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: