Inheritance

Chapter Overview

Inheritance is an essential genetics chapter in Class 12 Biology that investigates the transmission of biological traits across successive generations. It begins with classical Mendelian genetics, analyzing Gregor Mendel's monohybrid crosses which led to the Law of Segregation, and dihybrid crosses establishing the Law of Independent Assortment as a cornerstone generator of genetic variation. The chapter provides rigorous interpretations of 3:1 phenotypic and 1:2:1 genotypic ratios, 9:3:3:1 dihybrid distributions, the utility of diagnostic testcrosses, and the integration of mathematical rules of probability (product and sum rules) to calculate inheritance outcomes.

Extending beyond simple dominance, the text explores non-Mendelian dominance relationships including incomplete dominance in 4 o'clock plants where heterozygotes display intermediate phenotypes, and codominance where both alleles express independently without blending. Multiple allelism is exemplified through the human ABO blood group system governed by three alleles (I^A, I^B, and i), detailing transfusion compatibility and agglutination principles. The chapter further explores the Rh blood group system, the immunopathology of erythroblastosis foetalis (hemolytic disease of the newborn), polygenic continuous traits like human skin pigmentation and height, and epistatic gene interactions.

The final segment examines chromosomal inheritance patterns, analyzing gene linkage as a key limitation to independent assortment, and how crossing over during prophase I generates recombinant phenotypes. It covers chromosomal sex determination systems across animals (XX-XY in humans and Drosophila, XX-XO in grasshoppers, and ZW-ZZ in birds), followed by sex-linked inheritance. Particular emphasis is placed on X-linked recessive disorders in humans-hemophilia, red-green color blindness, and Duchenne muscular dystrophy-contrasted with sex-limited and sex-influenced physiological traits.