NCERT Solutions for Class 12 Biology Chapter 4 Principles of Inheritance and Variation

Short Note:

• Gregor Mendel, through his experiments on garden pea plants, formulated the Law of Dominance (one allele masks the expression of the other in a heterozygote), the Law of Segregation (the two alleles of a gene separate during gamete formation, so each gamete carries only one allele), and the Law of Independent Assortment (alleles of different genes assort independently of one another during gamete formation).

• A monohybrid cross studies the inheritance of a single trait, typically giving a 3:1 phenotypic ratio in the F2 generation, while a dihybrid cross studies two traits together, giving a 9:3:3:1 ratio in F2.

Short Note:

• Incomplete dominance occurs when the heterozygote shows an intermediate phenotype between the two parents (e.g. flower colour in Mirabilis jalapa), while co-dominance occurs when both alleles express themselves fully and simultaneously (e.g. AB blood group in the ABO system).

• Multiple alleles, such as the three alleles (IA, IB, i) controlling human ABO blood groups, and pleiotropy, where a single gene affects multiple phenotypic traits (e.g. phenylketonuria), are further examples of complexity beyond simple Mendelian ratios.

Short Note:

• The chromosomal theory of inheritance, proposed by Sutton and Boveri, states that genes are located on chromosomes, and their behaviour during meiosis parallels the segregation and assortment of Mendelian factors.

• In humans, sex is determined by sex chromosomes: females have XX and males have XY chromosomes; since sperm can carry either an X or a Y chromosome, the father's gamete determines the sex of the child.

Short Note:

• Mutations are sudden, heritable changes in DNA sequence; point mutations, such as in sickle cell anaemia, result from a single base pair change leading to an altered protein.

• Genetic disorders can be Mendelian (caused by a single gene mutation, e.g. haemophilia, sickle cell anaemia, colour blindness) or chromosomal (caused by changes in chromosome number or structure, e.g. Down's syndrome due to trisomy of chromosome 21, and Turner's and Klinefelter's syndromes involving sex chromosome abnormalities).