Principles of Inheritance and Variation – Class 12 Biology Notes | CBSE NCERT Based

 📚 Chapter: Principles of Inheritance and Variation – Class 12 Biology Notes



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🔸 1. Introduction


Genetics is the branch of biology that deals with heredity and variation.


Heredity: Transmission of characters from parents to offspring


Variation: Differences among individuals




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🔹 2. Mendel and His Experiments


Gregor Mendel is known as Father of Genetics


He worked on garden pea (Pisum sativum)


He selected 7 contrasting traits like tall/dwarf, round/wrinkled etc.




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🔹 3. Mendel’s Laws of Inheritance


a) Law of Dominance


Dominant allele expresses itself in F₁ generation



b) Law of Segregation


Alleles separate during gamete formation



c) Law of Independent Assortment


Genes for different traits assort independently




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🔹 4. Monohybrid Cross


One trait considered


Genotypic ratio: 1:2:1


Phenotypic ratio: 3:1




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🔹 5. Dihybrid Cross


Two traits considered


Phenotypic ratio: 9:3:3:1




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🔹 6. Incomplete Dominance


F₁ is intermediate


Example: Red × White flower = Pink (in Snapdragon)




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🔹 7. Codominance


Both alleles express together


Example: AB blood group (IA + IB = AB)




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🔹 8. Multiple Allelism


More than 2 alleles control a trait


Example: Blood group – IA, IB, i




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🔹 9. Sex Determination


XX-XY system in humans


XX = Female, XY = Male


Male determines the sex of baby




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🔹 10. Genetic Disorders


a) Mendelian Disorders


Haemophilia


Sickle Cell Anaemia


Colour blindness



b) Chromosomal Disorders


Down’s syndrome (Trisomy 21)


Turner’s syndrome (XO)


Klinefelter’s syndrome (XXY)




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📌 Conclusion:


Genetics helps us understa

nd how traits pass from parents to offspring and why differences occur. Mendel’s work laid the foundation for modern biology and medicine.


📊 Important Diagrams – Principles of Inheritance and Variation


1. ✅ Monohybrid Cross (Mendel's pea plant – single trait cross with 3:1 ratio)



2. ✅ Dihybrid Cross (Two trait cross showing 9:3:3:1 ratio)



3. ✅ Punnett Square (Used to predict genotype/phenotype ratio)



4. ✅ Law of Independent Assortment Diagram

(Visual of how different traits assort independently)



5. ✅ Incomplete Dominance Cross

(Red × White = Pink – Snapdragon example)



6. ✅ Codominance Example

(Blood group inheritance – AB from IA and IB)



7. ✅ Multiple Alleles Diagram

(Blood group chart – IA, IB, i combinations)



8. ✅ Sex Determination in Humans

(XX–XY system via Punnett square)



9. ✅ Pedigree Analysis Chart

(Symbols showing inheritance in family tree)



10. ✅ Chromosomal Disorders Karyotypes




Down's syndrome (47, +21)


Turner’s sy

ndrome (XO)


Klinefelter’s syndrome (XXY)


✍️ Written by: Shahid – Study with Shahid

🔗 Visit: https://shahidnotes12.blogspot.co

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