🧬 Concept: Inheritance of One Gene 🧬

Introduction

What is Inheritance of One Gene? 
Inheritance of one gene refers to the study of how a single trait is passed from parents to offspring. This type of inheritance is also called monohybrid inheritance, where only one pair of contrasting characters is considered at a time.
This concept was first clearly explained by Gregor Johann Mendel, whose experiments formed the basis of classical genetics.

Why Inheritance of One Gene is Important

  • It explains how traits are transmitted across generations
  • It forms the basis of Mendel’s laws
  • It helps in understanding genetic disorders

Basic Genetic Terms (For Beginners)

TermDefinition
GeneUnit of inheritance controlling a character/trait
AlleleAlternative form of same gene
CharacterGeneral feature (e.g., height)
TraitSpecific form of a character (e.g., tall or dwarf)
GenotypeGenetic makeup (TT, Tt, tt)
PhenotypePhysical appearance
HomozygousSame alleles (TT / tt)
HeterozygousDifferent alleles (Tt)
Dominant TraitExpressed in heterozygous
Recessive TraitExpressed only in homozygous

Mendel’s Monohybrid Cross:

The Foundation

To study inheritance of one gene, Mendel selected plant height in pea plants.
Contrasting Traits Studied
  • Tall plant (T) – Dominant
  • Dwarf plant (t) – Recessive

Parental Generation (P Generation)

Mendel crossed:
  • Pure tall plant → TT
  • Pure dwarf plant → tt
Both parents were pure breeding (homozygous).

First Filial Generation (F₁ Generation)

Cross:
  • TT × tt
Gametes:
  • Tall plant → T
  • Dwarf plant → t
Offspring Genotype:
  • All offspring → Tt
Observation:
  • All F₁ plants were tall
Conclusion:
The tall trait is dominant, while dwarf trait is recessive.
This observation led to the Law of Dominance.

Law of Dominance (Based on One Gene Inheritance)

Statement:
When two contrasting alleles are present together, only the dominant allele expresses itself, while the recessive allele remains hidden.

Self-Pollination of F₁ Generation (F₂ Generation)

Mendel allowed F₁ plants (Tt) to self-pollinate.
Cross:
  • Tt × Tt
Gametes Produced:
T and t from each parent

Punnett Square Analysis

Tt
TTTTt
tTttt

Results in F₂ Generation

Genotypic Ratio
  • 1 TT : 2 Tt : 1 tt
Phenotypic Ratio
  • 3 Tall : 1 Dwarf

Law of Segregation (Law of Purity of Gametes)

Statement:
The two alleles of a gene separate during gamete formation, so each gamete carries only one allele. Explanation:
  • Alleles do not blend
  • Each gamete is genetically pure
  • Recessive trait reappears in F₂ generation

Key Conclusions from Inheritance of One Gene:

  1. Characters are controlled by genes
  2. Genes exist in pairs (alleles)
  3. One allele may be dominant
  4. Alleles separate during gamete formation
  5. Traits follow definite ratios

Test Cross (Application of One Gene Inheritance)

A test cross is performed to determine the genotype of an organism showing dominant phenotype.
Cross:
  • Unknown tall plant × dwarf plant (tt)
Results:
  • If all offspring are tall → genotype is TT
  • If offspring are tall and dwarf → genotype is Tt

Chromosomal Basis of One Gene Inheritance:

  • Genes are located on chromosomes
  • Alleles separate during meiosis
  • Behavior of chromosomes explains Mendel’s laws
This connection was explained by Sutton and Boveri.

Limitations of One Gene Inheritance:

Inheritance of one gene explains only:
  • Traits with complete dominance
  • Single gene control
It does not explain:
  • Incomplete dominance
  • Codominance
  • Polygenic traits

Examples of One Gene Inheritance:

  • Plant height in pea plants
  • Seed shape (round/wrinkled)
  • Seed color (yellow/green)

Exam-Oriented Key Points (CBSE & NEET)

  • Monohybrid cross → one trait
  • Phenotypic ratio → 3:1
  • Genotypic ratio → 1:2:1
  • Law of segregation is universally applicable
  • Test cross always uses homozygous recessive parent

Frequently Asked Questions

What is inheritance of one gene?
It is the study of inheritance of a single character controlled by one gene.
Who proposed inheritance of one gene?
Gregor Johann Mendel.
Which ratio is obtained in F₂ generation?
Phenotypic ratio of 3:1.

Importance in Human Genetics:

Understanding one gene inheritance helps in:
  • Studying genetic diseases
  • Predicting inheritance patterns
  • Medical genetics and counseling

Conclusion:

Inheritance of one gene is the foundation of genetics. It explains how traits are passed from parents to offspring using simple rules and predictable ratios.