Inheritance is the process by which traits are passed from parents to offspring. Variation refers to the differences seen among individuals of the same species.
This chapter explains how traits are inherited, why variations occur, and how genetic disorders arise.
Gregor Johann Mendel, known as the Father of Genetics, performed experiments on pea plants (Pisum sativum) and laid the foundation of modern genetics.
This chapter is very important for board exams and NEET, as it includes:
- Mendel’s experiments
- Laws of inheritance
- Deviations from Mendel’s laws
- Sex determination
- Genetic disorders
Class 12 Principles of Inheritance and Variation Notes
Class 12 Principles of Inheritance and Variation Notes | CBSE Biology Chapter 5
1. Mendel’s Experiments on Pea Plant
Mendel selected pea plants because:
- They have distinct contrasting traits
- Short life cycle
- Self-pollinating but can be cross-pollinated
- Large number of seeds
Seven Contrasting Traits Studied by Mendel
| Character | Dominant | Recessive |
|---|---|---|
| Seed shape | Round | Wrinkled |
| Seed colour | Yellow | Green |
| Flower colour | Violet | White |
| Pod shape | Inflated | Constricted |
| Pod colour | Green | Yellow |
| Flower position | Axial | Terminal |
| Stem height | Tall | Dwarf |
2. Important Genetic Terms
- Gene: Unit of inheritance
- Allele: Alternative forms of a gene
- Genotype: Genetic makeup (TT, Tt, tt)
- Phenotype: Physical appearance
- Homozygous: Same alleles (TT / tt)
- Heterozygous: Different alleles (Tt)
- Dominant: Expressed even in heterozygous condition
- Recessive: Expressed only in homozygous condition
3. Monohybrid Cross
A cross involving one pair of contrasting traits.
Example: Plant Height
- Tall (TT) × Dwarf (tt)
F₁ Generation
- All offspring: Tall (Tt)
F₂ Generation
- Genotypic Ratio: 1 TT : 2 Tt : 1 tt
- Phenotypic Ratio: 3 Tall : 1 Dwarf
4. Mendel’s Laws of Inheritance
Law of Dominance
When two contrasting alleles are present together, only the dominant allele expresses itself.
👉 Example: T (Tall) dominates over t (Dwarf)
Law of Segregation
Alleles separate during gamete formation, so each gamete receives only one allele.
👉 Also called the Law of Purity of Gametes
Law of Independent Assortment
Alleles of different genes assort independently during gamete formation.
5. Dihybrid Cross
A cross involving two pairs of contrasting traits.
Example
- Seed Shape (Round/Wrinkled)
- Seed Colour (Yellow/Green)
Phenotypic Ratio in F₂
9 : 3 : 3 : 1
This proves the Law of Independent Assortment.
6. Deviations from Mendel’s Laws
Some inheritance patterns do not follow Mendel’s laws.
Incomplete Dominance
Neither allele is completely dominant.
👉 Example: Flower colour in Snapdragon
- Red × White → Pink (F₁)
Phenotypic ratio = 1 : 2 : 1
Codominance
Both alleles express equally.
👉 Example: AB blood group in humans
IA and IB both express.
Multiple Alleles
More than two alleles exist for a gene.
👉 Example: ABO Blood Group System
- IA, IB, i
7. Pleiotropy
One gene controls multiple traits.
👉 Example: Phenylketonuria (PKU)
8. Chromosomal Theory of Inheritance
Proposed by Sutton and Boveri.
States that:
- Genes are located on chromosomes
- Chromosome behavior explains Mendel’s laws
9. Linkage and Recombination
- Linkage: Genes close together on same chromosome are inherited together
- Recombination: Exchange of genes between homologous chromosomes during meiosis
Higher distance → higher recombination frequency
10. Sex Determination in Humans
Humans follow XX–XY mechanism.
| Parent | Chromosomes |
|---|---|
| Female | XX |
| Male | XY |
- Female produces only X gametes
- Male produces X and Y gametes
- Father determines sex of child
11. Mutation
Sudden change in DNA sequence leading to variation.
Types:
- Gene mutation
- Chromosomal mutation
12. Genetic Disorders
Caused by mutations or chromosomal abnormalities.
Mendelian Disorders
- Haemophilia (X-linked recessive)
- Colour Blindness
- Sickle Cell Anaemia
- Phenylketonuria
Chromosomal Disorders
- Down Syndrome (Trisomy 21)
- Turner Syndrome (XO)
- Klinefelter Syndrome (XXY)
Why This Chapter Is Important for Exams
✔ High weightage in CBSE boards
✔ Frequently asked in NEET
✔ Diagrams, ratios & laws are scoring
Quick Revision Points
- Mendel = Father of Genetics
- Monohybrid ratio = 3:1
- Dihybrid ratio = 9:3:3:1
- Blood group = codominance + multiple alleles
- Father determines sex of baby
⭐ Important & Repeated Questions
(Principles of Inheritance and Variation – Class 12 Biology)
🔹 Very Short Answer Questions (1 Mark)
- Who is known as the Father of Genetics?
- Define allele.
- What is genotype?
- What is phenotype?
- Name the law which states “alleles separate during gamete formation.”
- Write the phenotypic ratio of a monohybrid cross.
- What is codominance?
- Name a chromosomal disorder caused by trisomy of chromosome 21.
- Which chromosome determines sex in humans?
- Define mutation.
🔹 Short Answer Questions (2–3 Marks)
- Explain Mendel’s Law of Dominance with an example.
- What is incomplete dominance? Give one example.
- Differentiate between genotype and phenotype.
- Explain the ABO blood group system.
- What is pleiotropy?
- Write any two differences between linkage and recombination.
- Why did Mendel choose pea plant for his experiments?
- What is pedigree analysis?
- Define multiple alleles with example.
- Explain sex determination in humans.
🔹 Long Answer Questions (4–5 Marks)
- Describe Mendel’s monohybrid cross with a suitable diagram.
- Explain Mendel’s three laws of inheritance.
- Describe a dihybrid cross and state the phenotypic ratio obtained.
- Explain incomplete dominance and codominance with examples.
- Describe chromosomal theory of inheritance.
- Explain sex-linked inheritance taking haemophilia as an example.
- Describe any three Mendelian genetic disorders.
- Explain linkage and recombination.
- What is mutation? Explain its types.
- Describe chromosomal disorders in humans.
🔹 Assertion–Reason (Most Repeated)
- Assertion: Mendel’s Law of Segregation is called the law of purity of gametes.
Reason: Each gamete receives only one allele of a gene. - Assertion: In human beings, father determines the sex of the child.
Reason: Male produces two types of gametes (X and Y). - Assertion: Blood group AB is an example of codominance.
Reason: Both IA and IB alleles express equally.
🔹 Case-Based / NEET-Oriented Questions
- A cross between tall and dwarf pea plants produces tall plants in F₁ generation. Explain why.
- Why does sickle cell anaemia show incomplete dominance?
- How does recombination create variation?
- Why are X-linked disorders more common in males?
🧠 HOTS (Higher Order Thinking Questions)
- Why is the phenotypic ratio altered in incomplete dominance?
- How does linkage affect Mendel’s Law of Independent Assortment?
- Why is phenylketonuria called a pleiotropic disorder?
✅ Conclusion
The chapter “Principles of Inheritance and Variation” explains the basic rules of heredity and the genetic basis of variation in living organisms. Mendel’s experiments laid the foundation of genetics and helped us understand how traits are passed from one generation to the next.
Although Mendel’s laws explain most inheritance patterns, many traits show deviations such as incomplete dominance, codominance, linkage, and multiple alleles. The chapter also highlights the role of chromosomes, sex determination, and genetic disorders, making it highly important for understanding human genetics.
A strong grasp of this chapter helps students score well in CBSE board exams, NEET, and other competitive exams, and builds a solid foundation for advanced biology topics.
We are Completed Class 12 Principles of Inheritance and Variation Notes