CBSE Class 10 Science Heredity and Evolution Notes

2. Gregor Mendel and His Experiments

Gregor Mendel (1822–1884) was an Austrian monk and scientist who conducted systematic breeding experiments with pea plants (Pisum sativum) in the garden of his monastery in Brno (present-day Czech Republic). Over eight years (1856–1863), he grew and cross-pollinated thousands of pea plants, carefully recording the traits of parents and offspring over multiple generations. His work, published in 1866, established the fundamental laws of inheritance — but was largely ignored until 1900, when three scientists independently rediscovered and validated his findings.

Why Mendel chose pea plants: Pea plants were ideal experimental organisms because they have a short generation time (produce seeds within months), produce large numbers of offspring, have clearly distinguishable traits (tall/short, yellow/green), can self-pollinate (allowing pure lines to be established) or be cross-pollinated (allowing controlled crosses), and were cheap and easy to grow in large numbers.

2.1 Mendel's Seven Contrasting Traits in Pea Plants

2.2 Key Terminology in Genetics

3. Mendel's Laws of Inheritance

Mendel's observations led him to propose three fundamental laws that govern the inheritance of traits. These laws, though formulated over 150 years ago, remain the cornerstone of classical genetics and are fully consistent with our modern understanding of chromosomes and DNA.

3.1 Law of Dominance

Statement: When two homozygous individuals with contrasting traits are crossed, only one trait (the dominant trait) is expressed in the offspring (F1 generation). The other trait (the recessive trait) is suppressed but not lost.

3.2 Law of Segregation (Purity of Gametes)

Statement: The two alleles for any trait segregate (separate) from each other during gamete formation so that each gamete carries only ONE allele for that trait. The alleles are reunited randomly at fertilisation.

Why it works biologically: Alleles are located on homologous chromosomes. During meiosis (the cell division that produces gametes), homologous chromosomes separate — so each gamete receives only one copy of each chromosome, and therefore only one allele for each gene.

3.3 Law of Independent Assortment

Statement: When two or more pairs of traits are considered simultaneously, the alleles for different traits segregate independently of each other during gamete formation. The inheritance of one trait is not influenced by the inheritance of another trait (provided the genes are on different chromosomes).

Why it works biologically: Genes on different chromosomes assort independently because non-homologous chromosomes align randomly during meiosis I. The orientation of one chromosome pair has no effect on how another pair aligns.

Dihybrid Cross — Two Traits Simultaneously

4. Sex Determination

Sex determination refers to the biological mechanism that determines whether an individual develops as male or female. In humans and most mammals, sex is determined chromosomally — by a specific pair of chromosomes called sex chromosomes (or allosomes). All other chromosomes (non-sex chromosomes) are called autosomes.

4.1 Sex Determination in Humans — XX/XY System

Humans have 23 pairs of chromosomes (46 chromosomes total). Of these, 22 pairs are autosomes (identical in both sexes). The 23rd pair consists of the sex chromosomes, which differ between males and females.

4.2 Sex Determination in Other Organisms

5. Variations — Inherited and Acquired

Not all differences between individuals are passed on to the next generation. It is crucial to distinguish between variations that are encoded in DNA (inherited) and those caused purely by the environment (acquired). This distinction was one of the most important debates in the history of biology — the incorrect theory of Jean-Baptiste Lamarck proposed that acquired characteristics could be inherited, while the correct mechanism was revealed by Weismann's experiments.

5.1 Inherited Variations

Definition: Inherited variations are differences in traits that arise from changes in the DNA (mutations, recombination) and are therefore passed from parent to offspring through gametes. These are the variations that matter for evolution.

•         Sources: (1) Mutations — random errors in DNA copying during cell division. (2) Recombination — shuffling of chromosomes and crossing over during meiosis in sexual reproduction. (3) Random fertilisation — any sperm can fertilise any egg.

•         Examples: A child born with blue eyes (different from both brown-eyed parents), the occurrence of a genetic disease like sickle cell anaemia, or Mendel's tall/short pea plants — all inherited variations.

•         Significance for evolution: Only inherited variations can be passed to offspring and therefore accumulated in populations over generations. This is what drives evolution.

5.2 Acquired Variations (Acquired Characteristics)

Definition: Acquired variations are changes in an organism's body or behaviour that occur during its lifetime as a result of experience, use, or environmental conditions. These changes do NOT alter the DNA in the germ cells (eggs and sperm) and therefore CANNOT be passed to offspring.

•         Examples: A bodybuilder's enlarged muscles cannot be inherited by their children. A person's scar from an accident is not passed on. A plant that grows taller in a nutrient-rich soil — its children will not necessarily be taller (unless the nutrient conditions are the same).

•         Weismann's experiment (1880s): August Weismann cut off the tails of mice for 22 generations. In every generation, offspring were still born with normal tails. This directly disproved the idea that acquired modifications (cutting the tail) could be inherited — a powerful experimental refutation of Lamarck's theory.

5.3 Lamarck vs Darwin — The Great Debate

6. Evolution — Darwin's Theory and Natural Selection

Evolution is the change in heritable characteristics of biological populations over successive generations. Charles Darwin and Alfred Russel Wallace independently developed the theory of evolution by natural selection in the 1850s, and Darwin published his landmark work 'On the Origin of Species' in 1859. Their theory was the first scientifically rigorous explanation for the diversity of life and remains the foundational framework of all modern biology.

6.1 Darwin's Theory of Evolution by Natural Selection

Darwin's theory can be broken down into four observable facts and two deductions. These form a logical chain of reasoning that leads inevitably to the conclusion that natural selection occurs.

6.2 Genetic Drift — Random Evolution

Definition: Genetic drift is random change in allele frequencies in a population due to chance events — not due to natural selection. It is especially significant in small populations where a chance event can dramatically alter allele frequencies.

•         Founder Effect: When a small group of individuals breaks off from a larger population and establishes a new population elsewhere, the new population has a limited sample of the original gene pool. The allele frequencies in the new population may be very different from the original, simply by chance.

•         Bottleneck Effect: When a catastrophic event (disease, disaster) drastically reduces a population's size, the survivors may not be representative of the original gene pool. The reduced population's allele frequencies differ by chance, not selection.

•         Example: Cheetahs went through a severe bottleneck thousands of years ago — they are so genetically similar that skin grafts between unrelated cheetahs are accepted without immune rejection.

6.3 Speciation — How New Species Form

Definition: Speciation is the process by which one species splits into two or more distinct species that can no longer interbreed. It is the mechanism by which biodiversity increases over time.

7. Evidence for Evolution

Evolution is one of the most thoroughly evidenced theories in all of science. Evidence comes from multiple independent fields — palaeontology (fossils), comparative anatomy, embryology, biogeography, and molecular biology. The convergence of all these lines of evidence on the same conclusion gives evolution its exceptional scientific standing.

7.1 Fossil Record

What fossils are: Fossils are the preserved remains or impressions of organisms that lived in the past, found in sedimentary rocks. They form when an organism dies, is buried in sediment, and its hard parts (bones, shells, teeth) are gradually replaced by minerals over millions of years.

•         Evidence provided: Fossils show that life has changed dramatically over time. Older rock layers contain simpler organisms; newer layers contain more complex, modern-looking organisms. The fossil record shows a clear progression from simple to complex life forms over geological time.

•         Transitional fossils: Some fossils represent 'missing links' between major groups — they show intermediate features. Example: Archaeopteryx had both reptile features (teeth, clawed wings, long bony tail) and bird features (feathers), linking dinosaurs and modern birds.

•         Gaps in the fossil record: Not all organisms fossilise well (soft-bodied organisms rarely fossilise). Gaps in the fossil record reflect the limitations of fossilisation, not the absence of evolutionary intermediates.

7.2 Homologous Organs

Definition: Homologous organs are organs that have the same basic structural plan (same bones, same developmental origin) but have been modified to perform different functions in different organisms, due to adaptation to different environments.

•         Key principle: Same structure = common ancestry. Homologous organs prove that different species descended from a common ancestor — they inherited the same basic anatomical plan and modified it differently over time.

•         Classic example: The forelimb bones (humerus, radius, ulna, carpals, metacarpals, phalanges) of a human arm, a whale's flipper, a bat's wing, a horse's front leg, a cat's paw, and a bird's wing all follow the same pentadactyl (five-digit) pattern — modified for grasping, swimming, flying, running, etc.

•         Significance: Homologous organs are evidence of divergent evolution — one ancestral form diverging into multiple forms adapted to different lifestyles.

7.3 Analogous Organs

Definition: Analogous organs are organs that look similar and perform the same function in different organisms, but have completely different structural plans and different evolutionary origins. They evolved independently in response to similar environmental pressures.

•         Key principle: Same function, different structure = convergent evolution (independent evolution of similar solutions to similar environmental problems). No evidence of common ancestry.

•         Classic examples: Wings of a bat (modified forelimb bones, skin membrane) and wings of a butterfly (chitin exoskeleton, scale-covered membranes) both allow flight but have completely different structures and evolutionary origins.

•         Other examples: The streamlined body shape of dolphins (mammals) and sharks (fish) — both adapted for fast swimming but evolved independently. Sweet potato (modified root) and potato (modified stem) — both store starch but are structurally different.

7.4 Embryological Evidence

Key observation: The embryos of very different vertebrate animals (fish, frog, lizard, bird, human) look remarkably similar at early stages of development. All vertebrate embryos have gill slits, a tail, and a similar body plan in early development — even though adult forms are completely different.

•         Significance: The similarity of early embryos suggests that all vertebrates descended from a common ancestor. As development proceeds, the embryos diverge into their adult forms. Embryological similarity mirrors evolutionary similarity.

•         Karl Ernst von Baer's Law: The more closely related two species are, the more similar their embryos remain throughout development. Distantly related species diverge earlier in development; closely related species share embryonic features for longer.

7.5 Molecular Evidence (DNA)

The most powerful modern evidence for evolution comes from comparing DNA sequences across species. If all life evolved from a common ancestor, then the DNA of related species should be similar — and the more closely related the species, the more similar their DNA should be.

•         Humans and chimpanzees: Share approximately 98–99% of their DNA sequence. This is consistent with the evolutionary view that humans and chimps share a common ancestor that lived approximately 5–7 million years ago.

•         Universal genetic code: All living organisms use the same genetic code (DNA → RNA → protein, using the same codon assignments). This universal code could only have arisen through common descent from a single ancestral population — the Last Universal Common Ancestor (LUCA).

8. Human Evolution

Humans (Homo sapiens) did not evolve from chimpanzees — this is a common misconception. Rather, humans and chimpanzees share a common ancestor that lived approximately 5–7 million years ago. From that common ancestor, two lineages diverged: one leading to modern chimpanzees and bonobos, and another leading to the human lineage (hominins). The hominin lineage shows a progressive increase in brain size, upright posture, and complexity of tools and social organisation over millions of years.

8.1 Major Milestones in Human Evolution

9. Common Mistakes to Avoid

10. Key Definitions and Summary Table

11. Key Points to Remember

•         Mendel used pea plants because: short generation time, many offspring, clear contrasting traits, can be self- or cross-pollinated.

•         Dominant allele is expressed in both TT and Tt. Recessive allele is only expressed in tt.

•         F1 of monohybrid cross: ALL dominant phenotype. F2: 3 dominant : 1 recessive (3:1 phenotype ratio).

•         F1 of dihybrid cross: ALL double dominant phenotype. F2: 9:3:3:1 phenotype ratio.

•         Recessive trait is NOT destroyed in F1 — it reappears in F2. Law of Segregation.

•         Father determines child's sex: Mother always gives X. Father gives X (girl) or Y (boy).

•         Inherited vs acquired: Only inherited (genetic) variations are passed to offspring and drive evolution.

•         Weismann's experiment: Cutting tails of mice for 22 generations — offspring still born with tails. Disproves Lamarck.

•         Natural selection: Heritable variation + differential survival/reproduction = change in population over time.

•         Homologous: Same structure, different function (divergent evolution). Analogous: same function, different structure (convergent evolution).

•         Humans did NOT evolve from chimpanzees — both share a common ancestor ~5–7 million years ago.

•         Brain size increase in human evolution: Australopithecus ~450 cc → Homo habilis ~700 cc → Homo sapiens ~1350 cc.

12. Practice Questions

Modelled on CBSE board exam patterns. For Punnett square questions: always write the parental genotypes, list the gametes, draw the grid, state the genotype ratio, and state the phenotype ratio. For evolution questions: structure your answer as definition → mechanism → example.

12.1 — 1 Mark Questions (VSA)

1.       What is the phenotype ratio of the F2 generation in a monohybrid cross?

2.       Which chromosomes are responsible for determining sex in human beings?

3.       State the Law of Segregation in one sentence.

4.       Name the scientist who disproved Lamarck's theory of inheritance of acquired characteristics.

5.       What are homologous organs? Give one example.

6.       Which trait is recessive — round seeds or wrinkled seeds in pea plants?

7.       What is the phenotype ratio of F2 in a dihybrid cross?

8.       Name the transitional fossil that links reptiles and birds.

12.2 — 3 Mark Questions (SA)

9.       In a cross between tall (TT) and short (tt) pea plants: (a) Write the phenotype and genotype of the F1 generation. (b) What will be the result when F1 plants are self-fertilised? Show with a Punnett square. (c) What ratio of tall to short plants do you expect in F2?

10.   Explain how the sex of a child is determined in humans. Why is it scientifically incorrect to blame the mother for the sex of her child?

11.   Differentiate between homologous and analogous organs. Give one example of each and state what each type of organ is evidence for.

12.   Explain Darwin's theory of natural selection using the example of giraffes (or peppered moths).

13.   Distinguish between inherited and acquired variations. Describe Weismann's experiment and explain its significance.

14.   What is speciation? Explain the mechanism by which one species can give rise to two separate species.

12.3 — 5 Mark Questions (LA)

15.   (a) Define Mendel's Law of Segregation and Law of Independent Assortment. (b) In pea plants, yellow seed colour (Y) is dominant over green (y) and round seed shape (R) is dominant over wrinkled (r). Show with a Punnett square the outcome of crossing a RrYy plant with another RrYy plant. Write the F2 phenotype ratio.

16.   Describe the various types of evidence that support the theory of evolution. Include: fossil record, homologous organs, analogous organs, and embryological evidence. Explain what each type of evidence tells us.

17.   (a) What is natural selection? Explain with the example of industrial melanism in peppered moths. (b) What is genetic drift? How does it differ from natural selection? (c) What is speciation? Describe the role of geographic isolation in speciation.

18.   Trace the evolution of the human lineage from Dryopithecus to Homo sapiens sapiens. Describe four key changes that occurred during hominin evolution and explain their significance.

19.   (a) A pure breeding pea plant with violet flowers (VV) is crossed with a pure breeding white-flowered plant (vv). (i) What will the F1 plants look like? (ii) What happens when F1 plants are self-pollinated? Show with a Punnett square. (b) Explain what this experiment demonstrates about dominant and recessive traits, and how the recessive trait 'disappeared' in F1 and 'reappeared' in F2.