Chapter 4 — Principles of Inheritance and Variation

Chapter 4 of the Class 12 Biology NCERT textbook, "Principles of Inheritance and Variation", explains how Mendel's laws of dominance, segregation and independent assortment govern the passing of traits from parents to offspring, along with sex determination, mutation and genetic disorders.

• Mendel's foundational laws — The chapter builds genetics on Mendel's pea-plant experiments, deriving the laws of dominance, segregation and independent assortment as the rules by which traits are inherited and predictably reappear across generations.
• Beyond simple dominance — It extends Mendel with patterns like incomplete dominance, co-dominance, multiple alleles, polygenic inheritance and pleiotropy, showing that real inheritance is often more nuanced than a single dominant-recessive pair.
• How sex is determined — The chapter explains sex determination through XY, XO and ZW systems, treating chromosomes as the mechanism that decides an offspring's sex and linking certain traits to the sex chromosomes.
• Variation and genetic disorders — Finally it covers mutation and disorders, distinguishing single-gene Mendelian disorders like haemophilia and sickle-cell anaemia from chromosomal disorders such as Down's, Klinefelter's and Turner's syndromes.

1. 01Mendel studied 7 contrasting traits in pea plants over 7 years (1856–1863) and proposed three laws: Law of Dominance, Law of Segregation, and Law of Independent Assortment.
2. 02In a monohybrid cross, the F1 generation shows only the dominant trait; F2 produces a 3:1 phenotypic ratio and 1:2:1 genotypic ratio (TT:Tt:tt).
3. 03In a dihybrid cross, F2 offspring appear in a 9:3:3:1 phenotypic ratio, demonstrating that allele pairs segregate independently of each other.
4. 04ABO blood groups illustrate co-dominance (IA and IB both expressed) and multiple alleles (three alleles: IA, IB, i) controlling the same character.
5. 05Sex determination in humans is XY type: females are XX, males are XY; the sex of the child is determined by whether the sperm carries an X or Y chromosome.
6. 06Genetic disorders are classified as Mendelian (single-gene mutations — e.g., sickle-cell anaemia caused by Glu→Val substitution at position 6 of the beta globin chain) or chromosomal (e.g., Down's syndrome — trisomy of chromosome 21).