🧬 Chapter 4: Principles of Inheritance and Variation — Summery Notes | NCERT + NEET Focus

 

🌸 Chapter 2

 Human Reproduction 

(SUMMARY NOTES )

🌱 1. INTRODUCTION

• Genetics: Branch of biology dealing with heredity and variation.
• Heredity: Transmission of traits from parents to offspring.
• Variation: Differences among individuals of same species.
• Father of Genetics: Gregor Johann Mendel (1822–1884).
• Experimental plant: Pisum sativum (garden pea).
• Published results in 1865, rediscovered in 1900 by de Vries, Correns, and Tschermak.

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🧩 2. MENDEL’S EXPERIMENTS

🔹 Mendel chose 7 contrasting traits in pea:

Character	Dominant Trait	Recessive Trait
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

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⚖️ 3. KEY TERMS

• Gene: Unit of inheritance controlling a character.
• Alleles: Alternate forms of a gene.
• Dominant allele: Expresses in heterozygous condition.
• Recessive allele: Hidden in heterozygous condition.
• Genotype: Genetic constitution of an organism.
• Phenotype: Observable physical expression.
• Homozygous: Both alleles identical (TT or tt).
• Heterozygous: Different alleles (Tt).
• Monohybrid cross: Cross involving one character.
• Dihybrid cross: Cross involving two characters.

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⚗️ 4. MENDEL’S LAWS

Law 1 – Law of Dominance

One factor (allele) dominates the other in heterozygous condition.
Example: Tt → tall.

Law 2 – Law of Segregation (Law of Purity of Gametes)

Two alleles of a gene separate during gamete formation; each gamete receives only one allele.
F2 Ratio: Genotype 1:2:1, Phenotype 3:1.

Law 3 – Law of Independent Assortment

Alleles of different genes assort independently during gamete formation.
F2 Dihybrid Ratio: 9:3:3:1 (Round Yellow : Round Green : Wrinkled Yellow : Wrinkled Green)

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🧬 5. MONOHYBRID AND DIHYBRID CROSSES

🧩 Monohybrid Cross:

• F1 Generation: All heterozygous, showing dominant trait.
• F2 Generation: 3 (dominant) : 1 (recessive).

🧩 Dihybrid Cross:

• Traits: Seed shape and color.
• F1: All round yellow.
• F2: 9:3:3:1 (Round Yellow : Round Green : Wrinkled Yellow : Wrinkled Green).

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🧫 6. TEST CROSS

Crossing F1 hybrid with recessive parent.
Purpose: To determine genotype of unknown organism.
Ratio obtained: 1:1 (Phenotypic and Genotypic).

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⚗️ 7. NON-MENDELIAN INHERITANCE

Type	Example	Description
Incomplete dominance	Snapdragon (Antirrhinum majus)	Heterozygote shows intermediate phenotype (Red × White → Pink).
Co-dominance	ABO Blood group	Both alleles express equally (IAIB → AB blood group).
Multiple allelism	ABO system (IA, IB, i)	More than two alleles for a single gene.
Pleiotropy	Sickle cell anemia	One gene affects multiple traits.
Polygenic inheritance	Skin color, height	Traits controlled by multiple genes.

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🧫 8. BLOOD GROUP INHERITANCE

Genotype	Blood Group	Antigen	Antibody
IAIA / IAi	A	A	Anti-B
IBIB / IBi	B	B	Anti-A
IAIB	AB	A & B	None
ii	O	None	Anti-A, Anti-B

• Universal Donor: O
• Universal Recipient: AB

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🧬 9. SEX DETERMINATION

Species	Male	Female	Type
Human	XY	XX	XX–XY type
Drosophila	XY	XX	XX–XY type
Birds	ZZ	ZW	ZZ–ZW type
Grasshopper	XO	XX	XO–XX type

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🧩 10. SEX-LINKED INHERITANCE

Disorder	Gene Location	Inheritance Pattern
Colour blindness	X chromosome	X-linked recessive
Haemophilia	X chromosome	X-linked recessive
Thalassemia	Autosomal	Recessive
Sickle-cell anaemia	Autosomal	Co-dominant

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🧠 11. CHROMOSOMAL THEORY OF INHERITANCE

• Proposed by Sutton and Boveri (1902).
• Genes are located on chromosomes.
• Behavior of chromosomes during meiosis parallels gene segregation.
• Therefore, chromosomes are carriers of genes.

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🔗 12. LINKAGE AND RECOMBINATION

• Linkage: Tendency of genes located on same chromosome to be inherited together.
• Studied by T.H. Morgan in Drosophila.
• Recombination: Formation of new allele combinations due to crossing over.

Type of Linkage	Description
Complete linkage	Genes inherited together (no recombination)
Incomplete linkage	Some recombination occurs

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💢 13. MUTATION

• Mutation: Sudden heritable change in DNA sequence.
• Types:
• Point mutation – change in a single base (e.g., sickle cell).
• Chromosomal mutation – change in structure or number of chromosomes.

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🧬 14. PEDIGREE ANALYSIS

• Diagram showing inheritance pattern of a trait across generations.
• Used to study inheritance of traits in humans.

Symbols:
⬜ = Male
⚫ = Female
○ = Unaffected female
● = Affected female
■ = Affected male

Patterns:

• Autosomal dominant – appears in every generation.
• Autosomal recessive – skips generations.
• X-linked – mostly males affected.

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🧠 15. IMPORTANT DEFINITIONS (NCERT Keywords)

• Allele: Alternative form of a gene.
• Homozygote: Identical alleles.
• Heterozygote: Different alleles.
• Mutation: Sudden change in genetic material.
• Recombination: New gene combination after crossing over.
• Linkage: Inheritance of linked genes together.
• Pleiotropy: One gene → multiple effects.
• Polygenic inheritance: Multiple genes → one trait.
• Pedigree: Family tree analysis of inheritance.
• Genome: Complete set of genes in an organism.

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✏️ 16. DIAGRAMS TO PRACTICE

1.   Monohybrid cross (Mendel’s experiment)

2.   Dihybrid cross (Punnett square 9:3:3:1)

3.   Incomplete dominance in Snapdragon

4.   Co-dominance in ABO blood group

5.   Chromosomal theory (Sutton and Boveri)

6.   Linkage and recombination in Drosophila

7.   Pedigree chart symbols and examples

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🧾 17. QUICK RECALL TABLE

Concept	Example	Key Ratio
Monohybrid cross	Tall × Dwarf	3:1
Dihybrid cross	Round Yellow × Wrinkled Green	9:3:3:1
Incomplete dominance	Red × White (Snapdragon)	1:2:1
Co-dominance	Blood group AB	Both expressed
Test cross	F1 × recessive	1:1
Pleiotropy	Sickle-cell	One gene, multiple effects
Polygenic inheritance	Skin color	Continuous variation

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🧠 18. HOTS / NEET-FOCUSED KEY POINTS

• Alleles segregate during meiosis I (Anaphase I).
• Recombination frequency = % of crossing over.
• Chromosomal theory → parallels between genes & chromosomes.
• Mutations can be spontaneous or induced.
• Morgan = discovered linkage & recombination.
• Sickle cell anemia → mutation in β-globin gene (GAG → GUG).

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🩸 19. FORMULAS / DATA

• Recombination frequency (%) =
  (Number of recombinants ÷ Total offspring) × 100
• 1 map unit = 1% recombination = 1 centiMorgan (cM).

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🏁 20. SUMMARY IN ONE VIEW

Concept	Law/Example	Ratio
Law of Dominance	Tt → Tall	-
Law of Segregation	Monohybrid	3:1
Law of Independent Assortment	Dihybrid	9:3:3:1
Incomplete dominance	Snapdragon	1:2:1
Co-dominance	ABO blood group	-
Linkage	Morgan, Drosophila	-
Recombination	Crossing over	-
Pleiotropy	Sickle cell	-

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✅ Tip for Board + NEET:

• Focus on Mendel’s laws, Non-Mendelian inheritance, Blood groups, Linkage, Chromosomal theory, and Sex-linked disorders.
• Revise Punnett squares and ratios.
• Practice pedigree charts and conceptual MCQs.