📌 INTRODUCTION

Genetics is the branch of biology that studies heredity and variation.
Heredity: Transmission of traits from parents to offspring.
Variation: Differences in traits among individuals of a species.
Evolution: Gradual changes in organisms over generations, leading to new species.

🔬 PART 1: PRINCIPLES OF INHERITANCE AND VARIATION

🌿 Gregor Mendel – Father of Genetics

Conducted experiments on pea plants (Pisum sativum).
Proposed Laws of Inheritance.

🟢 Mendel’s Laws of Inheritance

1️⃣ Law of Dominance – One allele is dominant over the other.
2️⃣ Law of Segregation – Alleles separate during gamete formation.
3️⃣ Law of Independent Assortment – Genes for different traits assort independently.

🌼 Monohybrid Cross (One Trait)

Example: Tall (T) × Short (t) Pea Plant
F1 Generation: All Tall (Tt)
F2 Generation: 3 Tall : 1 Short

🌸 Dihybrid Cross (Two Traits)

Example: Yellow Round (YYRR) × Green Wrinkled (yyrr)
F2 Ratio: 9:3:3:1

🔹 Types of Inheritance

1️⃣ Incomplete Dominance – Neither allele is completely dominant.

Example: Red (RR) × White (rr) → Pink (Rr) in Snapdragon flower.
2️⃣ Co-dominance – Both alleles express equally.
Example: Blood Group AB (IAIB).
3️⃣ Multiple Alleles – More than two alleles for a gene.
Example: ABO Blood Group (IA, IB, i).
4️⃣ Polygenic Inheritance – Multiple genes control a single trait.
Example: Human skin colour.

🧬 Chromosomal Basis of Inheritance

Genes are located on chromosomes.
Morgan’s Experiment (Drosophila – Fruit Fly) proved linkage & recombination.
Sex Determination:
- XX-XY (Humans, Drosophila).
- ZZ-ZW (Birds, Reptiles).
- XO Type (Grasshoppers).

🧪 Mutations & Genetic Disorders

🔬 Types of Mutations

1️⃣ Point Mutation – Change in a single base. (Example: Sickle Cell Anaemia).
2️⃣ Chromosomal Mutation – Deletion, duplication, inversion, translocation.

🩺 Genetic Disorders

🦠 PART 2: MOLECULAR BASIS OF INHERITANCE

📜 DNA Structure & Replication

Discovered by Watson & Crick (Double Helix Model).
Nucleotides (A, T, G, C) make up DNA.
Chargaff’s Rule: A = T, G = C.
Replication: Semi-conservative process.

🧬 Central Dogma of Molecular Biology

DNA → RNA → Protein
(DNA Transcription → mRNA → Translation → Protein)

🧪 Gene Expression & Regulation

Lac Operon Model explains gene regulation in prokaryotes.
Exons (coding regions) & Introns (non-coding regions).

🌍 PART 3: EVOLUTION

🔹 Origin of Life

Oparin-Haldane Hypothesis: Life originated from simple molecules in early Earth.
Miller-Urey Experiment: Simulated early Earth conditions → formed organic molecules.

🦴 Evidence for Evolution

1️⃣ Fossils – Show progressive changes.
2️⃣ Comparative Anatomy – Homologous & Analogous structures.
3️⃣ Embryology – Similar embryonic development in vertebrates.
4️⃣ Molecular Evidence – DNA & protein similarities.

🔹 Theories of Evolution

1️⃣ Lamarck’s Theory of Use & Disuse – Disproven.
2️⃣ Darwin’s Natural Selection – Survival of the fittest.
3️⃣ Hardy-Weinberg Principle – Gene frequency remains constant in a population.

🐦 Darwin’s Natural Selection

Overproduction: More offspring are produced than can survive.
Variation: Individuals show differences.
Struggle for Existence: Competition for resources.
Survival of the Fittest: Best-adapted organisms survive & reproduce.

🌿 Types of Evolution

1️⃣ Convergent Evolution – Unrelated species evolve similarly. (Example: Wings in bats & birds).
2️⃣ Divergent Evolution – Common ancestors lead to different species. (Example: Darwin’s finches).
3️⃣ Adaptive Radiation – Rapid diversification from a common ancestor.

📌 Key points

✅ Mendel’s Laws explain heredity.
✅ Chromosomes carry genes; DNA codes for proteins.
✅ Mutations cause genetic disorders.
✅ Evolution explains diversity in life forms.
✅ Natural selection leads to adaptation & survival.

🌟 Genetic Disorders 🌟

🧬 Cystic Fibrosis:
- Cause: Mutation in the CFTR gene.
- Symptoms: Thick mucus, respiratory and digestive problems.
- Inheritance: Autosomal recessive.
🩸 Sickle Cell Anemia:
- Cause: Mutation in the HBB gene, causing sickle-shaped red blood cells.
- Symptoms: Pain, fatigue, and infections.
- Inheritance: Autosomal recessive.
🧠 Down Syndrome:
- Cause: Extra chromosome 21 (trisomy 21).
- Symptoms: Intellectual disability, distinct features.
- Inheritance: Mostly spontaneous, but maternal age increases risk.
🧩 Huntington’s Disease:
- Cause: Mutation in the HTT gene.
- Symptoms: Progressive neurodegeneration, motor issues.
- Inheritance: Autosomal dominant.
⚡ Hemophilia:
- Cause: Lack of clotting factors (Factor VIII or IX).
- Symptoms: Easy bruising, prolonged bleeding.
- Inheritance: X-linked recessive.

🧬 Monohybrid Cross 🧬

Example: Pea Plant Colour

Gene: Colour of the flowers (Purple vs White)
Alleles:
- P (Purple, dominant)
- p (White, recessive)

Parental Cross (P Generation):

Cross: Purple x White
Genotypes: PP x pp

F1 Generation:

All offspring will inherit one P allele and one p allele (Pp), making them purple-flowered (since purple is dominant).

F2 Generation (Crossing F1 individuals):

Cross: Pp x Pp
Punnett Square:

F2 Generation Ratios:

Genotypic ratio: 1 PP : 2 Pp : 1 pp
Phenotypic ratio: 3 Purple : 1 White

Class 12th Biology Genetics and Evolution Notes