genes complement each other to produce a phenotype. Both dominant alleles are required for the expression of a particular trait.

• Example: Flower colour in sweet pea (Lathyrus odoratus).
• • Genes C and P are required for purple flower colour.
  • Any plant lacking either gene (cc or pp) produces white flowers.
  • Phenotypic ratio: 9 purple: 7 white.
  • Explanation:
    C and P together → purple colour.
    cc or pp → white (no colour).

b) Epistasis

Epistasis occurs when one gene masks or suppresses the expression of another gene at a different locus. The gene that masks is called epistatic, and the one that gets masked is hypostatic.

(i) Dominant Epistasis (12:3:1 Ratio)

• A dominant allele of one gene suppresses the expression of another gene.
• Example: Fruit colour in squash.
• • Gene A (white colour) is epistatic over gene B (yellow colour).
  • Phenotypic ratio: 12 white : 3 yellow: 1 green.

(ii) Recessive Epistasis (9:3:4 Ratio)

• When the recessive allele of one gene masks the expression of another gene.
• Example: Coat colour in mice.
• • Gene C (pigment production) and gene A (distribution of pigment).
  • cc (no pigment) → white, regardless of gene A.
  • Phenotypic ratio: 9 agouti : 3 black: 4 white.

c) Duplicate Gene Interaction (15:1 Ratio)

When two genes perform the same function, and one dominant allele of either gene is sufficient to express the trait.

• Example: Fruit shape in Shepherd’s purse (Capsella bursa-pastoris).
• Phenotypic ratio: 15 triangular: 1 ovoid.

Explanation:
As long as at least one dominant allele (A or B) is present, the dominant trait appears.
Only the double recessive (aabb) shows the recessive phenotype.

d) Inhibitory Gene Interaction (13:3 Ratio)

A dominant gene suppresses the expression of a non-allelic gene, resulting in a modified 13:3 ratio.

• Example: Colour in chicken feathers.
• • Gene I inhibits colour expression controlled by gene C.
  • I_C_ or I_cc → white,
  • iiC_ → colored feathers.
  • Phenotypic ratio: 13 white : 3 colored.

e) Duplicate Recessive Epistasis (9:7 Ratio)

Sometimes, both genes must have at least one dominant allele for a trait to be expressed.
If either gene is homozygous recessive, the trait is not expressed.
This is the same as complementary gene action discussed earlier.

f) Polymeric Gene Interaction (9:6:1 Ratio)

When two genes interact additively to influence a trait, their combined effect differs from the sum of their individual effects.

• Example: Fruit shape in gourds.
• • A and B together → disk-shaped fruit,
  • A or B alone → spherical fruit,
  • aabb → long fruit.
  • Phenotypic ratio: 9 disk: 6 sphere: 1 long.

5.0Significance of Gene Interaction

• Explains deviations from simple Mendelian ratios.
• Demonstrates that traits are controlled by multiple genes rather than a single gene.
• Provides insight into complex inheritance patterns, such as skin colour, height, and intelligence, in humans.
• Helps breeders and geneticists predict phenotypic outcomes in hybridisation experiments.
• Forms the basis for quantitative genetics and polygenic inheritance.