🧬 Principles of Inheritance and Variation (Mendelian & Chromosomal)
| Category / Phenomenon | NCERT Examples | Key High-Yield Genetic Feature |
|---|---|---|
| Mendel's Study Plant | Garden Pea (Pisum sativum) | Studied 7 pairs of contrasting traits for 7 years (1856–1863). |
| Incomplete Dominance | • Antirrhinum majus (Snapdragon / Dog flower) • Mirabilis jalapa (Four o'clock plant) | F₂ generation shows identical Genotypic and Phenotypic ratios (1:2:1). Pink flowers from Red × White. |
| Co-dominance | • ABO Blood Grouping in humans • Coat colour in cattle (Roan pattern) | Plasma membrane of RBCs has sugar polymers controlled by gene I (\(I^A, I^B, i\)). Both \(I^{A}\) and \(I^{B}\) express fully. |
| Multiple Allelism | ABO Blood Grouping in humans | Three alleles control the trait; can only be studied in a population. |
| Pleiotropy | • Phenylketonuria (PKU) in humans • Starch grain size & shape in Pea seeds | A single gene mutation affects multiple phenotypic traits (e.g., mental retardation and hair reduction in PKU). |
| Polygenic Inheritance | Human Skin Colour, Human Height | Controlled by three or more genes; phenotype reflects the contribution of each allele (gradient effect). |
| Chromosomal Theory Organism | Fruit fly (Drosophila melanogaster) | Chosen by T.H. Morgan; easily grown on synthetic medium, short life cycle (2 weeks), clear sex differentiation. |
| XO Type Sex Determination | Grasshopper, True bugs | Males have only one X chromosome (XO), females have two (XX). Sperm decides the sex. |
| ZW Type Sex Determination | Birds (Fowl, Peacock, etc.), Butterflies | Females are heterogametic (ZW), males are homogametic (ZZ). Ovum decides the sex. |
| Haplodiploid Sex System | Honey bees | Males (Drones) develop parthenogenetically from unfertilized eggs (Haploid, \(n=16\)); Females are Diploid (\(2n=32\)). |
| Mendelian Disorders (Gene) | Haemophilia, Sickle-cell anaemia, Phenylketonuria, Thalassemia, Cystic fibrosis | • Haemophilia: X-linked recessive. • Sickle-cell: Autosomal recessive (Glu \(\rightarrow \) Val change at 6th position of \(\beta \)-chain). |
| Aneuploidy (Chromosomal) | • Down’s Syndrome: Trisomy of 21 • Klinefelter’s Syndrome: XXY (47) • Turner’s Syndrome: XO (45) | • Down's: Langdon Down (1866); furrowed tongue, mental retardation. • Klinefelter's: Sterile males with feminine traits (Gynaecomastia). • Turner's: Sterile females with rudimentary ovaries. |
🔬 Molecular Basis of Inheritance (DNA & Experimental Models)
| Category / Experiment | Organism / Model Used | High-Yield Landmark Discovery |
|---|---|---|
| Transforming Principle (1928) | Streptococcus pneumoniae & Mice | Frederick Griffith showed avirulent R-strain transforms into virulent S-strain via a heat-killed extract. |
| Biochemical Proof (1933-44) | Streptococcus enzymes | Avery, MacLeod, and McCarty proved DNA is the transforming agent using Protease, RNase, and DNase. |
| Unequivocal Proof (1952) | Bacteriophage (\(T_{2}\)) & E. coli | Hershey and Chase used radioactive Phosphorus (\(^{32}P\)) for DNA and Sulphur (\(^{35}S\)) for protein coat. |
| Semiconservative Replication | Escherichia coli (Bacteria) | Meselson and Stahl (1958) used heavy nitrogen (\(^{15}NH_4Cl\)) and Cesium Chloride (\(CsCl\)) density gradient centrifugation. |
| Chromosomal Semiconservative | Vicia faba (Faba beans) | Taylor and colleagues (1958) used radioactive thymidine to prove replication on plant chromosomes. |
| Polynucleotide Phosphorylase | Severo Ochoa enzyme | Synthesized RNA with defined sequences in a template-independent manner for cracking the genetic code. |
| Lac Operon Model | Escherichia coli | Jacob and Monod; Inducer is Lactose (or allolactose). Structural genes: z (beta-gal), y (permease), a (transacetylase). |
| Model Organisms in HGP | Bacteria, Yeast, Caenorhabditis elegans (nematode), Drosophila, Plants (Rice & Arabidopsis) | Used for sequencing non-human genomes during the Human Genome Project. |
🐒 Evolution (Evidences, Mechanism & History)
| Category / Concept | NCERT Examples | High-Yield Evolutionary Significance |
|---|---|---|
| Homologous Organs | • Animals: Forelimbs of whale, bat, cheetah, human; Vertebrate hearts/brains. • Plants: Thorns of Bougainvillea and Tendrils of Cucurbita. | Indicates Divergent Evolution (Shared ancestry, anatomy similar, functions adapt to different needs). |
| Analogous Organs | • Animals: Wings of butterfly & birds; Eyes of octopus & mammals; Flippers of penguins & dolphins. • Plants: Sweet potato (Root modification) & Potato (Stem modification). | Indicates Convergent Evolution (Different structures evolving for the same function due to similar habitat pressure). |
| Natural Selection (Industrial) | Peppered Moth (Biston betularia) in England | Pre-industrialisation (1850s): White moths dominant. Post-industrialisation (1920s): Dark/melanic moths dominant due to soot on tree trunks. |
| Anthropogenic Evolution | Antibiotic-resistant bacteria; Herbicide-resistant weeds | Evolution driven by human actions over a short timescale (months/years instead of millions of years). |
| Adaptive Radiation | • Darwin’s Finches (Galapagos Islands) • Australian Marsupials • Placental Mammals of North America | Evolution starting from a point in a geographical area and radiating to other habitats. |
| Convergent Adaptive Radiation | Placental Wolf vs. Tasmanian Wolf; Anteater vs. Spiny Anteater | Pairs of placental mammals and Australian marsupials showing striking structural similarities due to parallel niches. |
| Saltation (Mechanism) | Evening Primrose (Oenothera lamarckiana) | Hugo de Vries proposed that mutation (large single-step mutation called saltation) drives speciation, not gradual variations. |
🦖 Geological Time Scale & Human Evolution Highlights
- First Cellular Forms: Appeared around 2000 million years ago (mya).
- Invertebrates & Jawless Fish: Invertebrates formed around 500 mya; Jawless fish evolved around 350 mya.
- Coelacanth (Lobefin): Caught in South Africa (1938); thought to be extinct. Ancestor of modern amphibians.
- Reptiles to Mammals Lineage: Sauropsids evolved into lizards, snakes, and dinosaurs. Synapsids evolved into Pelycosaurs \(\rightarrow \) Therapsids \(\rightarrow \) Mammals.
- Mass Extinction: Dinosaurs suddenly disappeared around 65 mya (reasons unknown, possibly asteroid impact).
👤 Human Evolution Timeline
- Dryopithecus & Ramapithecus (~15 mya): Hairy, walked like gorillas. Dryopithecus was more ape-like; Ramapithecus was more man-like.
- Australopithecines (~2–3 mya): Lived in East African grasslands; hunted with stone weapons; essentially ate fruit.
- Homo habilis (First human-like hominid): Brain capacity 650–800 cc; did not eat meat.
- Homo erectus (~1.5 mya): Brain capacity 900 cc; ate meat.
- Neanderthal Man (100,000–40,000 years ago): Brain capacity 1400 cc; lived in Near East & Central Asia; buried their dead.
- Homo sapiens (10,000–75,000 years ago): Arose in Africa during the ice age. Prehistoric cave art developed around 18,000 years ago; agriculture started around 10,000 years ago.