CBSE Class 10 Science Chapter 9 Heredity and Evolution Class 10 Notes PDF

Heredity and Evolution Class 10 Notes explain how traits are passed from parents to offspring and how inherited variations can lead to evolution over many generations. This chapter helps students understand Mendel’s laws, monohybrid cross, dihybrid cross, sex determination, acquired and inherited traits, natural selection, fossils, speciation and human evolution.

In many NCERT editions, this topic appears as Chapter 9: Heredity and Evolution. In some updated CBSE Board learning materials, the heredity part may be treated separately under Heredity. Students should follow the chapter title and numbering used in their school textbook.

These notes are useful for NCERT revision, school exams, pre-board preparation, board exam preparation, MCQs, assertion-reason questions, case-based questions and quick last-minute revision.

Heredity and Evolution Class 10 Chapter Overview

Heredity explains how characters are transferred from parents to offspring. Evolution explains how inherited variations accumulate over many generations and may lead to the formation of new species.

This chapter is important because it connects genetics with evolution. Students learn how traits are inherited, why offspring resemble their parents but are not identical to them, how Mendel discovered the laws of inheritance, how sex is determined in humans and how variation helps in evolution.

Heredity and Evolution Class 10 Notes PDF Download

Students can download the Heredity and Evolution Class 10 Notes PDF for offline revision. The PDF includes definitions, Mendel’s experiments, Punnett squares, monohybrid and dihybrid crosses, important genetic terms, sex determination, acquired and inherited traits, evolution, natural selection, fossils, speciation, important questions, MCQs and FAQs.

Heredity — The Basics

Heredity is the transmission of characters/traits from parents to offspring. It is why a child resembles their parents, and why a pea plant grown from a tall parent tends to produce tall offspring. The branch of biology that studies heredity and variation together is called genetics. 

In sexually reproducing organisms, both parents contribute roughly equal amounts of genetic material (DNA) to the offspring, which is why offspring show a mix of features from both parents rather than being identical to either one.

Key Terms

Variation

Variation refers to the differences among individuals of the same species. No two individuals (other than identical twins) are exactly alike.

Why Variation Matters for Survival

Variation is the raw material on which natural selection acts. A population that has more variation is more likely to contain at least some individuals capable of surviving a sudden environmental change — heat, cold, disease, or a new predator.

Classic illustration

If a colony of bacteria is suddenly exposed to heat, most may die — but if a few individuals already carried a chance variation that gives heat tolerance, those survive and reproduce, gradually making the heat-tolerant form common in the population. This single idea links variation directly to evolution.

How Variations Arise

It's worth noting that asexually reproducing organisms also show variation (mainly from DNA copying errors), but it accumulates far more slowly than in sexually reproducing populations, where recombination constantly generates new combinations every generation.

Heredity vs Evolution

Memory hook: Heredity explains resemblance; evolution explains change.

Gregor Mendel and His Pea Plant Experiments

Gregor Johann Mendel, an Austrian monk, is called the Father of Genetics. Working in a monastery garden in the 1860s, he conducted controlled breeding experiments on garden pea plants (Pisum sativum) and derived the first mathematical laws of inheritance.

Why Mendel Chose Pea Plants

Mendel's Seven Contrasting Traits

Mendel first crossed plants differing in a single trait (monohybrid cross), then plants differing in two traits at once (dihybrid cross), and used the results to formulate his three laws of inheritance.

Monohybrid Cross

A monohybrid cross studies the inheritance of a single pair of contrasting traits.

Setup

Let T = allele for tallness (dominant), t = allele for dwarfness (recessive). Parental (P) generation: Pure tall (TT) × Pure dwarf (tt) Gametes: tall parent gives only T; dwarf parent gives only t. F₁ generation: All offspring are Tt (heterozygous) and are all tall, since T is dominant over t. F₂ generation: Self-pollinating F₁ plants — Tt × Tt

• Genotypic ratio: TT : Tt : tt = 1 : 2 : 1
• Phenotypic ratio: Tall : Dwarf = 3 : 1

What the Monohybrid Cross Proves

• Traits are controlled by paired factors (alleles), one from each parent.
• One allele can mask (dominate) the other in the heterozygote.
• The recessive trait does not vanish — it reappears in F₂, proving that alleles separate cleanly during gamete formation and recombine randomly at fertilisation.

F₁ and F₂ Generations

Dihybrid Cross

A dihybrid cross studies the simultaneous inheritance of two pairs of contrasting traits. Mendel crossed pea plants differing in both seed shape and seed colour: R = round (dominant), r = wrinkled (recessive) Y = yellow (dominant), y = green (recessive) Parental cross:RRYY (round, yellow) × rryy (wrinkled, green) F₁ generation: All RrYy — round, yellow (since both dominant traits are expressed). F₂ generation: Self-cross RrYy × RrYy. Each parent produces four types of gametes: RY, Ry, rY, ry.

F₂ Phenotypic Ratio

Ratio: 9 : 3 : 3 : 1

What the Dihybrid Cross Proves

It establishes the Law of Independent Assortment — the two pairs of traits (seed shape and seed colour) are inherited independently of each other; the allele a gamete receives for shape has no influence on which allele it receives for colour.

Monohybrid vs Dihybrid Cross

Mendel's Three Laws of Inheritance

Law of Dominance

When two contrasting alleles are present together (heterozygous condition), only one — the dominant allele — is expressed in the phenotype; the other, the recessive allele, remains hidden. Example: In Tt, only tallness (T) shows up.

Law of Segregation

The two alleles of a gene separate (segregate) during gamete formation, so each gamete carries only one allele of the pair. This is why a Tt plant produces two kinds of gametes, T and t, in equal proportion — explaining why the recessive trait can reappear in F₂.

Law of Independent Assortment

When two or more pairs of traits are considered together, each pair of alleles segregates independently of other pairs during gamete formation. This is demonstrated by the 9:3:3:1 ratio in the dihybrid cross. (A fourth idea sometimes listed alongside these is theLaw of Paired Factors— that every character is controlled by a pair of factors/alleles, one inherited from each parent.)

Important Genetic Terms — Glossary

Genotype vs Phenotype

Homozygous vs Heterozygous

How Traits Are Expressed: DNA → Gene → Protein → Trait

Genes are segments of DNA. Each gene carries the instructions to build a particular protein (an enzyme, structural protein, pigment, or hormone), and it is the protein's activity that ultimately produces the visible trait. 

DNA → Gene → Protein → Trait 

Example: A gene may control production of a plant growth hormone. More active hormone production can result in a taller plant; reduced or non-functional hormone production can result in a dwarf plant. 

So a small change at the DNA level can cascade into a visible phenotypic difference. During reproduction, DNA must be accurately copied and transmitted to offspring. Occasional copying errors create new variations — the raw material that, over generations, becomes the basis of evolutionary change.

Sex Determination in Humans

Humans have 23 pairs of chromosomes: 22 pairs of autosomes (same in both sexes) and 1 pair of sex chromosomes, which differ between males and females.

The mother produces eggs that always carry an X chromosome. The father produces two kinds of sperm in roughly equal numbers — one carrying X, one carrying Y.

Because the mother's contribution is always X, it is the father's sperm (X or Y) that determines the sex of the child — and since roughly half the sperm carry X and half carry Y, the probability of a male or female child is close to 50:50 at each conception. Important exam point: It is biologically incorrect to "blame" a mother for the sex of her child — she has no control over which type of sperm fertilises her egg.

Acquired Traits vs Inherited Traits

Why Cutting a Mouse's Tail Doesn't Produce Tailless Offspring

Cutting a mouse's tail changes only its body (somatic) cells, not the DNA in its germ cells (sperm/egg). Since the genetic information passed on is unaffected, the offspring of that mouse will still be born with a tail. This single example is frequently used to refute Lamarck's idea that acquired characters are inherited (see below).

Theories of Evolution: Lamarck and Darwin

Lamarck's Theory — Inheritance of Acquired Characters

Jean-Baptiste Lamarck, a French naturalist, was among the earliest scientists to propose a mechanism for evolution. His theory rested on two ideas:

1. Use and disuse of organs — organs that are used repeatedly become stronger and more developed; organs not used gradually weaken and may disappear.
2. Inheritance of acquired characters — these changes acquired during an individual's lifetime are passed on to offspring.

Example often cited: Lamarck proposed that giraffes developed long necks because successive generations stretched to reach higher leaves, and this acquired elongation was inherited. Lamarck's theory is now considered largely incorrect, because modern genetics shows that changes to body cells (somatic changes) do not alter the DNA of germ cells, so they cannot be passed to the next generation (see the mouse-tail example above).

Darwin's Theory — Natural Selection

Charles Darwin, an English naturalist, developed his theory after a five-year voyage aboard the survey ship HMS Beagle, during which his observations of wildlife — especially on the Galápagos Islands — formed the basis of his ideas, published in On the Origin of Species (1859).

Core Argument

This is often summarised (somewhat loosely) as "survival of the fittest" — though "fittest" means best suited to the current environment, not necessarily strongest. A small, well-camouflaged insect may survive better than a larger, more conspicuous one.

Worked Example: The Peppered Moth

Before industrial pollution in England, tree trunks were pale, and light-coloured peppered moths were well camouflaged while dark moths were easily spotted and eaten by birds. As soot from factories darkened tree trunks, the advantage reversed — dark moths became better camouflaged, and their numbers rose sharply in polluted areas. This is a textbook case of natural selection acting on a pre-existing variation in response to a changed environment.

Worked Example: Beetles on Bushes

In a beetle population on green bushes, suppose most beetles are red and a few are green due to natural variation. Birds spot and eat red beetles far more easily than green ones. Over many generations, surviving and reproducing green beetles become more common, gradually shifting the population's colour composition — again, natural selection in action, with the population (not any single beetle) undergoing change.

Genetic Drift

Natural selection is not the only force that changes a population's gene pool. Genetic drift is a random change in the frequency of genes in a population, unrelated to whether the gene is advantageous. For instance, if a chance event (a flood, a predator attack, a natural disaster) wipes out a large fraction of a population regardless of which variation individuals carried, the surviving population's gene frequencies may shift purely by chance — not because any trait conferred a survival advantage. Genetic drift is especially significant in small, isolated populations.

Gene Flow

Gene flow is the movement of genes from one population into another, usually through migration of individuals who then interbreed with the new population. This introduces new variations into a population and can counteract the differences that geographic isolation might otherwise create between two groups.

Lamarck vs Darwin — Quick Comparison

Evidence of Evolution

Homologous Organs

Homologous organs have the same basic structure and developmental origin but may perform different functions in different organisms. Their similarity points to a common ancestor. Classic example: the forelimbs of humans, bats, whales, and horses all share the same underlying bone arrangement, despite being used for grasping, flying, swimming, and running respectively.

Analogous Organs

Analogous organs have different basic structures and origins but perform a similar function due to similar environmental pressures (not shared ancestry). Classic example: the wings of birds and the wings of insects both enable flight, but they evolved independently from entirely different structures. Similarly, the wing of a bat and the wing of a butterfly are analogous, not homologous, because their structural design and developmental origin are completely different even though both fly.

Homologous vs Analogous Organs

Fossils

Fossils are preserved remains, impressions, or traces of organisms that lived in the past — bones, shells, footprints, or leaf impressions, often preserved when minerals gradually replace buried organic remains. Fossils are useful because:

• They help trace the evolutionary history of organisms.
• Deeper rock layers generally contain older fossils, and shallower layers contain more recent ones, letting scientists build a timeline of change.
• Comparing fossils such as Archaeopteryx (which shows both reptilian and bird-like features) helps establish evolutionary links between groups.
• They allow approximate dating of rock layers.

Molecular/DNA Evidence

Beyond anatomy and fossils, modern biology compares DNA sequences between species — this is called molecular phylogeny. Species with more similar DNA sequences are inferred to share a more recent common ancestor, while greater DNA differences suggest more distant relatedness. This molecular approach has confirmed and refined many relationships first proposed from anatomical evidence — for example, the close genetic similarity between humans and chimpanzees.

Speciation

Speciation is the process by which a new species forms from an existing one.

How Speciation Occurs

Worked Example

A population of beetles living on a mountain feeds on a particular bush. If some individuals begin feeding on a nearby, separate population of bushes, they may become geographically/behaviourally separated from the original group. 

Over many generations, different variations accumulate independently in each group due to their different conditions; if a river later separates a population into two groups entirely, each may accumulate enough genetic difference to eventually become a distinct species. Important note: A self-pollinating species, isolated as a single small population, may also undergo speciation purely from accumulated mutations and genetic drift, even without geographic separation. 

Geographic isolation is the most common route, but not the only one — and the number of individuals an isolated population starts with (genetic drift is more powerful in smaller populations) and the type of reproduction (sexual vs asexual) both influence how quickly speciation can occur.

Human Evolution

Human evolution traces the gradual development of modern Homo sapiens from earlier ancestral primates over several million years.

Key Points

• All living humans today belong to a single species, Homo sapiens, regardless of differences in appearance, size, or skin colour — because all humans can interbreed and produce fertile offspring, which is the standard biological test for belonging to the same species.
• Human evolutionary studies rely on excavation, time-dating techniques, and fossil comparison.
• Evidence points to human origins in Africa, with subsequent migration of populations across West Asia, Central Asia, South Asia, East Asia, and eventually into Indonesia, Australia, and the Americas — a process that involved repeated movement in and out of regions, not a single, one-way march.
• Humans share a close genetic relationship with chimpanzees and other primates, indicating a relatively recent common ancestor — but importantly, humans did not evolve from modern apes or monkeys; both lineages diverged from a shared ancestral primate population.

Approximate Stages (often cited)

This sequence is a simplified summary; actual human evolution involved many side branches, overlapping populations, and is still an active area of research, so textbooks present a generalised picture rather than a strict ladder.

How Evolution and Classification Are Linked

The degree of similarity or difference between organisms' characteristics is itself a clue to how closely related they are in evolutionary terms — this is essentially why biological classification and evolutionary study are interconnected. Organisms grouped together in classification typically share a more recent common ancestor, and the further back you must go to find a shared ancestor between two groups, the more different their characteristics tend to be. 

Evolutionary studies also generally show that body design has become more complex over time — for instance, in evolutionary terms, bacteria represent some of the simplest body designs, while organisms like chimpanzees represent considerably more complex ones, although "simple" does not mean "poorly adapted to its environment."

Common Mistakes to Avoid

MCQs for Practice

1. The transmission of traits from parents to offspring is called: A. Evolution B. Heredity C. Speciation D. Mutation Answer: B
2. Who is called the father of genetics? A. Charles Darwin B. Gregor Mendel C. Lamarck D. Watson Answer: B
3. In a monohybrid cross, the F₂ phenotypic ratio is: A. 1:2:1 B. 3:1 C. 9:3:3:1 D. 2:1 Answer: B
4. In a dihybrid cross, the F₂ phenotypic ratio is: A. 3:1 B. 1:2:1 C. 9:3:3:1 D. 1:1 Answer: C
5. A pea plant with genotype Tt will be: A. Tall B. Dwarf C. Neither D. Both Answer: A
6. Which chromosome combination produces a male child? A. XX B. XY C. YY D. XO Answer: B
7. Organs with the same basic structure but different functions are called: A. Analogous organs B. Homologous organs C. Vestigial organs D. Fossils Answer: B
8. Fossils are chiefly useful as evidence of: A. Food chains B. Evolution C. Photosynthesis D. Respiration Answer: B
9. The formation of a new species is called: A. Heredity B. Speciation C. Variation D. Dominance Answer: B
10. Who proposed the theory of "inheritance of acquired characters"? A. Darwin B. Mendel C. Lamarck D. Watson Answer: C
11. The ship on which Darwin made his famous voyage was the: A. HMS Beagle B. Titanic C. Mayflower D. HMS Victory Answer: A
12. A random, non-selective change in gene frequency within a population is called: A. Natural selection B. Genetic drift C. Speciation D. Mutation Answer: B

Assertion–Reason Questions

Q1. Assertion: Mendel selected pea plants for his experiments. 

Reason: Pea plants show many contrasting traits and can both self-pollinate and be cross-pollinated. 

Answer: Both true; Reason correctly explains Assertion. 

Q2. Assertion: A Tt pea plant is tall. 

Reason: The allele for tallness is dominant over the allele for dwarfness. 

Answer: Both true; Reason correctly explains Assertion. 

Q3. Assertion: Acquired traits are not inherited. 

Reason: Acquired traits do not alter the DNA of reproductive (germ) cells. 

Answer: Both true; Reason correctly explains Assertion. 

Q4. Assertion: The father determines the sex of the child in humans. 

Reason: The father produces sperm carrying either an X or a Y chromosome, while the mother's egg always carries X. 

Answer: Both true; Reason correctly explains Assertion. 

Q5. 

Assertion: Lamarck's theory of evolution is not accepted by modern science. 

Reason: Changes acquired in body cells during an organism's lifetime cannot alter the genetic information passed on through germ cells.

 Answer: Both true; Reason correctly explains Assertion.

21. Case-Based Questions

Case 1: Monohybrid Cross

A pure tall pea plant was crossed with a pure dwarf pea plant. All F₁ offspring were tall. When two F₁ plants were crossed, the F₂ generation showed tall and dwarf plants in a 3:1 ratio.

1. Which trait is dominant? — Tallness
2. What is the genotype of F₁ plants? — Tt
3. What is the F₂ phenotypic ratio? — 3 tall : 1 dwarf
4. Why did the dwarf trait disappear in F₁? — It is recessive and masked by the dominant tall allele
5. Which law explains its reappearance in F₂? — Law of Segregation

Case 2: Sex Determination

In humans, the mother's eggs always carry an X chromosome, while the father's sperm carry either X or Y.

1. Chromosome combination of a female child? — XX
2. Chromosome combination of a male child? — XY
3. Which parent determines the child's sex? — The father
4. Why doesn't the mother determine sex? — She always contributes an X chromosome regardless of the child's eventual sex
5. What happens if an X-bearing sperm fertilises the egg? — The child will be female

Case 3: Natural Selection

A beetle population on green leaves is mostly red, with a few green individuals due to natural variation. Birds spot red beetles more easily than green ones. Over many generations, the proportion of green beetles in the population increases.

1. What is the useful variation here? — Green body colour
2. Why do green beetles survive better? — Better camouflage against green leaves, less predation
3. What evolutionary process does this illustrate? — Natural selection
4. Does this change occur in a single beetle or in the population? — In the population, over generations
5. What might happen over many more generations? — Green colouring could become the dominant trait in the population