Class 12 Biology Chapter 18 Inheritance Notes | Punjab Board PDF
Chapter-wise notes covering Mendel’s Laws, Dominance Relations, Blood Groups (ABO & Rh), Polygenic Inheritance, Epistasis, Linkage & Crossing Over, Sex Determination Systems, and Sex-Linked Disorders – aligned with PCTB / PECTAA syllabus for Class 12 HSSC Part 2.
Chapter Overview
Class 12 Biology Chapter 18 Inheritance explores classical and modern genetics, detailing how traits are passed from parents to offspring. The chapter begins with Gregor Mendel's landmark experiments on pea plants, leading to the Law of Segregation and the Law of Independent Assortment, supported by testcrosses and mathematical probability rules (product and sum rules).
Extensions of Mendelian genetics are thoroughly covered, including incomplete dominance, co-dominance, multiple alleles (ABO blood groups and Rh factor leading to erythroblastosis foetalis), polygenic inheritance, and epistasis. The physical basis of inheritance is further elaborated through gene linkage, chromosomal crossing over, and sex determination mechanisms across species (XX-XY, XX-XO, and ZW-ZZ systems).
The final section addresses sex-linked inheritance in humans, explaining X-linked recessive disorders such as haemophilia, colour blindness, and Duchenne muscular dystrophy, alongside sex-limited and sex-influenced traits. For HSSC Part 2 pre-medical students across Lahore Board, Rawalpindi Board, Gujranwala Board, Multan Board, and other Punjab boards, Chapter 18 is one of the most high-yielding conceptual topics in board exams. These notes follow the PCTB / PECTAA 2026-2027 syllabus with complete solved genetic cross problems, MCQs, short questions, and long questions.