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Principles of Inheritance and Variation – Complete NCERT Notes | Class 12 Biology | NEET 2026



Keywords




Principles of Inheritance and Variation Notes, Mendelian Genetics Notes, Class 12 Genetics Notes, Inheritance and Variation NCERT Notes, Blood Group Genetics, Sex Linked Inheritance Notes, Genetic Disorders Notes, NEET 2026
Concept Definition / NCERT Point Example Ratio / Result
Monohybrid Cross Cross involving one character. TT × tt F₁: All Tall (Tt)
F₂ Genotypic: 1:2:1
Phenotypic: 3:1
Law of Dominance One allele expresses itself and suppresses the other. T dominant over t Dominant trait appears in F₁.
Law of Segregation Alleles separate during gamete formation. Law of Purity of Gametes Each gamete receives one allele.
Dihybrid Cross Cross involving two characters. Round Yellow × Wrinkled Green F₂ Ratio = 9 : 3 : 3 : 1
Law of Independent Assortment Genes of different characters assort independently during gamete formation. Dihybrid Cross Produces new combinations.
Incomplete Dominance Neither allele is completely dominant. Snapdragon (Red × White) Pink (F₁)
F₂ Ratio = 1 Red : 2 Pink : 1 White
Codominance Both alleles express simultaneously. Human ABO Blood Group IA and IB are codominant.
Multiple Alleles More than two alleles govern a character. ABO Blood Group Alleles: IA, IB, i
Blood Group Genotype A = IAIA or IAi
B = IBIB or IBi
AB = IAIB
O = ii
Pleiotropy One gene influences multiple traits. Phenylketonuria Single gene → Multiple effects
Chromosomal Theory Genes are located on chromosomes. Sutton & Boveri Basis of modern genetics.
Linkage Genes on the same chromosome tend to inherit together. Thomas Hunt Morgan Linked genes show reduced recombination.
Recombination Formation of new gene combinations. Crossing Over Occurs during meiosis.
Human Sex Determination Chromosomal mechanism Male = XY
Female = XX		 Father determines sex of offspring.




Principles of Inheritance and Variation is one of the highest-scoring chapters in NEET Biology. Questions are frequently asked from Mendel's laws, monohybrid cross, dihybrid cross, incomplete dominance, codominance, blood groups, linkage, and genetic disorders.

Introduction

Inheritance is the transmission of traits from parents to offspring.

Gregor Johann Mendel
Known as:
Father of Genetics

Experimented on:
Garden Pea (Pisum sativum)

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Why Mendel Chose Pea Plant?

Easy cultivation
Short life cycle
Self-pollinating
Distinct contrasting characters

Seven Contrasting Traits
Character
Dominant
Recessive
Seed Shape
Round
Wrinkled
Seed Color
Yellow
Green
Flower Color
Violet
White
Pod Shape
Inflated
Constricted
Pod Color
Green
Yellow
Flower Position
Axial
Terminal
Stem Height
Tall
Dwarf
Important Terms

Gene
Unit of inheritance.

Alleles
Alternative forms of a gene.
Example:
T = Tall
t = Dwarf

Homozygous
Same alleles.
Examples:
TT
tt

Heterozygous
Different alleles.
Example:
Tt

Monohybrid Cross

Cross involving one character.
Example
TT × tt

F₁ Generation
All Tall (Tt)

F₂ Generation
Genotypic Ratio:
1 TT : 2 Tt : 1 tt
Phenotypic Ratio:
3 Tall : 1 Dwarf

Law of Dominance


States that in a heterozygote, one allele expresses itself and suppresses the other.
Example
T dominates over t.

Law of Segregation

Alleles separate during gamete formation.

NCERT Point
Also called Law of Purity of Gametes.

Dihybrid Cross

Cross involving two characters.
Example
Round Yellow × Wrinkled Green
F₂ Phenotypic Ratio
9 : 3 : 3 : 1

Law of Independent Assortment

Genes of different characters assort independently during gamete formation.
Derived From
Dihybrid cross.

Incomplete Dominance

Neither allele completely dominant.

Example

Snapdragon (Antirrhinum)
Red × White
↓
Pink
F₂ Ratio
1 Red : 2 Pink : 1 White

Codominance

Both alleles express simultaneously.
Example
Human ABO Blood Group
IA and IB are codominant.

Multiple Alleles

Example
ABO Blood Group
Alleles:
IA
IB
i
Blood Groups
Blood Group
Genotype
A
IAIA or IAi
B
IBIB or IBi
AB
IAIB
O
ii

NCERT Point

AB blood group shows codominance.

Pleiotropy

One gene influences multiple traits.
Example
Phenylketonuria

Chromosomal Theory of Inheritance

Proposed by:
Walter Sutton
Theodor Boveri
Main Idea
Genes are located on chromosomes.
Linkage
Genes present on same chromosome tend to inherit together.
Discovered by
Thomas Hunt Morgan

Recombination
Formation of new gene combinations.
Occurs due to crossing over.

Sex Determination

Human Sex Determination
Male:
XY
Female:
XX

NCERT Point
Father determines sex of child.

Sex-Linked Inheritance

Genes present on sex chromosomes.
Haemophilia
Cause
Recessive gene on X chromosome.
Symptoms
Blood clotting defect.

Colour Blindness
Cause
X-linked recessive trait.
Symptoms
Inability to distinguish colors.

Pedigree Analysis

Study of inheritance pattern across generations.
Symbols

Square = Male
Circle = Female
Filled Symbol = Affected

Mendelian Disorders
Haemophilia
Blood clotting disorder.
Colour Blindness
Vision disorder.
Phenylketonuria

Metabolic disorder.
Sickle Cell Anaemia
Mutation in ฮฒ-globin gene.

NCERT Point

Autosomal recessive disorder.
Chromosomal Disorders

Down Syndrome
Cause
Trisomy 21
Chromosomes
47

Klinefelter Syndrome
Karyotype
XXY
Chromosomes
47

Turner Syndrome
Karyotype
XO
Chromosomes
45

Most Important NCERT Points

✔ Mendel is Father of Genetics.
✔ Monohybrid ratio = 3:1
✔ Dihybrid ratio = 9:3:3:1
✔ Law of Segregation = Purity of Gametes.
✔ AB blood group shows codominance.
✔ Snapdragon demonstrates incomplete dominance.
✔ Father determines sex of child.
✔ Sickle cell anaemia is autosomal recessive.

NEET PYQ Practice Questions

Q1. Father of Genetics is:
A. Darwin
B. Mendel
C. Morgan
D. Sutton
✅ Answer: B. Mendel

Q2. Phenotypic ratio in monohybrid cross:
A. 9:3:3:1
B. 1:2:1
C. 3:1
D. 1:1
✅ Answer: C. 3:1

Q3. Example of incomplete dominance:
A. Pea Plant
B. Snapdragon
C. Blood Group
D. Colour Blindness
✅ Answer: B. Snapdragon

Q4. AB Blood Group exhibits:
A. Dominance
B. Recessiveness
C. Codominance
D. Linkage
✅ Answer: C. Codominance

Q5. Down Syndrome results from:
A. Trisomy 21
B. XO
C. XXY
D. Mutation
✅ Answer: A. Trisomy 21

Last-Minute NEET Revision

Must Revise
Mendel's Laws
Monohybrid Cross
Dihybrid Cross
Blood Groups
Linkage
Pedigree Analysis
Sex-linked Inheritance
Genetic Disorders
Chromosomal Disorders

Conclusion

Principles of Inheritance and Variation is one of the most important NEET chapters. Questions are directly asked from Mendelian genetics, blood groups, linkage, and genetic disorders. Master the ratios, examples, and NCERT terminology to score easy marks in NEET Biology.
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