Principles of Inheritance and Variation – NEET Notes with Charts & Tricks

Principles of Inheritance and Variation

NCERT-Based NEET Biology Notes, Charts & Tricks

📚 Quick Summary

Principles of Inheritance and Variation is one of the most important chapters of Class 12 Biology for NEET preparation. The chapter explains how genetic information is transmitted from parents to offspring and how variations arise among individuals.

Important topics include Mendel's experiments, laws of inheritance, monohybrid and dihybrid crosses, incomplete dominance, codominance, multiple alleles, pleiotropy, polygenic inheritance, linkage, crossing over, sex determination, pedigree analysis and genetic disorders.

These notes are designed for NCERT revision, NEET 2027 preparation, quick concept review and MCQ practice.

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Principles of Inheritance and Variation: Introduction

Principles of Inheritance and Variation explains the basic principles by which characters are inherited from parents to offspring. Inheritance refers to the transmission of genetic information, while variation refers to differences among individuals of the same species.

For NEET Biology, this chapter is especially important because questions can test Mendelian ratios, genetic crosses, terminology, linkage, sex determination and genetic disorders.

Chapter at a Glance

Topic	Key Point
Mendel's Experiments	Foundation of genetics
Law of Dominance	Dominant allele expresses in heterozygote
Law of Segregation	Alleles separate during gamete formation
Independent Assortment	Alleles of different genes may assort independently
Incomplete Dominance	Heterozygote shows intermediate phenotype
Codominance	Both alleles express in heterozygote
Linkage	Genes on the same chromosome tend to be inherited together
Genetic Disorders	Single-gene and chromosomal disorders

1. Mendel's Experiments

Gregor Johann Mendel performed his famous experiments on the garden pea, Pisum sativum. His experiments established the basic principles of inheritance.

Why Did Mendel Select Pea Plants?

Short generation time
Many clearly distinguishable traits
Several contrasting characters
Easy to cultivate
Natural self-pollination
Cross-pollination can be performed easily

🧠 NEET Memory Trick:
TASS → Traits, Annual, Self-pollinating, Simple to grow

2. Important Genetic Terms

Term	Meaning
Gene	Functional unit of heredity
Allele	Alternative form of a gene
Genotype	Genetic constitution of an individual
Phenotype	Observable characteristics
Homozygous	Both alleles are identical
Heterozygous	Two alleles are different
Dominant	Allele expressed in a heterozygote
Recessive	Allele masked in a heterozygote

3. Mendel's Laws of Inheritance

1️⃣ Law of Dominance

When two contrasting alleles are present together in a heterozygote, only one allele may express itself phenotypically. The expressed allele is called the dominant allele.

2️⃣ Law of Segregation

The two alleles of a gene separate from each other during gamete formation. Each gamete receives only one allele of a gene.

NEET Point: The law of segregation is also called the law of purity of gametes.

3️⃣ Law of Independent Assortment

During gamete formation, alleles of different genes can assort independently, provided the genes are not linked in a way that prevents independent assortment.

4. Monohybrid Cross

A monohybrid cross involves the inheritance of a single character.

Example: Tall pea plant × Dwarf pea plant

TT × tt

↓

F₁: All Tt (Tall)

↓ Selfing of F₁

Tt × Tt

↓

F₂ Genotypic Ratio = 1 TT : 2 Tt : 1 tt

F₂ Phenotypic Ratio = 3 Tall : 1 Dwarf

🧠 NEET Must Remember:
Genotypic ratio → 1 : 2 : 1
Phenotypic ratio → 3 : 1

5. Dihybrid Cross

A dihybrid cross studies the inheritance of two characters simultaneously.

Mendel studied seed shape and seed colour in pea plants.

A typical F₂ dihybrid cross gives the phenotypic ratio:

9 : 3 : 3 : 1

Dominant-Dominant : Dominant-Recessive : Recessive-Dominant : Recessive-Recessive

NEET Trick:
Dihybrid F₂ → 9 : 3 : 3 : 1

6. Test Cross

A test cross is performed between an individual showing a dominant phenotype and a homozygous recessive individual to determine the unknown genotype.

Example: Tt × tt

Expected phenotypic ratio: 1 Tall : 1 Dwarf

7. Back Cross

A back cross involves crossing the F₁ hybrid with either of its parents.

A test cross is a specific type of back cross in which the F₁ or unknown dominant phenotype is crossed with the recessive parent.

8. Incomplete Dominance

In incomplete dominance, neither allele completely dominates the other. The heterozygote shows an intermediate phenotype.

A classic example is flower colour in snapdragon.

Genotype	Phenotype
RR	Red
Rr	Pink
rr	White

F₂ phenotypic ratio = 1 : 2 : 1

9. Codominance

In codominance, both alleles express themselves simultaneously in the heterozygote. There is no blending of the two alleles.

The AB blood group in humans is a classic example. The I^A and I^B alleles are codominant.

NEET Trick: Co = Co-exist → both alleles express

10. Multiple Alleles

When a gene has more than two alternative forms in a population, the condition is described as multiple allelism.

The human ABO blood group system is an important example. The alleles are I^A, I^B and i.

Genotype	Blood Group
I^AI^A or I^Ai	A
I^BI^B or I^Bi	B
I^AI^B	AB
ii	O

11. Pleiotropy

Pleiotropy occurs when a single gene influences multiple phenotypic traits.

An important example is the HbS allele, which is associated with sickle-cell anaemia and can influence multiple phenotypic effects.

Mnemonic: Pleiotropy = Plenty of Effects

12. Polygenic Inheritance

In polygenic inheritance, a single trait is controlled by two or more genes.

Human skin colour is a commonly discussed example of polygenic inheritance and shows continuous variation.

Mnemonic: Polygenic = Pile of Genes

13. Linkage and Crossing Over

Linkage refers to the tendency of genes located close together on the same chromosome to be inherited together.

Genes located farther apart on the same chromosome have a greater probability of being separated by crossing over.

Crossing Over

Crossing over is the exchange of genetic material between homologous chromosomes during pachytene of prophase I of meiosis. It contributes to genetic recombination.

NEET Tip:
Closer genes → stronger linkage
Farther genes → greater chance of recombination

14. Sex Determination in Humans

Humans have XX chromosomes in females and XY chromosomes in males.

The female produces only X-bearing ova, whereas the male produces X-bearing and Y-bearing sperm.

X sperm + X ovum → XX female

Y sperm + X ovum → XY male

Therefore, the sperm determines the chromosomal sex of the offspring.

15. Pedigree Analysis

A pedigree is a diagram used to study the inheritance of a particular trait through several generations of a family.

Square → Male
Circle → Female
Shaded symbol → Usually affected individual
Unshaded symbol → Usually unaffected individual
Horizontal line → Mating relationship
Vertical line → Parent-offspring relationship

16. Mendelian Disorders

Mendelian disorders are generally caused by alterations in a single gene and may follow specific inheritance patterns.

Disorder	Inheritance	Key Point
Haemophilia	X-linked recessive	Blood clotting disorder
Colour blindness	X-linked recessive	Difficulty distinguishing certain colours
Sickle-cell anaemia	Autosomal recessive	Associated with HbS
Phenylketonuria	Autosomal recessive	Metabolic disorder
Thalassemia	Autosomal recessive	Reduced globin-chain synthesis

17. Chromosomal Disorders

Chromosomal disorders result from abnormalities in chromosome number or structure. Numerical abnormalities can arise due to non-disjunction during cell division.

Disorder	Karyotype	Abnormality
Down syndrome	47, +21	Trisomy 21
Klinefelter syndrome	47, XXY	Extra X chromosome
Turner syndrome	45, XO	Monosomy X
Edwards syndrome	47, +18	Trisomy 18
Patau syndrome	47, +13	Trisomy 13

🧠 NEET Trisomy Trick

Down → 21
Edwards → 18
Patau → 13

Remember: 21 – 18 – 13

🔥 NEET High-Yield Revision Points

Mendel worked on Pisum sativum.
Monohybrid F₂ phenotypic ratio → 3 : 1.
Monohybrid F₂ genotypic ratio → 1 : 2 : 1.
Dihybrid F₂ phenotypic ratio → 9 : 3 : 3 : 1.
Test cross with a heterozygote → 1 : 1 phenotypic ratio.
Incomplete dominance → intermediate phenotype.
Codominance → both alleles express.
AB blood group demonstrates codominance and multiple allelism.
Pleiotropy → one gene affects multiple traits.
Polygenic inheritance → multiple genes contribute to one trait.
Linked genes are located on the same chromosome.
Crossing over occurs during prophase I of meiosis.
Human female → XX.
Human male → XY.
Down syndrome → Trisomy 21.
Klinefelter syndrome → 47, XXY.
Turner syndrome → 45, XO.
Edwards syndrome → Trisomy 18.
Patau syndrome → Trisomy 13.

18. NEET Practice MCQs

1. Mendel performed his experiments mainly on:

A. Drosophila
B. Pisum sativum
C. Neurospora
D. E. coli

✅ Answer: B. Pisum sativum

2. The phenotypic ratio of a typical monohybrid F₂ generation is:

A. 1 : 1
B. 1 : 2 : 1
C. 3 : 1
D. 9 : 3 : 3 : 1

✅ Answer: C. 3 : 1

3. The genotypic ratio in a typical monohybrid F₂ generation is:

A. 3 : 1
B. 1 : 1
C. 1 : 2 : 1
D. 9 : 3 : 3 : 1

✅ Answer: C. 1 : 2 : 1

4. The F₂ phenotypic ratio of a typical dihybrid cross is:

A. 3 : 1
B. 1 : 2 : 1
C. 1 : 1
D. 9 : 3 : 3 : 1

✅ Answer: D. 9 : 3 : 3 : 1

5. AB blood group is an example of:

A. Complete dominance only
B. Codominance
C. Incomplete dominance only
D. Polygenic inheritance

✅ Answer: B. Codominance

6. Which condition represents incomplete dominance?

A. ABO blood group
B. Snapdragon flower colour
C. Haemophilia
D. Down syndrome

✅ Answer: B. Snapdragon flower colour

7. A single gene affecting multiple traits is called:

A. Polygenic inheritance
B. Pleiotropy
C. Linkage
D. Codominance

✅ Answer: B. Pleiotropy

8. Genes located close together on the same chromosome show:

A. Independent assortment
B. Linkage
C. Mutation only
D. Polyploidy

✅ Answer: B. Linkage

9. Down syndrome is associated with:

A. Trisomy 13
B. Trisomy 18
C. Trisomy 21
D. Monosomy X

✅ Answer: C. Trisomy 21

10. The karyotype 45, XO represents:

A. Down syndrome
B. Turner syndrome
C. Klinefelter syndrome
D. Patau syndrome

✅ Answer: B. Turner syndrome

📌 One-Minute Genetics Revision

Mendel → Pea plant
Monohybrid → 3 : 1
Genotype → 1 : 2 : 1
Dihybrid → 9 : 3 : 3 : 1
Test cross → 1 : 1
Incomplete dominance → Intermediate phenotype
Codominance → Both alleles express
Multiple alleles → ABO blood group
Pleiotropy → One gene, many effects
Polygenic → Many genes, one trait
Linkage → Genes on same chromosome
Crossing over → Prophase I
Female → XX
Male → XY
Down → 21
Edwards → 18
Patau → 13
Turner → XO
Klinefelter → XXY

🌿 NEET Biology for Odisha Students

Students preparing for NEET in Odisha can use these NCERT-based genetics notes for Class 12 Biology revision and regular MCQ practice.

For students in Cuttack, Bhubaneswar, Puri, Sambalpur and other parts of Odisha, a strong NCERT-first strategy combined with previous-year questions and repeated revision can make Genetics easier to manage.

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