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STUDY NOTES

Chapter – Genetics & Evolution (Class 12 Biology)

1. Genetics – Basic Terms

Genetics

→ study of

heredity

variation

Heredity

→ passing traits from parents → offspring

Variation

→ differences seen in individuals

Gene

→ unit of inheritance

Alleles

→ alternative forms of a gene

Genotype

→ TT, Tt,

(genetic makeup)

Phenotype

→ Tall / Dwarf (physical appearance)

Homozygous

→ TT /

Heterozygous

→ Tt

2. Mendel’s Laws (Important)

(i) Law of Segregation

Alleles separate during gamete formation

Gametes are

pure

Example → Tt produces

(ii) Law of Independent Assortment

Genes assort independently

Dihybrid ratio →

9 :

3 : 3 : 1

3. Monohybrid Cross (Handwritten Look)

TT (Tall) × tt (Dwarf)

→ F₁ = Tt (All Tall)

Selfing: Tt × Tt

Genotype ratio → 1 : 2 : 1 Phenotype ratio → 3 Tall : 1 Dwarf

4. Dihybrid Cross (Simple Notes)

RRYY × rryy → F₁ = RrYy Selfing → Phenotype = 9:3:3:1

Shows → Independent Assortment

5. Incomplete Dominance

Neither allele dominant

Example:

Snapdragon

RR (Red) ×

(White)

F₁ → Rr (Pink)

6. Codominance + Multiple Alleles

ABO blood group

→ 3 alleles

IA & IB

→ codominant

Genotypes:

A → IAIA /

IAi

B → IBIB /

IBi

AB → IAIB

O → ii

7. Sex-Linked Inheritance

Gene present on X chromosome

Example →

Colour

blindness,

Haemophilia

More common in males (XY)

8. Sex Determination in Humans

Female →

Male →

Father decides sex of baby

X sperm → girl

Y sperm → boy

9. Mutation (Very Important)

Sudden, heritable change in DNA

Types:

Point mutation

Frameshift

Chromosomal mutation

Example →

Sickle cell

anaemia

10. Polygenic Inheritance

Trait controlled by many genes

Shows continuous variation

Examples → Skin

colour

, Height

11. Linkage & Recombination

Linkage

→ genes close together stay together

Recombination

→ new combinations by crossing over

Opposite effects

12. Hardy–Weinberg Law

Population remains in equilibrium if:

No mutation

No migration

No selection

Random mating

Large population

Formula: p² + 2pq + q² = 1

EVOLUTION NOTES

13. Darwin’s Theory of Natural Selection

Steps:

1️⃣ Overproduction 2️⃣ Struggle for existence 3️⃣ Variations 4️⃣ Survival of the fittest 5️⃣ Natural selection → new species

14. Evidence for Evolution

Fossils

→ ancient remains (Archaeopteryx)

Homologous organs

→ same origin (arm & wing)

Analogous organs

→ same function (bird wing & insect wing)

Embryology

→ similar embryos

Molecular evidence

→ DNA similarity

15. Genetic Drift

Random

change in allele frequen

Seen in

small populations

Types:

Founder Effect

Bottleneck Effect

16. Speciation

Formation of new species

Occurs by reproductive isolation + evolution

17. Adaptive Radiation

One ancestor → many species

Example → Darwin’s finches