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 Biology Notes, Mendel Laws PYQ.
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.
Variation refers to differences among individuals of the same species.
Gregor Johann Mendel
Known as:
Father of Genetics
Experimented on:
Garden Pea (Pisum sativum)
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
More than two alleles govern a character.
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 β-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.
✔ Down syndrome = Trisomy 21.
✔ 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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