Heredity and Evolution
Introduction
Heredity and Evolution is one of those Class 10 Biology chapters that connects genetics with long-term species change. The first half focuses on how traits are inherited (Mendel's experiments, monohybrid and dihybrid crosses, sex determination), while the second half explains how species evolve (variations, natural selection, speciation, fossils, homologous/analogous organs). This chapter consistently appears through "Mendel's cross diagrams," "differentiate between acquired and inherited traits," "explain natural selection," and "homologous vs analogous organs" type questions. Understanding the Punnett square method for crosses and the logic behind evolution makes this a high-scoring chapter for Board 2026.
What is heredity?
Heredity is the transmission of genetic characteristics (traits) from parents to offspring through genes.
Gene: The functional unit of inheritance that controls one or more characteristics in living organisms. Genes are located on chromosomes.
Chromosome: Thread-like structures in the nucleus
that carry genes. Humans have 23 pairs (46 total) of chromosomes.
History: THE NINETEENTH CENTURY (1815-1914) Chapter 3: The Making of a Global World — NCERT
Mendel's contribution to genetics
Gregor Johann Mendel (1822-1884) is called the "Father of Genetics." He performed experiments on garden pea plants (Pisum sativum) and discovered the fundamental laws of inheritance.
Why did Mendel choose pea plants?
· Easy to grow and short life cycle
· Produces many offspring
· Has several contrasting visible traits (tall/dwarf, round/wrinkled seeds)
· Self-pollination possible, but cross-pollination can be done artificially
· True-breeding varieties available (pure lines)
Inherited traits vs acquired traits
|
Feature |
Inherited traits |
Acquired traits |
|
Definition |
Traits controlled by genes, passed from parents to offspring |
Traits developed during lifetime due to environment/use |
|
Inheritance |
Passed to next generation |
NOT passed to next generation |
|
Cause |
Genetic (DNA changes) |
Environmental factors, use/disuse of organs |
|
Examples |
Eye color, blood group, height, skin color |
Muscular body (exercise), language, scars, hair loss |
Why are acquired traits not inherited? Acquired traits do not cause changes in DNA of germ cells (reproductive cells), so they cannot be passed to offspring.
Mendel's experiments and laws
Monohybrid cross
Monohybrid cross studies inheritance of ONE pair of contrasting traits.
Experiment: Mendel crossed pure tall pea plants (TT) with pure dwarf pea plants (tt).
Results:
· F1 generation: All plants were tall (Tt) → Tallness is dominant
· F2 generation (F1 selfed): Tall and dwarf plants appeared in ratio 3:1
Genotypic ratio in F2: TT : Tt : tt = 1 : 2 : 1
Phenotypic ratio in F2:
Tall : Dwarf = 3 : 1
Explanation:
· Each trait is controlled by a pair of alleles (genes)
· Dominant allele (T): Expresses itself even when paired with recessive allele
· Recessive allele (t): Expresses only when both alleles are recessive (tt)
· Alleles separate during gamete formation (Law of Segregation)
Mendel's Law of Segregation
The two alleles of a gene separate during gamete formation, so each gamete receives only one allele. During fertilization, alleles from both parents combine to restore the pair.
Dihybrid cross
Dihybrid cross studies inheritance of TWO pairs of contrasting traits simultaneously.
Experiment: Mendel crossed pea plants with round-yellow seeds (RRYY) with wrinkled-green seeds (rryy).
Results:
· F1 generation: All plants had round-yellow seeds (RrYy) → Round and yellow are dominant
· F2 generation (F1 selfed): Four types appeared in ratio 9:3:3:1
o Round-yellow: 9
o Round-green: 3
o Wrinkled-yellow: 3
o Wrinkled-green: 1
Mendel's Law of Independent Assortment
When two pairs of traits are considered together, the inheritance of one trait is independent of the other. Alleles of different genes assort independently during gamete formation.
Example: The probability of a seed being round
or wrinkled is independent of whether it will be yellow or green.
Class 10 Science – Chapter: How Do Organisms Reproduce? complete notes
Sex determination in humans
Humans have 23 pairs of chromosomes (46 total): 22 pairs are autosomes (non-sex chromosomes), and 1 pair is sex chromosomes.
Sex chromosomes:
· Females: XX (both are X chromosomes)
· Males: XY (one X, one Y chromosome)
How is sex determined?
|
Gamete |
Mother (XX) |
Father (XY) |
Child |
|
Egg |
Always X |
— |
— |
|
Sperm |
— |
50% have X, 50% have Y |
— |
|
If sperm = X |
X from mother |
X from father |
XX = Girl |
|
If sperm = Y |
X from mother |
Y from father |
XY = Boy |
Conclusion: Father's sperm determines the sex of the child (50% chance boy, 50% chance girl).
History: THE PRE-MODERN WORLD (Before 1800s) Chapter 3: The Making of a Global World.
Evolution
Evolution is the gradual change in inherited characteristics of a population over successive generations, leading to formation of new species.
How do variations lead to evolution?
Variation is the difference in traits among individuals of a species caused by:
· Errors in DNA copying during reproduction
· Sexual reproduction (mixing of genetic material from two parents)
· Genetic recombination during gamete formation
Importance of variation:
· Provides raw material for evolution
· Helps species adapt to changing environment
· Variations that give survival advantage are selected by nature
Example: If temperature of water increases
suddenly, bacteria with heat-resistant variation will survive better than
others.
Class 10 Science – Chapter 6: Control and Coordination
Darwin's theory of natural selection
Charles Darwin proposed that evolution occurs through natural selection.
Natural selection:
- In any population, more offspring are produced than can survive (struggle for existence)
- Individuals with variations suited to their environment survive and reproduce (survival of the fittest)
- Advantageous traits are passed to next generation
- Over many generations, these changes accumulate, leading to formation of new species
Example: Giraffe's long neck evolved because
individuals with slightly longer necks could reach higher leaves, survived
better, and passed this trait to offspring.
History: THE SENSE OF COLLECTIVE BELONGING (Chapter 2: Nationalism in India — NCERT)
Evidence of evolution
1. Homologous organs
Homologous organs have the same basic structure and origin but perform different functions.
Examples:
· Forelimbs of humans, cats, whales, bats (same bone structure, different functions)
· Thorns of Bougainvillea and tendrils of Cucurbita (both are modified stems)
Significance: Homologous organs indicate common ancestry (divergent evolution).
2. Analogous organs
Analogous organs have different structure and origin but perform similar functions.
Examples:
· Wings of birds and wings of insects (both for flying, different structures)
· Sweet potato (modified root) and potato (modified stem) (both for storage)
Significance: Analogous organs indicate convergent evolution (different species adapted similarly to same environment).
Table: Homologous vs Analogous organs
|
Feature |
Homologous organs |
Analogous organs |
|
Basic structure |
Same |
Different |
|
Origin |
Same |
Different |
|
Function |
Different |
Same |
|
Evolutionary significance |
Common ancestry (divergent evolution) |
Convergent evolution |
|
Example |
Human arm and bat wing |
Bird wing and insect wing |
3.
Fossils
Fossils are preserved remains or traces of organisms that lived in the past, found in rocks.
How fossils form:
· Dead organism gets buried in sediment
· Over millions of years, sediment hardens into rock
· Organism's remains are preserved as fossil
Importance of fossils:
· Provide evidence of evolution
Show how organisms looked in the past
Help determine age of organisms (deeper fossils are older)
Show evolutionary relationships between species
Example: Archaeopteryx fossil shows
characteristics of both reptiles (teeth, tail) and birds (feathers, wings),
proving birds evolved from reptiles.
Class 10 Students (Board Exam 2026)! Chapter 6 Life Processes complete notes.
Speciation
Speciation is the formation of new species from existing ones due to evolution.
How does speciation occur?
1. Genetic drift: Random changes in gene frequency in small populations
Geographic isolation: Physical barrier (river, mountain) separates populations
Reproductive isolation: Separated populations cannot interbreed
Accumulation of variations: Each population adapts to its environment differently
New species: After many generations, populations become so different they can no longer interbreed
Example: Darwin's finches on Galapagos Islands
evolved from common ancestor into different species with different beak shapes
suited to different foods.
Class 10 Science Chapter 4 Carbon and its Compounds
Human evolution
Humans belong to genus Homo and species Homo sapiens.
Stages of human evolution:
1. Apes (common ancestor of humans and apes)
2. Ramapithecus (15 million years ago)
3. Australopithecus (4 million years ago, walked upright)
4. Homo habilis (2 million years ago, used tools)
5. Homo erectus (1.5 million years ago, discovered fire)
6. Homo sapiens (modern humans, 200,000 years ago)
Evidence:
· Fossils show gradual changes in skull size, brain capacity, posture
· DNA studies show humans share 98.6% DNA with chimpanzees
· Humans evolved in Africa and migrated to other continents
Class 10 Science Chapter 3: Metals and Non-metals.
MCQs (PYQ)
1. The exchange of genetic material takes
place in:
(a) Vegetative reproduction
(b) Asexual reproduction
(c) Sexual reproduction
(d) Budding
Answer: (c) Sexual reproduction. (CBSE 2020)
2.
In
a monohybrid cross between tall pea plants (TT) and dwarf pea plants (tt), the
ratio of tall to dwarf plants in F2 generation is:
(a) 1:3
(b) 3:1
(c) 1:1
(d) 2:1
Answer: (b) 3:1. (CBSE 2024)
3.
The
number of pairs of sex chromosomes in the zygote of humans is:
(a) One
(b) Two
(c) Three
(d) Four
Answer: (a) One (23rd pair). (CBSE 2023)
4.
A
trait which is expressed only in homozygous condition is called:
(a) Dominant
(b) Recessive
(c) Codominant
(d) Incomplete dominant
Answer: (b) Recessive. (CBSE 2020)
5.
Which
of the following is an example of homologous organs?
(a) Wings of bird and insect
(b) Forelimbs of human and bat
(c) Sweet potato and potato
(d) Tendril and spine
Answer: (b) Forelimbs of human and bat. (CBSE 2024)
6.
The
theory of evolution was proposed by:
(a) Mendel
(b) Darwin
(c) Lamarck
(d) Morgan
Answer: (b) Charles Darwin. (CBSE 2020)
7.
Fossils
found in deeper layers of earth are:
(a) Older
(b) Younger
(c) Same age
(d) Cannot determine
Answer: (a) Older fossils are in deeper layers. (CBSE 2023)
8.
In
pea plants, round seed (R) is dominant over wrinkled seed (r). A plant with
genotype Rr will have:
(a) Round seeds
(b) Wrinkled seeds
(c) Both
(d) Neither
Answer: (a) Round seeds (dominant expressed). (CBSE 2020)
9.
Which
determines the sex of a child in humans?
(a) Mother's X chromosome
(b) Father's X or Y chromosome
(c) Both equally
(d) Environment
Answer: (b) Father's sperm (X or Y). (CBSE 2024)
10.
Analogous
organs indicate:
(a) Common ancestry
(b) Convergent evolution
(c) Divergent evolution
(d) No evolution
Answer: (b) Convergent evolution. (CBSE 2020)
Class 10 History (NCERT) Chapter 2 — Section 2: Differing Strands within the Movement.
Short Answer Questions (PYQ with answers)
Q1. Why are acquired traits not
inherited?
Answer: Acquired traits are developed during
an organism's lifetime due to environmental factors or use/disuse. They do not
cause changes in DNA of germ cells (reproductive cells), so cannot be passed to
offspring. Example: Bodybuilder's muscles won't be inherited by children. (CBSE
2020)
Q2. Differentiate between homologous
and analogous organs with examples.
Answer: Homologous organs have same basic
structure and origin but different functions (e.g., human arm and bat wing -
common ancestry). Analogous organs have different structure and origin but same
function (e.g., bird wing and insect wing - convergent evolution). (CBSE
2024)
Q3. How does sex determination occur
in humans?
Answer: Humans have 23 pairs of chromosomes. Females have XX sex
chromosomes; males have XY. Mother always gives X. Father gives either X (child
is girl) or Y (child is boy). Thus father's sperm determines sex with 50%
probability each. (CBSE 2023)
Q4. What is speciation? How does
geographic isolation lead to speciation?
Answer: Speciation is formation of new
species. Geographic isolation separates a population by physical barrier
(river, mountain). Isolated groups cannot interbreed, accumulate different
variations suited to their environments. Over generations, they become so
different they cannot interbreed even if barrier removed - new species formed. (CBSE
2020)
Q5. State Mendel's law of
independent assortment.
Answer: When two pairs of traits are inherited together, inheritance of
one trait is independent of the other. Alleles of different genes assort
independently during gamete formation. Example: In dihybrid cross,
round/wrinkled is independent of yellow/green, giving 9:3:3:1 ratio in F2. (CBSE
2020).
Science Chapter 2: Acids, Bases & Salts.
Long Answer Questions (PYQ with answers)
Q1. Explain Mendel's monohybrid
cross experiment with diagram (in words). What is law of segregation?
Answer: Mendel crossed pure tall (TT) with pure dwarf (tt) pea plants. F1: All tall (Tt) - tallness dominant. F1 selfed → F2: 3 tall : 1 dwarf. Genotypic ratio: TT:Tt:tt = 1:2:1. Law of Segregation: Two alleles of a gene separate during gamete formation; each gamete gets only one allele. During fertilization, alleles from both parents combine. (CBSE 2020, 2023)
Q2. How do fossils provide evidence
for evolution?
Answer: Fossils are preserved remains of organisms from the past. They show: (1) How organisms looked millions of years ago, (2) Deeper fossils are older, showing time sequence of evolution, (3) Transitional fossils like Archaeopteryx (reptile + bird features) show evolutionary links, (4) Help trace evolutionary relationships between species. Fossils prove organisms have changed over time. (CBSE 2024)
Q3. Explain Darwin's theory of
natural selection.
Answer: Darwin's theory states: (1) Organisms produce more offspring than can survive - struggle for existence, (2) Variations exist within population, (3) Individuals with advantageous variations survive better - survival of fittest, (4) Survivors reproduce, passing beneficial traits to offspring, (5) Over many generations, favorable traits accumulate leading to evolution. Example: Giraffes with longer necks could reach high leaves, survived better, passed trait to offspring. (CBSE 2020)
Q4. A cross was made between pure-breed
pea plants - one with round, green seeds (RRyy) and another with wrinkled,
yellow seeds (rrYY). (a) What will be F1 phenotype? (b) What is F2 phenotypic
ratio when F1 is selfed?
Answer: (a) F1 will be RrYy - all round, yellow seeds (both dominant traits expressed). (b) F2 phenotypic ratio: Round-yellow : Round-green : Wrinkled-yellow : Wrinkled-green = 9:3:3:1. This follows law of independent assortment. (CBSE 2020)
Q5. (a) Why are human males
considered heterogametic? (b) A child has blood group O. If father has blood
group A and mother has blood group B, work out genotypes of parents.
Answer: (a) Human males have XY sex
chromosomes (two different types), producing two types of sperms (50% X, 50%
Y), hence heterogametic. Females have XX (homogametic). (b) Child O means
genotype OO. Father A can be IA IA or IA IO; Mother B can be IB IB or IB IO.
For child to have OO, both parents must be heterozygous: Father IA IO, Mother
IB IO. (CBSE 2023).
Complete Notes on Nationalism in India (First World War, Khilafat & Non-Cooperation Movement).
Conclusion
Heredity and Evolution becomes manageable when you divide it into two parts: (1) Heredity - Mendel's experiments (monohybrid 3:1 ratio, dihybrid 9:3:3:1 ratio), laws of segregation and independent assortment, sex determination (XX/XY), acquired vs inherited traits; (2) Evolution - variations, natural selection, homologous vs analogous organs, fossils, speciation. NCERT emphasizes Punnett square diagrams for crosses, comparison tables (homologous vs analogous, acquired vs inherited), and "explain" questions (Darwin's theory, speciation). This conceptual chapter rewards understanding and can easily give 5-6 marks in Board exams.
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