Principles of Inheritance and Variation — Class 12 Biology

This chapter introduces genetics through Mendel's experiments on the garden pea, establishing the principles of inheritance. Mendel's monohybrid crosses gave the Law of Dominance and the Law of Segregation, while dihybrid crosses gave the Law of Independent Assortment, expressed through phenotypic ratios such as 3:1 and 9:3:3:1. Deviations from Mendelian ratios are explained by incomplete dominance (Snapdragon), co-dominance (ABO blood groups), multiple allelism, pleiotropy and polygenic inheritance (human skin colour). The chromosomal theory of inheritance, proposed by Sutton and Boveri, links genes to chromosomes, supported by Morgan's work on Drosophila showing linkage and recombination, and the construction of genetic maps. Sex determination mechanisms (XX-XY, XX-XO, ZZ-ZW) and the chromosomal basis of sex are explained, along with the role of environment. Mutations, sudden heritable changes in DNA, and the resulting variations are described, including chromosomal aberrations (deletion, duplication) and point mutations like in sickle-cell anaemia. The chapter concludes with the genetic basis of human disorders, classifying them as Mendelian (haemophilia, colour blindness, thalassaemia, sickle-cell anaemia, phenylketonuria) and chromosomal (Down's syndrome, Klinefelter's syndrome, Turner's syndrome).