🧬 Chapter 4: Principles of Inheritance and Variation – Class 12 Biology Notes | NCERT + NEET Focus

 

🌸 Chapter 4

Principles of Inheritance and Variation

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🧬 Introduction

Inheritance is the process by which characters or traits are passed from parents to progeny or offspring. It is the basis of heredity.

Variation is the degree by which progeny differ from their parents.
Variation refers to the differences among individuals of the same species or between parents and offspring.

🌱 4.1. MENDEL'S LAWS OF INHERITANCE

Father of Genetics – Gregor Johann Mendel (1822–1884)
Gregor Johann Mendel, conducted hybridisation experiments on garden pea plant (Pisum sativum) for 7 years (1856-1863) and proposed the laws of inheritance in living organisms.

Mendel’s Experimental Work

Mendel chose pea plant (Pisum sativum) because:

• Short life cycle and easy to grow.
• Clearly distinguishable contrasting traits.
• Can be self-pollinated and cross-pollinated easily.
• Large number of seeds produced.

Mendel investigated characters in the garden pea plant that were manifested as two opposing traits ,e.g.,tall or dwarf plants, yellow or green seeds. This allowed him to set up a basic framework of rules governing inheritance.

Seven Pairs of Contrasting Traits studied by Mendel In Pea

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🌾 INHERITANCE OF ONE GENE

Hybridisation experiment carried out by Mendel where he crossed tall and dwarf pea plants to study the inheritance of one gene.

Steps in making a cross in pea:

Fig : Steps in making a cross in pea

He collected the seeds produced as a result of this cross and grew them to generate plants of the first hybrid generation.This generation is also called the Filial₁ Progeny or the F_1.

Mendel observed that all the F₁ progeny plants were tall like one of its parents ; none were dwarf.

Diagramatic representation of monohybrid cross:

Based on his observations on monohybrid crosses Mendel propsed 2 general rules to consolidate his understanding of inheritance in monohybrid crosses.

Today these rules are called the principles or Laws of Inheritance.

Mendel’s Laws of Inheritance:

1. the First law or Law of Dominance

• Characters are controlled by discrete units called factors.
• Factors occur in pairs.
• In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive)
• The law of dominance is used to explain the expression of only one of the parental characters in a monohybrid cross in the F₁  and the expression of the both in the F₂ .It also explains the proportion of 3:1 obtained at the F₂

• In a heterozygote (Aa), only one allele (dominant) is expressed while the other (recessive) is masked.
  Example: TT (tall) × tt (dwarf) → Tt (tall)

2. the Second Law or Law of Segregation (Purity of gametes)

This law is based on the fact that the alleles do not show any blending and that both the characters are recovered as such in the F₂ genaration through one of these is not seen at the F₁ stage .

•  Alleles separate during gamete formation, and each gamete receives only one allele of a pair.
• Represented by Monohybrid cross.

Monohybrid Cross Example:
TT × tt → F₁: Tt (all tall)
F₁ selfed → F₂: 1 TT : 2 Tt : 1 tt → Genotypic ratio = 1:2:1; Phenotypic ratio = 3:1

3. Law of Independent Assortment

• Genes for different traits assort independently of each other during gamete formation.
• Demonstrated by Dihybrid cross.

Example:
R = round, r = wrinkled
Y = yellow, y = green
Parental: RRYY × rryy
F₁: RrYy (Round Yellow)
F₂ ratio → Phenotypic: 9:3:3:1

Traits	Genotype	Count
Round Yellow	R_Y_	9
Round Green	R_yy	3
Wrinkled Yellow	rrY_	3
Wrinkled Green	rryy	1

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🔬 Deviations from Mendelism

1. Incomplete Dominance

• Neither allele is completely dominant; the phenotype is intermediate.
  Example: Snapdragon (Antirrhinum majus)
  RR (red) × rr (white) → F₁: Rr (pink)
  Ratio: Genotypic = Phenotypic = 1:2:1

2. Co-dominance

• Both alleles express equally in the heterozygote.
  Example: ABO blood group – IA and IB alleles both express → AB blood group.

3. Multiple Alleles

• More than two allelic forms of a gene.
  Example: ABO blood group system (IA, IB, i).
• IA and IB are co-dominant; both dominant over i.

4. Pleiotropy

• One gene affects multiple traits.
  Example:
• Sickle cell anemia (HbS gene affects shape + oxygen capacity)
• Phenylketonuria (affects brain + skin pigmentation)

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🧫 Key Genetic Terms

Term	Meaning
Gene	Unit of inheritance controlling a trait
Alleles	Alternative forms of a gene
Homozygous	Same alleles (TT or tt)
Heterozygous	Different alleles (Tt)
Genotype	Genetic constitution (TT, Tt, tt)
Phenotype	Observable character (Tall, Dwarf)
Dominant	Expressed in heterozygous condition
Recessive	Expressed only in homozygous condition
Hybrid	Offspring from cross of different parents
Test Cross	Cross between F₁ hybrid and recessive parent
Back Cross	Cross between hybrid and any parent
Punnett Square	Diagram showing possible genetic combinations

🧬 Chromosomal Theory of Inheritance

Proposed by: Sutton and Boveri (1902)
Main idea: Genes are present on chromosomes; behaviour of chromosomes during meiosis explains Mendel’s laws.

Evidence:

• Both chromosomes and genes occur in pairs.
• Both segregate during gamete formation and recombine during fertilization.

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🧩 Linkage and Recombination

• Linkage: Tendency of genes located on the same chromosome to be inherited together.
• Discovered by: T.H. Morgan (Drosophila)
• Example: Body colour and wing size in fruit fly.
• Recombination: Formation of new gene combinations due to crossing over.

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🧠 Sex Determination

Definition: Mechanism by which sex of an organism is established.

Different Mechanisms:

Type	Example	Male	Female
XX–XY	Humans, Drosophila	XY	XX
ZZ–ZW	Birds	ZZ	ZW
XO	Grasshopper	XO	XX
Haplo–diploidy	Honeybee	Haploid (16)	Diploid (32)

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⚕️ Sex-linked Inheritance

Genes present on sex chromosomes.

1. X-linked disorders

• Colour blindness – recessive X-linked (mother → son)
• Haemophilia – blood clotting disorder due to recessive X-linked gene
• Duchenne Muscular Dystrophy

2. Y-linked traits

• Holandric inheritance – transmitted from father to son (e.g., hairy ears).

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🧬 Genetic Disorders

1. Mendelian Disorders (single gene defect)

Disorder	Defective Gene	Effect
Sickle cell anemia	HbS (β-globin mutation)	RBCs become sickle-shaped
Phenylketonuria	PAH gene	Mental retardation, skin pigmentation loss
Cystic fibrosis	CFTR gene	Thick mucus in lungs
Thalassemia	β-globin gene	Reduced hemoglobin
Haemophilia	X-linked	Lack of clotting factor

2. Chromosomal Disorders

Caused by abnormal number or structure of chromosomes.

Disorder	Cause	Karyotype	Features
Down’s Syndrome	Trisomy 21	47, +21	Mental retardation, short height
Turner’s Syndrome	Monosomy X	45, XO	Female, sterile
Klinefelter’s Syndrome	Extra X in male	47, XXY	Male, tall, sterile

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🧾 Summary Table (Quick Revision)

Concept	Example	Key Ratio
Monohybrid Cross	Tall × Dwarf	3:1
Dihybrid Cross	Round Yellow × Wrinkled Green	9:3:3:1
Incomplete Dominance	Snapdragon	1:2:1
Co-dominance	Blood group AB	Both alleles express
Multiple Alleles	ABO blood group	IA, IB, i
Pleiotropy	Sickle cell	Multiple effects
Linkage	Drosophila	Inherited together
Recombination	Crossing over	New variations

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🧭 Important Diagrams (must draw in exams)

1.   Monohybrid Cross (Punnett Square)

2.   Dihybrid Cross showing 9:3:3:1

3.   Sex Determination in Humans (XX–XY)

4.   Pedigree Chart for Sex-linked Inheritance

5.   Chromosomal Theory (pairing and segregation)

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📘 Exam Tips (Board + NEET)

✅ Always mention scientist name + example + ratio.
✅ Draw cleanly labeled diagrams (2–3 marks guaranteed).
✅ Learn exceptions to Mendel’s laws (frequent NEET questions).
✅ Practice Pedigree analysis (important for NEET).
✅ Revise genetic disorders table – usually a 1-mark question in boards and 1 NEET MCQ.

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🧩 NCERT Keywords (must remember for NEET)

Inheritance, Variation, Gene, Allele, Dominant, Recessive, Segregation, Independent Assortment, Genotype, Phenotype, Linkage, Recombination, Co-dominance, Incomplete Dominance, Multiple Alleles, Pleiotropy, Sex-linked, Trisomy, Monosomy, Mutation.

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