Ch 3Inheritance and Variation — Class 12 Biology, concept-first.

Heredity is the process by which parents pass genetic information to their offspring, generation after generation.

Before working through Mendel's laws it helps to fix the vocabulary geneticists use. The book defines the following terms, in this order:

Working purely from the numerical patterns in his pea-plant crosses, Mendel arrived at three fundamental postulates about how heredity works.

1. Law of Dominance :

2. Law of segregation (Law of purity of gametes) :

3. Law of Independent Assortment :

Two related crosses are used to test an individual's genotype against its two parents.

Mendel arrived at a few generalisations on the basis of his garden-pea experiments, such as:

a. Incomplete dominance :

b. Co-dominance :

c. Multiple alleles :

d. Pleiotropy :

Mendel published his findings in 1866, but for decades the work went almost unnoticed, partly because scientific communication was slow in that era and partly because his mathematical, statistical app…

Chromosomes are thread-like (filamentous) bodies found in the nucleus of eukaryotic cells; the name itself (Greek chromo = colour, soma = body) was coined by W.

Linkage : Linkage describes the tendency of genes that sit close together on the same chromosome to be inherited together, rather than assorting independently as Mendel's third law predicts for genes…

Human somatic (diploid, 2n) cells carry 23 pairs of chromosomes in total. Functionally these split into one pair of sex chromosomes, which determine sex, and the remaining 22 pairs, called autosomes,…

Genes located on the non-homologous regions of the sex chromosomes are called sex-linked genes, the traits they control are sex-linked traits, and their transmission from parent to offspring is sex-li…

Sex determination is the mechanism that establishes the sexual phenotype of an organism. Some species carry both male and female reproductive organs in the same individual and are described as bisexua…

Genetic disorders fall into two broad categories. Mendelian disorders arise from alteration or mutation of a single gene and follow the classic dominant/recessive inheritance patterns covered earlier…

Q.1 of the textbook exercise (p68): nine multiple-choice questions on the chapter. Pick the most appropriate option; each answer explains why the option is correct.

Q.2 of the textbook exercise (p68): explain the two statements (1a, 1b), define the four terms (2a-2d) and answer questions 3 to 9 in one or two lines.

Q.3 of the textbook exercise (pp68-69): short-answer questions 1 to 9, and question 10 on the printed pedigree chart (parts a and b).

Q.4 of the textbook exercise (p69): match the four items of Column-I with Column-II and re-write the matching pairs.

Q.5 of the textbook exercise (p69): eight long-answer questions.

The textbook's end-of-chapter Project (p69): a hands-on class survey of a genetic trait. There is no single correct answer; a guided approach is given.

Sample papers and previous-year board questions for this subject.