HEREDITY AND EVOLUTION – FULL CHAPTER ARTICLE (NCERT CLASS 10)

Here is a complete, detailed, easy-to-teach article on NCERT Class 10 Science – Chapter 9: Heredity and Evolution, covering all topics exactly as per NCERT

⭐ HEREDITY AND EVOLUTION – FULL CHAPTER ARTICLE (NCERT CLASS 10)

By Slatepencil Biology Classes – Sandeep Sen

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🔷 INTRODUCTION

Every living organism resembles its parents, but not completely. Why?
Because traits pass from one generation to the next through a process called heredity.
At the same time, living organisms also develop differences over many generations — this leads to evolution.

This chapter explains how traits are inherited, how variations arise, and how life evolves over time.

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🧬 PART 1: HEREDITY

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1️⃣ ACCORDING TO DARWIN – VARIATION IS NATURAL

Every organism shows small differences from others of its species.
These differences are called variations, and they become the raw material for evolution.

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2️⃣ HEREDITY – HOW TRAITS ARE PASSED

Heredity is the transmission of characters from parents to offspring.
These traits are controlled by DNA, present in the nucleus.

Examples of traits:

Eye color

Blood group

Height

Shape of nose

Hair type

Some traits are inherited, some are acquired.

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🔹 INHERITED vs ACQUIRED TRAITS

✔ Inherited Traits

Passed from parents to children through genes.
Example:

Tongue rolling

Blood group

Height

Eye color

These changes involve DNA.

✔ Acquired Traits

Develop during life due to environment or experience.
Example:

Learning music

Bodybuilding

Losing a limb in accident

These do NOT get passed to the next generation because they don’t change DNA.

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🧬 MENDEL’S EXPERIMENTS – LAWS OF INHERITANCE

Gregor Mendel worked on pea plants and discovered how traits are inherited.

Mendel selected 7 contrasting traits, such as:

Tall (T) vs Dwarf (t)

Round seed (R) vs Wrinkled seed (r)

Green pod vs Yellow pod
etc.

 

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3️⃣ MONOHYBRID CROSS – ONE TRAIT

Mendel crossed Tall (TT) with Dwarf (tt).

F1 Generation:

All plants were Tall (Tt)
→ Because T (tall) is dominant, t (dwarf) is recessive.

F2 Generation:

Ratio obtained:

3 Tall : 1 Dwarf

Genotype ratio = 1 TT : 2 Tt : 1 tt

🟦 Important Law:

LAW OF SEGREGATION

Genes separate during gamete formation.

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4️⃣ DIHYBRID CROSS – TWO TRAITS

Crossing Round Yellow seeds (RRYY) with Wrinkled Green seeds (rryy).

F2 Ratio:

9 : 3 : 3 : 1

This proved:

LAW OF INDEPENDENT ASSORTMENT

Traits are inherited independently.

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🔬 SEX DETERMINATION

Sex of offspring is determined by the type of chromosomes they receive.

Humans have XX (female) and XY (male).

✔ Female gametes: all eggs are X

✔ Male gametes: 50% X, 50% Y

If sperm contributes X → Girl (XX)

If sperm contributes Y → Boy (XY)

👉 Father determines the sex of the child.

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🧬 PART 2: EVOLUTION

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1️⃣ WHAT IS EVOLUTION?

Evolution is the gradual change in living organisms over generations, leading to the development of new species.

It explains:

Why humans look different from their ancestors

How complex organisms evolved from simple ones

 

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🌱 2️⃣ SPECIATION – FORMATION OF NEW SPECIES

A new species forms when populations become separated and stop interbreeding.

Major causes:

Geographical isolation (mountains, rivers)

Reproductive isolation

Genetic variation

Natural selection

Over time, these differences accumulate → forming new species.

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🧬 3️⃣ EVOLUTION AND CLASSIFICATION

Classification is based on similarities in traits.

More similarities → organisms are closely related.
Fossils also help track evolutionary relationships.

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🦴 4️⃣ FOSSILS AND EVOLUTION

Fossils are preserved remains of ancient organisms.

They help us understand:

Age of species

Transitional forms (e.g., dinosaurs → birds)

Fossil Dating Methods:

Relative dating (deeper fossils are older)

Carbon dating

 

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🧬 5️⃣ EVOLUTION BY STAGES

Complex organs evolved step-by-step.

Examples:

Eye evolved from simple light-sensitive spots.

Feathers first evolved for warmth, later used for flight.

Limbs of vertebrates show homology (same basic structure).

 

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🧬 6️⃣ EVOLUTIONARY EVIDENCE

✔ Homologous organs (same structure, different function)

Example:

Human arm, cat leg, whale flipper, bat wing
→ Show common ancestry

✔ Analogous organs (different structure, same function)

Example:

Wings of birds & insects
→ Show different ancestry

 

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🌳 7️⃣ HUMAN EVOLUTION

Human evolution started in Africa.

Order of evolution:

1. Dryopithecus

2. Australopithecus

3. Homo habilis

4. Homo erectus

5. Neanderthals

6. Homo sapiens

 

Humans migrated from Africa to Asia, Europe, and the world.

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✅ CONCLUSION

This chapter teaches that:

Heredity controls traits through DNA.

Evolution is a slow, continuous process.

Variations and natural selection drive evolution.

Humans evolved through multiple stages.