NCERT Class 12 Biology Chapter 4: Principles of Inheritance and Variation PDF Notes Download

Chapter 4 of Class 12 Biology, Principles of Inheritance and Variation, explains how traits are passed from one generation to another. It talks about Mendel’s laws of inheritance, genetic crosses, linkage, recombination, sex determination, mutation, and genetic disorders. These concepts form the base of classical genetics and help us understand how genes influence physical traits

Chapter 4 of Class 12 Biology, Principles of Inheritance and Variation, explains how traits are passed from one generation to another. It talks about Mendel’s laws of inheritance, genetic crosses, linkage, recombination, sex determination, mutation, and genetic disorders. These concepts form the base of classical genetics and help us understand how genes influence physical traits and diseases.

I’m writing about this chapter because it is one of the most frequently asked sections in both CBSE board exams and NEET. Understanding inheritance patterns and variations is not just important for scoring marks but also for understanding how living organisms evolve and why genetic differences exist. Most students find this chapter slightly tricky in the beginning due to the number of terms and genetic crosses, but once understood clearly, it becomes one of the most scoring chapters. I have tried to make the explanations simple and relatable. Also, I have shared the link to download the NCERT PDF for this chapter so that you can revise the concepts at your convenience.

Mendel’s Experiments and Laws of Inheritance

Gregor Mendel, known as the father of genetics, performed hybridisation experiments on pea plants. His work laid the foundation of classical genetics.

Key Terms

Gene: Unit of inheritance
Allele: Different forms of a gene
Homozygous: Same alleles (TT or tt)
Heterozygous: Different alleles (Tt)
Phenotype: Physical appearance
Genotype: Genetic makeup

Mendel’s Laws

Law of Dominance: In a heterozygous pair, the dominant allele expresses itself
Law of Segregation: Alleles separate during gamete formation
Law of Independent Assortment: Traits assort independently during inheritance

Mendel’s Crosses

Monohybrid Cross: Involves one trait (e.g. tall vs. dwarf)
- F1 Generation: All tall (Tt)
- F2 Generation Ratio: 3 Tall : 1 Dwarf (Phenotypic), 1:2:1 (Genotypic)
Dihybrid Cross: Involves two traits (e.g. seed shape and colour)
- F2 Generation Phenotypic Ratio: 9:3:3:1

Quick Summary Table

Concept	Example/Details
Mendel’s Laws	Dominance, Segregation, Independent Assortment
Incomplete Dominance	Pink flowers in Mirabilis jalapa
Codominance	AB blood group
Multiple Alleles	ABO blood system
Sex Determination	XX-XY system in humans
Mutation	Sickle cell anaemia, Down’s syndrome

Download PDF – NCERT Class 12 Biology Chapter 4

To download the official PDF of this chapter for free:

NCERT Class 12 Biology Chapter 4

NCERT Class 10 Math Chapter 14: प्रायिकता PDF Download

NCERT Class 10 Math Chapter 14 प्रायिकता (Probability) introduces students to the concept of chance and likelihood of events. In this chapter, students learn how to calculate the probability of simple events using the formula P(E) = Number of favourable outcomes ÷ Total number of outcomes. The chapter deals with real-life examples like tossing a

NCERT Class 10 Math Chapter 14 प्रायिकता (Probability) introduces students to the concept of chance and likelihood of events. In this chapter, students learn how to calculate the probability of simple events using the formula P(E) = Number of favourable outcomes ÷ Total number of outcomes. The chapter deals with real-life examples like tossing a coin, rolling a dice, or drawing cards, which makes the subject more interesting and practical. Since probability questions are common in board exams and are generally considered easy, this chapter is highly important for scoring well.

I am writing about this topic because probability is not only an important part of the Class 10 syllabus but also a concept that students will use in higher studies and real life. From predicting weather conditions to calculating risks in business, probability plays a key role. Many students initially find it confusing, but NCERT presents it in a simple and easy-to-understand manner. By practising from the NCERT book, students can build a strong foundation and develop confidence in solving probability problems. Having the PDF makes it easier for learners to access the chapter anytime, revise formulas, and attempt practice questions before exams.