👨‍👧Dominance, Multiple Alleles, and Gene Interaction

Why Dominance and Gene Interaction Matter in Agriculture

When a plant breeder crosses two white-flowered sweet pea varieties and gets coloured flowers in F1, Mendel’s simple 3:1 ratio cannot explain it — gene interaction (complementary genes, 9:7 ratio) can. When a hybrid maize plant outperforms both parents in yield, overdominance is one explanation. And when breeders use self-incompatibility alleles in Brassica crops to produce hybrid seed without manual emasculation, they are exploiting multiple alleles. Understanding these concepts is essential for predicting breeding outcomes beyond simple Mendelian inheritance.

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Types of Dominance

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Co-dominance (No Dominance)

• Both alleles express simultaneously in the heterozygote — neither masks the other.
• Example: AB blood group — I^A I^B genotype produces both antigen A and antigen B on red blood cells.
• Phenotypic ratio = genotypic ratio = 1 : 2 : 1 (each genotype produces a distinct phenotype).

Incomplete (Partial) Dominance

• The dominant allele does not fully suppress the recessive allele; the heterozygote is intermediate.
• Example: Snapdragon (Antirrhinum majus) — Red (RR) x White (rr) → Pink (Rr) in F1. F2 = 1 Red : 2 Pink : 1 White.
• Pink arises because a single dose of the red allele produces only half the pigment.

Complete Dominance

• One allele completely masks the other. Heterozygote (Tt) looks exactly like homozygous dominant (TT).
• Example: Round vs. wrinkled seeds in pea — F2 ratio = 3:1 (Mendel’s observation).

Overdominance

• The heterozygote is phenotypically superior to either homozygous parent.
• Example: In maize, heterozygotes show more pigmentation than either homozygote.
• Overdominance is one proposed genetic explanation for hybrid vigour (heterosis) — why hybrid crops often outyield their inbred parents.

Agricultural significance: Understanding dominance type determines how a breeder handles selection. With complete dominance, a test cross is needed to distinguish TT from Tt. With incomplete dominance, all three genotypes are phenotypically distinct.

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Multiple Alleles

A single gene may have more than two alleles in a population. While any individual carries at most two alleles, the population can harbour many.

Characteristic Features

Examples of Multiple Alleles

1. ABO Blood Groups in Humans

• Discovered by Karl Landsteiner (1900) — Nobel Prize.
• Controlled by gene “I” with three alleles: I^A, I^B, and i.
• I^A and I^B are co-dominant to each other; both are dominant over i.

2. Coat Colour in Rabbits

• Four alleles of gene ‘C’ produce four phenotypes.
• Dominance hierarchy: C (agouti) > c^ch (chinchilla) > c^h (himalayan) > c (albino).

3. Self-Incompatibility Alleles in Plants

• Gene ‘S’ with multiple alleles (S₁, S₂, S₃, S₄, …) — some species have hundreds of S alleles.
• When pollen carries the same S allele as the pistil, pollen tube growth is inhibited → prevents self-fertilisation.
• First described in tobacco; also found in Brassica, radish, tomato, potato.

Agricultural application: Plant breeders exploit self-incompatibility in Brassica crops (cabbage, cauliflower, mustard) to produce hybrid seed without manual emasculation — saving enormous labour costs.

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Penetrance and Expressivity

• Complete penetrance: All individuals with the genotype show the phenotype.
• Incomplete penetrance: Some individuals with the genotype do not show the phenotype.
• Example of incomplete penetrance: Genes for diabetes mellitus — not everyone carrying the genes develops the disease (environmental factors like diet and lifestyle also matter).
• Example of variable expressivity: Polydactyly (extra fingers) — may appear on one hand but not the other in the same individual.
• Both are influenced by genetic background and environment.

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Pleiotropy

• A single gene influences more than one phenotypic trait — called multiple effect of a single gene.
• Occurs because the gene product (protein/enzyme) may participate in multiple biochemical pathways.
• Example: Phenylketonuria (PKU) — a single defective gene causes accumulation of phenylalanine, affecting brain development, skin pigmentation, and other metabolic processes simultaneously (collectively called a syndrome).

Agricultural example: In soybean, the gene controlling seed coat colour also affects hilum colour and flower colour — a classic case of pleiotropy that breeders must account for.

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Lethality

• Lethal genes cause death of certain genotypes prematurely.
• Example: In mice, dominant allele Y (yellow coat) is lethal in homozygous state (YY die).
• Expected 1:2:1 ratio is modified to 2 : 1 (only Yy yellow and yy non-yellow survive).

Agricultural note: Lethal genes exist in crops too. In barley, certain chlorophyll-deficient mutants are lethal in homozygous state — seedlings die because they cannot photosynthesise.

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Gene Interaction — Modified Dihybrid Ratios

When two or more genes affect the same trait, the classic 9:3:3:1 dihybrid ratio gets modified. ^{UPPSC 2021}

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Complementary Genes — 9 : 7

• Discovered by Bateson & Punnett in sweet pea (Lathyrus odoratus).
• Both dominant alleles (from two genes) must be present together for colour. If either is homozygous recessive → white.

Epistasis — One Gene Masks Another

Key distinction: Dominance = interaction between alleles of the same gene (intragenic). Epistasis = interaction between alleles of different genes (intergenic).

Inhibitory Gene Action — 13 : 3

• One dominant inhibitory gene suppresses expression of another dominant gene (does not produce its own phenotype).
• Example: Seed colour in maize — 13 white : 3 red.

Duplicate Genes (Duplicate Epistasis) — 15 : 1

• Either gene in dominant state independently produces the same phenotype.
• Only double homozygous recessive shows the alternative phenotype.
• Observed by G.H. Shull in shepherd’s purse (Capsella) — 15 triangular : 1 top-shaped. ^{RRB-SO 2019}

Polymeric/Additive Gene Action — 9 : 6 : 1

• Both genes contribute additively to the phenotype.
• Example: Duroc-Jersey pig coat colour — 9 red : 6 sandy : 1 white.
• Both dominants present = full expression (red). One dominant from either gene = intermediate (sandy). Neither = minimal (white).

Agricultural significance: Polymeric/additive gene action is the foundation of quantitative genetics — most crop traits (yield, grain weight, plant height) are governed by many genes with small additive effects.

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Summary Cheat Sheet