Class 10 Science Chapter 8 — Heredity and Evolution (International Edition)

NCERT / RBSE Syllabus 2026–27 · Learn by doing · Includes three labelled diagrams

Author: Surendra Singh Chouhan — former Education Officer, Rajasthan. 34 years (1991–2025) in teaching, training and administration.
Based on: NCERT Class 10 Science textbook, Chapter 8, and the Board of Secondary Education, Rajasthan syllabus 2026–27.
Prepared: Compiled from the textbook, organised with AI assistance, reviewed by the author. All diagrams are original.
Published: 24 September 2026 · Last updated: 24 September 2026

In 1856, an Austrian monk began growing pea plants in his monastery garden. No one imagined this hobbyist's experiment would one day explain why children resemble their parents, yet are never exactly identical.

That monk was Gregor Mendel — today called the "father of genetics," and these Class 10 Science Chapter 8 notes on heredity and evolution begin exactly where his pea-plant experiments did.

How to read this — first you think or try it, then comes the explanation:
🔍 Wonder · 🧪 Try it · 📖 Understand · 🔗 Apply · ✅ Check yourself
"Check yourself" has three levels — 1, 2, 3.

Contents

1. Foundation check
2. Where variation comes from
3. Mendel's work — the rules of heredity
4. Sex determination
5. What is evolution
6. Acquired versus inherited traits
7. Homologous and analogous organs
8. Fossils and the evidence for evolution
9. Evolution is not progress
10. Your ladder
11. How to draw the diagrams
12. Marking scheme
13. Common mistakes
14. Final test
15. Glossary
16. For parents and teachers
17. Beyond NCERT

Foundation check

1. What is variation? (Chapter 7)
2. In which organelle is DNA found?
3. Where do an offspring's traits come from in sexual reproduction?

Answers

1. A slight difference that appears from one generation to the next. 2. The nucleus. 3. From both parents — half the DNA from the mother, half from the father. This chapter examines exactly how that "half-and-half" actually works.

1. Where variation comes from

🔍 Wonder — Why aren't even two children of the same parents ever quite identical?

🧪 Try it — Guess the heights of ten classmates. Think first — is this difference only about diet, or is something else at work too?

📖 Understand — Chapter 7 explained that sexual reproduction mixes genes from two parents, producing variation in the offspring. This variation comes from two sources: (1) a fresh combination of the parents' genes, and (2) mutation — a random change in DNA, arising from a small copying error, that can create an entirely new genetic variation not present in either parent.

🔗 Apply — This same variation later becomes the raw material for evolution (the second half of this chapter) — without variation, evolution simply could not happen.

✅ Check yourself

Level 1 What are the two sources of variation? — Answer: A fresh combination of genes and mutation.
Level 2 What is the relationship between variation and evolution? — Answer: Variation is the raw material natural selection acts on — no species can change over time without it.

2. Mendel's work — the rules of heredity

🔍 Wonder — Why did Mendel choose pea plants of all things — why not roses or mangoes?

🧪 Try it — Take a few pea seeds — some round, some (if you can find them) wrinkled. Think first — if you crossed a round-seeded plant with a wrinkled-seeded one, what would the next generation look like — half round and half wrinkled, or something else entirely?

Mendel's real answer was surprising — in the F1 generation (the first offspring generation), every plant was round — the wrinkled trait seemed to vanish entirely. But when these F1 plants were crossed with each other (producing the F2 generation), the wrinkled trait reappeared — in a precise ratio of 3 round : 1 wrinkled.

📖 Understand — Mendel explained that every trait is controlled by two factors — today we call these genes — one from the mother, one from the father. When the two differ (like T and t), one dominant trait is expressed and the other stays recessive, hidden but not gone — it can reappear in a later generation.

Type of cross	Number of traits	F2 ratio
Monohybrid	1 (e.g. height alone)	3 : 1
Dihybrid	2 (e.g. seed shape + colour)	9 : 3 : 3 : 1

A dihybrid cross example — crossing a round, yellow-seeded plant with a wrinkled, green-seeded plant produces four combinations in the F2 generation: round-yellow, round-green, wrinkled-yellow, wrinkled-green — in a 9:3:3:1 ratio. This told Mendel that the two traits are inherited independently of each other. The 9:3:3:1 ratio appears only when the two genes assort independently — a condition worth remembering, since it is exactly what changes if the genes involved sit very close together on the same chromosome (see Level 3 below).

🔗 Apply — This is exactly why siblings with the same parents still look different — every trait's combination is formed independently, like two separate rolls of dice.

✅ Check yourself

Level 1 What is the difference between a dominant and a recessive trait? — Answer: A dominant trait is expressed even with just one copy of the gene; a recessive trait is expressed only when both copies are recessive.
Level 2 Why did the wrinkled trait not appear in the F1 generation? — Answer: Every F1 plant carried one round (dominant) gene and one wrinkled (recessive) gene — being dominant, only the round trait showed, while the wrinkled gene stayed hidden.
Level 3 If, in a dihybrid cross, the two traits sat very close together on the same chromosome, would you still get a 9:3:3:1 ratio? — Answer: Not usually — that ratio assumes the two traits are inherited independently. If they sat very close on the same chromosome, they would tend to be inherited together, and the ratio would deviate from 9:3:3:1.

3. Sex determination

🔍 Wonder — Whose "doing" is it that a baby turns out to be a boy or a girl — the mother's or the father's?

📖 Understand — A human cell has 23 pairs of chromosomes — 22 pairs of autosomes, and 1 pair of sex chromosomes. In females this pair is XX, in males XY. This is the basis of sex determination in Class 10 biology — one of the chapter's most frequently tested topics.

Every egg from the mother carries an X — there is no alternative. But the father's sperm are split — half carry X, half carry Y. In the human XX/XY system, whichever sperm succeeds in fertilisation contributes its chromosome, resulting in an XX or XY embryo:

X (mother) + X (sperm) = XX → daughter   ·   X (mother) + Y (sperm) = XY → son

🔗 Apply — So the answer to the opening question is scientifically clear: the sex chromosome carried by the fertilising sperm is what decides XX or XY — the mother's egg has no alternative to offer besides X in the first place.

✅ Check yourself

Level 1 How many pairs of chromosomes do humans have? — Answer: 23 pairs.
Level 2 What determines whether a baby is a boy or a girl? — Answer: The sex chromosome (X or Y) present in the father's sperm — the mother always contributes X.
Level 3 If a mother's egg occasionally carried a Y chromosome, what would change? — Answer: Sex determination would no longer depend solely on the father, becoming more variable. In reality, every cell in the mother's body is XX, so her eggs can never carry a Y — which is exactly why this responsibility falls entirely on the father.

4. What is evolution

🔍 Wonder — Did a giraffe's neck grow long because it kept stretching to reach high leaves?

🧪 Try it — Imagine a bacterial population where some bacteria happen to be naturally somewhat resistant to a particular drug (an antibiotic) — and others are not. Now that drug is applied to the whole population. Think first — what will the surviving population look like afterward?

What happens? The non-resistant bacteria die off, and only the resistant ones survive and multiply — the whole population gradually becomes "drug-resistant." Notice — no individual bacterium "tried" to become resistant; the trait already existed in the population's variation, and nature simply selected for it.

📖 Understand — This is natural selection, the principle proposed by Charles Darwin. Evolution is the gradual process by which, over time, traits already present as variation within a population that suit the environment survive and spread more successfully — eventually giving rise to new species.

The real answer to the giraffe question: ancestral giraffes already had natural variation in neck length. Those with slightly longer necks survived better by reaching higher leaves and left more offspring — and over many generations, this trait spread. No single giraffe's neck stretched longer within its own lifetime — that is the subject of the next section.

🔗 Apply — Natural selection needs three things: (1) variation within a population, (2) that variation must be heritable (in the DNA), and (3) environmental pressure that lets some traits survive better than others.

✅ Check yourself

Level 1 Who proposed the theory of natural selection? — Answer: Charles Darwin.
Level 2 What three conditions does natural selection require? — Answer: Variation in a population, that variation being heritable, and environmental pressure that favours the survival of certain traits.
Level 3 Does a bacterium "deliberately" develop drug resistance? — Answer: No. The resistance already existed as random variation. The drug simply selects who survives — this is not willpower or effort, but the blind process of natural selection.

5. Acquired versus inherited traits

🔍 Wonder — If a person builds strong muscles through a lifetime of exercise, will their child be born with strong muscles too?

Answer — No. This is a very common misconception.

📖 Understand — Acquired traits develop during a lifetime because of environment or practice (muscles from exercise, skin tanned by sun) — they affect the body's ordinary cells, never the DNA of reproductive cells (egg/sperm). So they are not passed to the next generation.

Inherited traits arise from changes in DNA and reach offspring only through reproductive cells — such as eye colour or blood group.

🔗 Apply — This is exactly why a giraffe's neck did not "stretch" longer generation after generation (that would be acquired) — instead, natural selection picked out already-longer-necked giraffes from existing variation (that is inherited).

✅ Check yourself

Level 1 Are acquired traits passed to offspring? — Answer: No.
Level 2 What is the core difference between acquired and inherited traits? — Answer: Inherited traits arise from DNA changes and pass through reproductive cells; acquired traits form during a lifetime because of environment and do not affect reproductive-cell DNA.

6. Homologous and analogous organs

🔍 Wonder — A human arm, a bat's wing, and a whale's flipper look completely different — so why is their underlying bone structure nearly identical?

📖 Understand — Organs that share the same basic structure but different functions are called homologous organs. This is strong evidence that these organisms share a common ancestor, whose original structure gradually adapted for different jobs over time.

The opposite case — organs with the same function but different underlying structure — are called analogous organs. For example, a bat's wing (made of bone) and a butterfly's wing (made of chitin, no bone at all) — both fly, but their origins are completely different.

Type	How to recognise	Example	What it shows
Homologous organs	Same structure, different function	Human arm, bat wing, whale flipper	Common ancestry
Analogous organs	Same function, different structure	Bat wing and butterfly wing	Convergent evolution — different lineages independently arriving at a similar solution

🔗 Apply — The opening question now has a clear answer: the forelimb bones of humans, bats and whales match because all three descended from a very ancient shared ancestor, and that same basic framework was reshaped differently — for grasping, for flying, for swimming.

✅ Check yourself

Level 1 What are homologous organs? — Answer: Organs that share the same basic structure but differ in function.
Level 2 What is the key difference between homologous and analogous organs? — Answer: Homologous organs share structure but differ in function; analogous organs share function but differ in structure.
Level 3 Carrots and radishes are both underground roots that store food — are they homologous or analogous? — Answer: Both their basic root structure and function match, and both come from closely related plant families — they are considered homologous.

7. Fossils and the evidence for evolution

🔍 Wonder — How do we know which organisms lived on Earth millions of years ago, when no one was around to see them?

📖 Understand — Fossils are preserved remains or impressions of ancient organisms — often buried within layers of rock. The deeper a rock layer, the older it generally is — so a fossil's depth gives a rough estimate of its age and where it fits in the sequence of evolution.

A famous example is Archaeopteryx — one of the best-known transitional fossils, showing both reptile-like features (a toothed jaw, a long bony tail) and bird-like features (feathers, wings) at once, illustrating the gradual evolutionary path connecting reptiles and birds.

🔗 Apply — Fossils, homologous organs, and DNA similarity together form three independent lines of evidence that all support evolution.

✅ Check yourself

Level 1 How is a fossil's age estimated? — Answer: By how deep it is found within rock layers.
Level 2 Why is the Archaeopteryx fossil significant? — Answer: It shows features of both reptiles and birds, providing evidence for the gradual evolution of birds from reptiles.

8. Evolution is not progress

📖 Understand — A common misconception is that evolution means moving "from simple to complex" or "getting better." This is wrong. Evolution only shows which trait succeeded at surviving and reproducing in a particular environment — it is not a certificate of being "superior" or "advanced."

Bacteria are still on Earth today, thriving in their own environment just as humans thrive in theirs. Biologists avoid ranking species as "more" or "less" evolved — instead, all living species have been evolving for the same length of time since their common ancestors, simply along different paths.

Human evolution — Homo sapiens is also the outcome of a long evolutionary chain that began in Africa and passed through several intermediate human species over millions of years. This too is not a "ladder of progress" but a branching evolutionary tree, most of whose branches — such as the Neanderthals — no longer exist today.

✅ Check yourself

Level 1 Does evolution mean "getting better"? — Answer: No, only adapting to the environment.
Level 2 Are bacteria "less evolved" than humans? — Answer: No — biologists avoid that ranking. Both have been evolving for the same length of time since their common ancestors, just along different paths suited to different environments.

Your ladder

Level	Total	If you got this many right	You are
Level 1	10	8+	Familiar
Level 2	10	7+	Proficient — board-exam ready
Level 3	4	3+	Skilled

How to draw the diagrams

1. Punnett square (Fig. 1) — Draw a 2×2 grid, write the parents' genes (T, t) across the top and down the left side, then fill in the four boxes (TT, Tt, Tt, tt). Labels required: dominant/recessive gene, the 3:1 ratio.

2. Sex determination (Fig. 2) — On the left, draw the mother (XX) with two arrows, both pointing to "X". On the right, draw the father (XY) with two arrows — one to "X", one to "Y". Below, show the two possible offspring (XX daughter, XY son). Common mistake: writing that the mother can also contribute X or Y — the mother always contributes only X.

3. Homologous organs (Fig. 3) — Draw three different outlines (arm, wing, flipper), each with the same sequence of bones (one long upper bone → two bones → several small bones), highlighted in the same colour across all three to make the similarity obvious.

Marking scheme

(An example of a typical examiner's approach — not an official scheme.)

Question (5 marks): Draw a Punnett square for Mendel's monohybrid cross and define dominant and recessive traits.

Correctly drawn Punnett square. 2 marks
Definition of dominant/recessive. 1 mark
Explaining what appeared in F1 versus F2. 1 mark
Stating the final 3:1 ratio. 1 mark

Common mistakes

Common mistake	Correct fact
"The mother determines the baby's sex"	In the human XX/XY system, the sperm's X or Y chromosome decides the outcome — the mother's egg always contributes X.
"Muscles built through exercise are passed to offspring"	Acquired traits are not inherited — only genetic (DNA-based) traits are.
"Evolution means getting better"	Evolution only means adapting to the environment, not "improving."
"An organism deliberately changes itself (e.g. a giraffe stretching its neck)"	The variation already existed; natural selection only selected it — no organism tried to change itself.
"Homologous and analogous organs are the same thing"	Homologous organs share structure, differ in function; analogous organs share function, differ in structure.

Final test — mixed

8+ correct = chapter mastered.

1. Which plant did Mendel use for his experiments?
2. What is the F2 ratio in a monohybrid cross?
3. What determines whether a baby is a boy or a girl?
4. How many pairs of autosomes do humans have?
5. Is tanned skin passed on to the next generation?
6. What kind of organs are a human arm and a bat's wing?
7. Which two groups does the Archaeopteryx fossil link?
8. Does evolution mean "getting better"?
9. (Chapter 7) Name one source of variation.
10. (Chapter 6) What role does the brain play in a reflex arc?

Answers — 1. Pea plant | 2. 3:1 | 3. The sex chromosome (X or Y) carried by the fertilising sperm | 4. 22 pairs | 5. No | 6. Homologous organs | 7. Reptiles and birds | 8. No, only adapting to the environment | 9. A fresh combination of genes, or mutation | 10. None — the decision is made in the spinal cord

Glossary

English	Hindi	English	Hindi
Dominant trait	प्रभावी लक्षण	Homologous organs	समजात अंग
Recessive trait	अप्रभावी लक्षण	Analogous organs	असमजात अंग
Gene	जीन	Fossil	जीवाश्म
Autosome	दैहिक गुणसूत्र	Natural selection	प्राकृतिक चयन
Sex chromosome	लिंग गुणसूत्र	Acquired trait	अर्जित लक्षण
Heredity	अनुवांशिकता	Speciation	स्पीशिएशन

For parents and teachers

Five-minute check: Have them draw a Punnett square and explain the 3:1 ratio. Ask what determines a boy or girl. Ask the difference between acquired and inherited traits. Ask for examples of homologous and analogous organs.

To try at home: compare one trait (such as attached versus free earlobes) across three generations of your family — seeing Mendel's principle play out in real life. Once practice is done, revisit the Class 10 Science Mega Test (200 MCQs) for revision.

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Beyond NCERT — For Curious Readers

This section goes beyond the RBSE/NCERT Class 10 syllabus and is not required for the board exam. It draws only on government and internationally reputed university sources, for curious readers who want to see where this chapter leads.

The world ignored Mendel's work for 34 years

Mendel published his research in 1865–66, but the scientific community largely overlooked it. Historians point to a key reason: Mendel's mathematical, ratio-based approach was unusual for biologists of that era. It was only in 1900 — after Mendel's death — that three European scientists, working independently, rediscovered the same rules, and only then was Mendel's original work finally credited.

The physical basis of the gene — the journey to DNA

Mendel spoke of "factors" but never explained what they physically were. Twentieth-century scientists showed that genes sit on chromosomes, and in 1953 the double-helix structure of DNA was proposed — a discovery built on the combined work of James Watson, Francis Crick, Rosalind Franklin and Maurice Wilkins, with Franklin's X-ray crystallography images proving decisive evidence for the structure. This revealed that genes are, in fact, specific segments of DNA.

Two scientists discovered natural selection at the same time

Darwin's name is most closely associated with natural selection, but a naturalist named Alfred Russel Wallace independently developed a strikingly similar theory around the same time. In 1858, both scientists' work was presented together at London's Linnean Society, and Darwin published his famous book On the Origin of Species the following year, in 1859.

Fossil research in India

India's Geological Survey of India conducts fossil research across the country, including notable discoveries of fossilised dinosaur eggs in central India. This is a direct, homegrown illustration of the very principle this chapter introduces — that deeper rock layers hold the record of older life.

Modern genetics — from Mendel's peas to the genome

The journey that began with Mendel's pea plants has today reached the Human Genome Project, which mapped nearly all 3 billion base pairs of human DNA. India's Department of Biotechnology now builds on this very foundation to research genetic disease identification and crop improvement — a highly sophisticated, modern extension of Mendel's original principle.

Sources for this section

• ncert.nic.in — NCERT, Government of India
• ePathshala — Ministry of Education, Government of India
• gsi.gov.in — Geological Survey of India

Note: nothing from these sources has been copied into this text; all statements above are written in the author's own words for general educational understanding, and are not medical or professional advice.

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Continue reading
← Previous: Chapter 7 — How Do Organisms Reproduce? (8 marks)
→ Next: Chapter 9 — Light: Reflection and Refraction

Related material

• Chapter 6 — Control and Coordination (English)
• Chapter 7 — How Do Organisms Reproduce? (English)
• Chapter 8 — अनुवांशिकता एवं जैव विकास (Hindi)
• Class 10 Science Mega Test — 200 MCQs

Based on the Class 10 Science (Code 07) syllabus 2026–2027 of the Board of Secondary Education, Rajasthan, Ajmer — Chapter 8, marks weightage 7. Textbook: Science, NCERT. All diagrams are original.