Chapter 5: Principles of Inheritance and Variation – Study Modules with Revision Notes
CBSE Class 12 Biology Study Module (NCERT Based): Principles of Inheritance and Variation – Unit II Genetics and Evolution
Course Details (As Per CBSE & NCERT)
- Course: CBSE Class 12 Biology
- Unit: Unit II – Genetics and Evolution
- Chapter: Chapter 5 – Principles of Inheritance and Variation
- Prescribed Textbook: NCERT Biology Class XII
- Examination: CBSE Class 12 Board Examination (Theory & Competency-Based Questions)
Introduction to the Chapter
The chapter Principles of Inheritance and Variation forms the foundation of classical and modern genetics. It explains how traits are transmitted from one generation to the next and why variations occur among individuals of the same species. This chapter integrates experimental genetics, chromosomal behavior, and molecular changes leading to genetic disorders. Questions from this chapter frequently appear in long answer, case-based, numerical, and diagram-based formats in CBSE Board Examinations.
1. Mendel’s Laws of Inheritance
Genetics as a scientific discipline began with the experiments of Gregor Mendel, who studied inheritance patterns in garden pea (Pisum sativum). Mendel selected pea plants due to their distinct contrasting traits, short life cycle, and ease of controlled pollination.
Key Terms
- Gene: Functional unit of inheritance
- Allele: Alternative form of a gene
- Genotype: Genetic constitution
- Phenotype: Observable expression
- Homozygous / Heterozygous
Mendel’s Three Laws
- Law of Dominance
In a heterozygous condition, one allele (dominant) expresses itself, while the other (recessive) remains masked. - Law of Segregation (Law of Purity of Gametes)
Allele pairs separate during gamete formation, ensuring that each gamete receives only one allele. - Law of Independent Assortment
Alleles of different gene pairs assort independently during gamete formation (valid only for genes on different chromosomes or far apart).
2. Inheritance of One Gene (Monohybrid Cross)
A monohybrid cross involves a single pair of contrasting traits.
Example
- Trait: Plant height
- Dominant: Tall (T)
- Recessive: Dwarf (t)
Generational Results
- F₁ Generation: All Tall (Tt)
- F₂ Generation Phenotypic Ratio: 3 Tall : 1 Dwarf
- Genotypic Ratio: 1 TT : 2 Tt : 1 tt
Significance
- Demonstrates the Law of Dominance and Law of Segregation
- Commonly asked in numerical problems
3. Inheritance of Two Genes (Dihybrid Cross)
A dihybrid cross considers two pairs of contrasting traits simultaneously.
Example
- Seed shape: Round (R) / Wrinkled (r)
- Seed color: Yellow (Y) / Green (y)
F₂ Phenotypic Ratio
9 : 3 : 3 : 1
Significance
- Establishes the Law of Independent Assortment
- Valid only when genes are unlinked
4. Deviations from Mendelian Inheritance
Not all traits follow Mendelian ratios due to interactions between alleles.
4.1 Incomplete Dominance
- Neither allele is completely dominant
- Heterozygote shows an intermediate phenotype
Example: Flower color in Mirabilis jalapa
- Red × White → Pink (F₁)
- Phenotypic ratio equals genotypic ratio (1:2:1)
4.2 Codominance
- Both alleles express equally in heterozygous condition
Example:
- ABO blood group system (IA and IB are codominant)
5. Multiple Alleles and Pleiotropy
Multiple Alleles
- More than two alleles exist for a gene in a population
- Individual carries only two alleles
Example: ABO blood group (IA, IB, i)
Pleiotropy
- A single gene influences multiple phenotypic traits
Example: Phenylketonuria affects mental development, skin pigmentation, and metabolism.
6. Chromosomal Theory of Inheritance
Proposed by Sutton and Boveri, this theory correlates Mendel’s laws with chromosomal behavior.
Key Postulates
- Genes are located on chromosomes
- Chromosomes segregate during meiosis
- Independent assortment corresponds to non-homologous chromosomes
Significance
- Provides cytological evidence for Mendel’s principles
- Forms the basis of modern genetics
7. Linkage and Recombination
Linkage
- Genes located close together on the same chromosome tend to be inherited together
- Linked genes do not assort independently
Types
- Complete Linkage: No crossing over
- Incomplete Linkage: Some recombination occurs
Recombination
- Exchange of genetic material during crossing over in prophase I of meiosis
- Produces new gene combinations
Importance
- Generates variation
- Used in gene mapping
8. Sex Determination Mechanisms
Sex determination varies among organisms.
8.1 XX–XY Mechanism (Humans)
- Female: XX
- Male: XY
- Male is heterogametic
8.2 XX–XO Mechanism (Grasshopper)
- Male lacks one sex chromosome
8.3 ZW–ZZ Mechanism (Birds)
- Female is heterogametic (ZW)
8.4 Haplodiploidy (Honeybees)
- Males are haploid, females are diploid
9. Mutation
A mutation is a sudden, heritable change in genetic material.
Types
- Gene Mutation – Alteration in DNA sequence
- Chromosomal Mutation – Structural or numerical changes
Causes
- Radiation
- Chemicals
- Errors during DNA replication
Significance
- Source of variation
- Drives evolution
- Can cause genetic disorders
10. Genetic Disorders
Genetic disorders arise due to mutations or chromosomal abnormalities.
10.1 Mendelian Disorders
Caused by mutations in single genes and follow Mendelian inheritance patterns.
Examples
- Haemophilia: X-linked recessive disorder affecting blood clotting
- Colour Blindness: X-linked recessive
- Sickle Cell Anaemia: Autosomal recessive disorder caused by abnormal haemoglobin
10.2 Chromosomal Disorders
Caused by absence, excess, or abnormal arrangement of chromosomes.
Examples
- Down’s Syndrome: Trisomy 21
- Turner’s Syndrome: XO condition in females
- Klinefelter’s Syndrome: XXY condition in males
CBSE Board Examination Focus
High-Weightage Areas
- Monohybrid and dihybrid crosses
- Deviations from Mendelian inheritance
- Genetic disorders with causes
- Sex determination mechanisms
- Linkage vs recombination
Frequently Asked Question Types
- Explain with crosses (5 marks)
- Case-based genetics problems
- Pedigree analysis
- Assertion–Reason questions
- Numerical ratios
Quick Revision Highlights
- Mendelian ratios are fundamental but not universal
- Incomplete dominance ≠ codominance
- Linkage contradicts independent assortment
- Mutations are raw material for evolution
- Many genetic disorders are sex-linked
Conclusion
The chapter Principles of Inheritance and Variation bridges classical genetics with modern biological understanding. A strong grasp of Mendelian principles, chromosomal behavior, and inheritance patterns is essential for scoring well in CBSE Class 12 Biology. This NCERT-aligned study module ensures conceptual clarity, examination relevance, and structured revision, making it an ideal resource for board exam preparation.
✔ Designed strictly as per NCERT syllabus
✔ Fully aligned with CBSE Class 12 Board Examination standards
✔ Suitable for theory, numericals, and case-based questions