Chapter 1: Mendel's Experimental Organism and Methodology

Mendel used Pisum sativum for four key reasons: self-pollinating nature, large and rapidly produced offspring, short generation time, and seven clearly distinguishable contrasting traits. He first established true-breeding lines through multiple rounds of self-pollination, ensuring all experimental parents were homozygous. He then performed artificial cross-pollination by emasculating the female parent flower and dusting pollen from the chosen male. He studied F1 and F2 generations, counting thousands of offspring and applying probability theory to establish reproducible ratios.

Chapter 2: Mendel's Three Laws

The Law of Dominance states that F1 offspring of two contrasting pure lines express only the dominant trait. The Law of Segregation (First Law) states that paired alleles separate during meiosis; each gamete carries one allele; the F2 monohybrid ratio is 3:1 phenotypic and 1:2:1 genotypic. The Law of Independent Assortment (Second Law) states that alleles of different genes on different chromosomes assort freely during meiosis; the dihybrid F2 ratio is 9:3:3:1. A test cross (unknown × homozygous recessive) reveals genotype: all dominant → TT; 1:1 ratio → Tt.

Chapter 3: Deviations from Complete Dominance

Incomplete dominance produces a blended F1 (pink from red × white); F2 phenotypic ratio = 1:2:1 (equals genotypic ratio). Co-dominance produces both parental phenotypes simultaneously in F1 (neither blended nor suppressed); classic example: $I^A$ $I^B$ = blood group AB. The critical distinction: blended intermediate = incomplete; both distinct phenotypes present simultaneously = co-dominance.

Chapter 4: ABO Blood Group and Multiple Alleles

Three alleles ($I^A$, $I^B$, i) produce six genotypes and four phenotypes. $I^A$ and $I^B$ are co-dominant; both are dominant over i. Genotype ii = group O (universal donor: no antigens, both antibodies). Genotype $I^A$ $I^B$ = group AB (universal recipient: both antigens, no antibodies). Cross $I^A$ i × $I^B$ i produces all four blood groups in 1:1:1:1 ratio. Cross AB × O produces only groups A and B (no O or AB offspring possible).

Chapter 5: Pleiotropy and Polygenic Inheritance

Pleiotropy: single HbS gene allele → multiple phenotypic effects (sickle haemoglobin, RBC sickling, anaemia, multi-organ damage). Polygenic: multiple genes → single trait with continuous variation (skin colour, height follow bell-curve distribution). The distinction is directional: pleiotropy is 1→many; polygenic is many→1.

Chapter 6: Chromosomal Basis of Inheritance and Linkage

Sutton and Boveri (1902): genes on chromosomes — meiotic behaviour of chromosomes mirrors Mendelian allele behaviour. Morgan (Drosophila): genes on same chromosome are linked; crossing over during Prophase I creates recombinants. Recombination frequency = physical distance in centimorgans. Law of Independent Assortment is VIOLATED by linked genes. Modified dihybrid ratios (9:7, 9:3:4, 12:3:1, 15:1, 13:3) arise from gene interaction (epistasis).