Chapter 4: Principles of Inheritance and Variation | Summary

In a monohybrid cross between a tall (TT) and dwarf (tt) pea plant, Mendel observed that all F1 plants were tall (Tt). When the F1 plants were self-pollinated, the F2 generation showed a 3:1 phenotypic ratio (3 tall : 1 dwarf) and a 1:2:1 genotypic ratio (1 TT : 2 Tt : 1 tt). The Punnett square, developed by Reginald Punnett, is a graphical tool used to calculate the probability of all possible genotypes in offspring from a genetic cross.

To determine whether a dominant phenotype is homozygous or heterozygous, Mendel used a test cross — crossing the organism showing the dominant phenotype with a homozygous recessive parent. If all progeny are dominant phenotype, the organism is homozygous; if 50% are dominant and 50% recessive, it is heterozygous. Incomplete dominance, as seen in Antirrhinum (snapdragon), produces an intermediate phenotype (pink) in F1 when red (RR) is crossed with white (rr), yielding a 1:2:1 ratio at F2.

Co-dominance occurs when both alleles in a heterozygote are fully expressed simultaneously. ABO blood grouping in humans is a classic example, controlled by the gene I with three alleles — IA, IB, and i. IA and IB are co-dominant with each other but dominant over i. Six genotypes and four blood group phenotypes (A, B, AB, O) are possible. This is also an example of multiple alleles, where more than two alleles control the same trait at the population level.

Pleiotropy refers to the ability of a single gene to influence multiple phenotypic traits. Phenylketonuria is a human example, where mutation in the gene coding for phenylalanine hydroxylase leads to both mental retardation and reduction in hair and skin pigmentation. Polygenic inheritance, such as skin colour in humans, involves three or more genes whose alleles act additively to produce a continuous range of phenotypes, and is also influenced by the environment.

In a dihybrid cross between RRYY (round, yellow) and rryy (wrinkled, green) pea plants, F1 plants were all RrYy (round, yellow). Self-pollination of F1 produced a 9:3:3:1 phenotypic ratio in F2, demonstrating the Law of Independent Assortment. This ratio arises because the segregation of the R/r pair is independent of the Y/y pair during meiosis, generating four gamete types (RY, Ry, rY, ry) in equal frequency.

Walter Sutton and Theodore Boveri proposed the Chromosomal Theory of Inheritance (1902), noting that chromosomes behave parallel to genes — both occur in pairs, segregate during gamete formation, and different pairs assort independently. Thomas Hunt Morgan experimentally verified this theory using Drosophila melanogaster. He discovered linkage (physical association of genes on the same chromosome) and recombination (generation of non-parental combinations through crossing over). Alfred Sturtevant used recombination frequencies to construct the first genetic maps.

Henking (1891) first observed the X body (later identified as the X-chromosome) in insects. In XO type (e.g., grasshopper), males have one X chromosome while females have two. In XY type (e.g., humans, Drosophila), females are XX and males are XY. In humans, 22 pairs are autosomes and the 23rd pair are sex chromosomes. Males produce two types of sperm (X-bearing and Y-bearing), while females produce only X-bearing eggs. The sex of the child is determined by the type of sperm that fertilises the egg.

Honey bees follow a haplodiploid sex-determination system, where fertilised eggs develop into diploid females (32 chromosomes) and unfertilised eggs develop into haploid males (16 chromosomes) through parthenogenesis. Males produce sperm by mitosis. In birds, females are heterogametic (ZW) and males are homogametic (ZZ) — the opposite of the mammalian pattern. This is called female heterogamety.

Mutation is a phenomenon resulting in alteration of DNA sequences, leading to changes in the genotype and phenotype of an organism. Point mutation involves a change in a single base pair of DNA; sickle-cell anaemia is a classical example where a single base change (GAG to GUG) substitutes glutamic acid with valine at the sixth position of the beta-globin chain. Loss or gain of segments of DNA (deletions, insertions, duplications) can cause chromosomal aberrations, often observed in cancer cells. Frame-shift mutations result from insertion or deletion of base pairs.

Mutations can be induced by physical and chemical factors called mutagens. UV radiation is a well-known physical mutagen. Polyploidy, an increase in whole sets of chromosomes, and aneuploidy, a change in the number of individual chromosomes, are also forms of mutation. Polyploidy is commonly observed in plants and has been exploited in agriculture. Aneuploidy underlies many chromosomal disorders in humans.

Genetic disorders are classified into Mendelian disorders (caused by single-gene mutations) and chromosomal disorders (caused by abnormalities in chromosome number or structure). Common Mendelian disorders include haemophilia, sickle-cell anaemia, colour blindness, phenylketonuria, thalassemia, and cystic fibrosis. Pedigree analysis is used to trace the inheritance pattern of a trait across generations in a family, helping determine if a disorder is dominant or recessive, autosomal or sex-linked.

Colour blindness and haemophilia are X-linked recessive disorders. In haemophilia, a clotting protein is defective, causing uncontrolled bleeding. Sickle-cell anaemia is autosomal recessive, with abnormal haemoglobin causing RBCs to sickle. Thalassemia is a quantitative defect in globin chain synthesis, unlike sickle-cell anaemia which is qualitative. Chromosomal disorders include Down's syndrome (trisomy 21), Klinefelter's syndrome (XXY, 47 chromosomes), and Turner's syndrome (XO, 45 chromosomes).

Chapter 4 establishes the fundamental principles governing how traits are transmitted from one generation to the next. Mendel's three laws — Dominance, Segregation, and Independent Assortment — form the bedrock of classical genetics and are explained at the molecular level through alleles, genotypes, and phenotypes. Extensions such as incomplete dominance, co-dominance, linkage, polygenic inheritance, and sex-linked traits reveal the complexity beyond simple Mendelian ratios. Mutation and chromosomal aberrations connect genetic variation to hereditary disorders, while pedigree analysis provides a practical tool for tracing these traits in human families.

For CBSE board examinations, this chapter is among the highest-weightage topics in Class 12 Biology. Students should master solving Punnett square problems, predicting F1 and F2 ratios, distinguishing between types of dominance and sex-determination mechanisms, and explaining genetic disorders with their molecular basis. Concepts like sickle-cell anaemia, chromosomal disorders, and linkage vs. independent assortment are frequently tested in short-answer, long-answer, and application-based question formats.