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CBSE Class 10 Science Chapter 9 Heredity and Evolution — Complete Notes, MCQs and Board Exam Tips
Heredity and Evolution is one of the most conceptually rich chapters in CBSE Class 10 Science. This chapter typically carries 4-5 marks in the board examination and tests students on Mendel’s Laws, sex determination, Darwin’s theory of Natural Selection, and human evolution. This comprehensive guide covers all key concepts, important questions, and board exam tips to help you score full marks.
Chapter Importance in Board Exam
- Marks Weightage: 4-5 marks (typically 1 short answer + 1 long answer question)
- Unit: Heredity and Evolution falls under the “How do Organisms Reproduce?” and “Heredity” unit
- Frequently Tested: Mendel’s Laws, sex determination in humans, Darwin vs Lamarck, speciation
- Diagram Questions: Monohybrid and dihybrid crosses using Punnett square may be asked
Mendel’s Experiments with Pea Plants
Gregor Johann Mendel (1822-1884), known as the “Father of Genetics,” conducted his famous experiments on the garden pea plant (Pisum sativum) over 8 years (1856-1863). He chose pea plants because they have clearly contrasting traits, short life cycle, and can be easily cross-pollinated.
The Seven Pairs of Contrasting Traits Studied by Mendel
| Trait | Dominant Form | Recessive Form |
|---|---|---|
| Seed Shape | Round | Wrinkled |
| Seed Color | Yellow | Green |
| Pod Shape | Inflated / Full | Constricted |
| Pod Color | Green | Yellow |
| Flower Color | Violet / Purple | White |
| Flower Position | Axial (on stem) | Terminal (at tip) |
| Plant Height | Tall | Dwarf |
Laws of Inheritance
Law of Dominance
When two organisms with contrasting traits are crossed, only one trait (the dominant trait) appears in the F1 generation. The other trait (recessive) is hidden but not lost. Example: When a tall plant (TT) is crossed with a dwarf plant (tt), all F1 plants are tall (Tt) — tall is dominant.
Law of Segregation (Law of Purity of Gametes)
Each organism has two alleles for each trait. During gamete (sex cell) formation, these alleles segregate or separate so that each gamete carries only one allele for each trait. When fertilization occurs, the offspring receive one allele from each parent, restoring the pair.
Example: In a monohybrid cross of Tt × Tt, gametes are T and t. Possible offspring: TT, Tt, Tt, tt — giving a phenotypic ratio of 3 Tall : 1 Dwarf.
Law of Independent Assortment
When two pairs of traits are combined in a cross (dihybrid cross), the alleles for each trait segregate independently of the alleles for the other trait. The phenotypic ratio in F2 of a dihybrid cross is 9:3:3:1.
Sex Determination in Humans
In humans, sex is determined by a pair of sex chromosomes. Females have two X chromosomes (XX) while males have one X and one Y chromosome (XY).
- Eggs (Ova): Females produce only X-bearing eggs
- Sperms: Males produce two types — X-bearing sperms and Y-bearing sperms
- If an X-bearing sperm fertilizes the egg → XX → Female child
- If a Y-bearing sperm fertilizes the egg → XY → Male child
Key point: The sex of the child is determined by the father’s sperm, not the mother’s egg. The mother always contributes an X chromosome.
Evolution
Darwin’s Theory of Evolution (Natural Selection)
Charles Darwin proposed that evolution occurs through Natural Selection. The key principles are:
- Overproduction: Organisms produce more offspring than can survive
- Variation: Natural variations exist among individuals of a species
- Struggle for Existence: Organisms compete for limited food, space, and mates
- Survival of the Fittest: Organisms with advantageous variations survive and reproduce more successfully
- Inheritance: Advantageous traits are passed to offspring over generations, leading to gradual change
Example of Natural Selection: Industrial Melanism in Peppered Moths — before industrialization, light-coloured moths were common (camouflaged on light tree bark). After industrialization, bark turned black with soot. Dark moths survived better; light moths were eaten by birds. Dark moths became dominant — an example of natural selection in action.
Lamarck’s Theory (Inheritance of Acquired Characters)
Jean-Baptiste Lamarck proposed that traits acquired during an organism’s lifetime can be inherited by offspring. Example: Giraffes stretched their necks to reach leaves, and this longer neck was passed to offspring. This theory has been disproved — acquired characters are NOT inherited genetically.
Speciation
Speciation is the process by which new species form from an existing species. It occurs when populations of the same species become reproductively isolated from each other (due to geographic barriers, genetic drift, or natural selection) and diverge genetically over many generations until they can no longer interbreed. Example: Darwin’s finches on the Galapagos Islands — a common ancestor species diversified into 14 different species adapted to different food sources.
Human Evolution Timeline
| Ancestor | Approximate Period | Key Feature |
|---|---|---|
| Dryopithecus | ~15 million years ago | Common ancestor of apes and humans; lived in forests |
| Ramapithecus | ~14 million years ago | More human-like; walked semi-erect |
| Australopithecus | ~2-4 million years ago | Walked upright; used stone tools; brain size ~500 cc |
| Homo habilis | ~2 million years ago | “Handy man”; first tool users; brain size ~600-700 cc |
| Homo erectus | ~1.5 million years ago | Fully upright; used fire; brain size ~900 cc |
| Homo neanderthalensis | ~1,00,000-40,000 years ago | Lived in caves; buried dead; brain size ~1400 cc |
| Homo sapiens (Modern Human) | ~75,000 years ago to present | Language, art, agriculture; brain size ~1350-1500 cc |
Board Exam Important Questions
- What is the difference between inherited and acquired traits? Give one example of each.
- Explain Mendel’s Law of Segregation with a suitable example and Punnett square.
- How is sex determined in human beings? Why is it incorrect to blame the mother for the sex of the child?
- Distinguish between Darwin’s theory of Natural Selection and Lamarck’s theory of Inheritance of Acquired Characters.
- Define speciation. What factors could lead to the rise of a new species?
- In a cross between a tall (TT) and dwarf (tt) pea plant, what will be the ratio of tall to dwarf plants in F2 generation? Show with a Punnett square.
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Practice Quiz — Heredity and Evolution
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Frequently Asked Questions — Heredity and Evolution (Board Exam)
Q1. What is the difference between dominant and recessive traits?
A dominant trait is one that appears in the offspring even when only one copy of the allele is present (heterozygous condition). A recessive trait appears in the offspring only when both copies of the allele are recessive (homozygous recessive condition). In Mendel’s experiment, tallness (T) is dominant over dwarfness (t).
Q2. Why is Mendel called the Father of Genetics?
Mendel is called the Father of Genetics because he was the first scientist to systematically study the inheritance of traits through controlled experiments on pea plants. His findings — the Laws of Dominance, Segregation, and Independent Assortment — laid the foundation of modern genetics. His work, though initially ignored, was rediscovered in 1900 and validated by subsequent research.
Q3. How does natural selection differ from artificial selection?
Natural selection is the process in nature where individuals with advantageous traits survive and reproduce more, driven by environmental pressures. Artificial selection (selective breeding) is performed by humans, deliberately choosing organisms with desired traits for breeding — as in agriculture (high-yield wheat varieties) and animal husbandry (milk-producing cattle breeds).
Q4. What is the significance of variations in evolution?
Variations are differences among individuals of the same species. They arise due to errors in DNA copying (mutations), sexual reproduction, and environmental factors. Variations are the raw material for natural selection — without variation, all individuals would be identical and any environmental change could wipe out the entire species. Variations ensure species adaptability and survival.
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