Chapter 8 — Heredity and Evolution — is one of the most important chapters in CBSE Class 10 Science. It carries 8-10 marks in the board exam and the questions are predictable if you know what to focus on.
This chapter appears in every board exam — it's never skipped. Questions typically include 1 long answer (5 marks), 1-2 short answers (2-3 marks each), and 1 MCQ. A well-prepared student can score full marks here.
Key Concepts You Must Know
1. Heredity — The Basics
Heredity is the transmission of characteristics from parents to offspring. The physical basis of heredity is DNA (Deoxyribonucleic Acid), found in chromosomes inside the nucleus of every cell.
| Term | Definition | Example |
|---|---|---|
| Gene | A unit of inheritance — a segment of DNA that codes for a specific protein/trait | Gene for eye colour |
| Allele | Different forms of the same gene | Tall (T) and short (t) |
| Dominant | Trait expressed even in heterozygous condition | Tt shows Tall |
| Recessive | Trait expressed only in homozygous condition | tt shows short |
| Homozygous | Both alleles are same (TT or tt) | Pure tall or pure short |
| Heterozygous | Alleles are different (Tt) | Hybrid tall |
2. Mendel's Experiments
Gregor Mendel worked with pea plants (Pisum sativum) and studied 7 contrasting traits. His work established the foundation of genetics.
- Seed shape: Round vs Wrinkled
- Seed colour: Yellow vs Green
- Flower colour: Purple vs White
- Flower position: Axial vs Terminal
- Pod shape: Inflated vs Constricted
- Pod colour: Green vs Yellow
- Plant height: Tall vs Short
Monohybrid Cross (One trait):
| Generation | Cross | Result |
|---|---|---|
| P (Parent) | Tall (TT) × Short (tt) | — |
| F1 | All Tall (Tt) | Dominant trait expressed |
| F1 × F1 | Tt × Tt | — |
| F2 | TT : Tt : Tt : tt | 3 Tall : 1 Short |
Phenotypic ratio: 3:1 | Genotypic ratio: 1:2:1 (TT : Tt : tt)
Dihybrid Cross (Two traits):
- Cross involving two traits simultaneously
- F2 ratio: 9:3:3:1
- This proves the Law of Independent Assortment — traits are inherited independently of each other
3. Sex Determination in Humans
| Feature | Female | Male |
|---|---|---|
| Sex chromosomes | XX | XY |
| Type | Homogametic | Heterogametic |
| Gametes | All eggs carry X | Sperm carry X or Y |
| Sex determined by | — | Father's sperm |
Many students incorrectly write that the mother determines the sex of the child. The father's sperm determines sex — if X sperm fertilises the egg → girl (XX); if Y sperm fertilises → boy (XY). This is a socially important point that CBSE loves to test.
4. Evolution
Evolution is the gradual change in the characteristics of a species over generations.
| Evidence Type | What It Shows | Example |
|---|---|---|
| Homologous organs | Same structural plan, different functions | Human hand vs whale flipper |
| Analogous organs | Different structure, same function | Bird wing vs insect wing |
| Fossils | Preserved remains showing transitional forms | Archaeopteryx (reptile → bird) |
| DNA comparison | Common ancestry through genetic similarity | Human & chimp DNA 98% similar |
- Natural selection (Darwin): Organisms with favourable variations survive and reproduce more
- Genetic drift: Random changes in gene frequency — significant in small, isolated populations
- Speciation: Formation of new species due to geographical isolation + genetic drift + natural selection
5. Human Evolution
"Humans evolved from monkeys" — this is wrong. Humans and present-day apes share a common ancestor. We did not descend from any living primate species.
Key facts about human evolution:
- All races of humans belong to a single species — Homo sapiens
- Evolution involved changes in tools, brain size, and posture over millions of years
- Migration out of Africa spread humans across the globe
10 Most Important Questions
These question types appear repeatedly in CBSE board exams:
Short Answer (2-3 marks)
Homologous vs Analogous organs — differences with examples
Draw a comparison table. Homologous = same origin, different function (human hand / whale flipper). Analogous = different origin, same function (bird wing / insect wing).
How is sex determined in human beings? Explain with diagram.
Show the cross: Father (XY) × Mother (XX). Draw a Punnett square showing 50% XX (girl) and 50% XY (boy). Mention that the father's sperm determines sex.
Explain Mendel's monohybrid cross across two generations.
Describe TT × tt → F1 all Tt → F2 self-cross gives 3:1 ratio. Mention dominant and recessive. Draw the Punnett square.
What are fossils? How do they provide evidence for evolution?
Define fossils as preserved remains/traces. Explain they reveal organisms from different time periods, showing gradual changes. Mention Archaeopteryx as example.
Distinguish between acquired and inherited traits with examples.
Acquired traits (muscles from exercise) — not coded in DNA, not passed on. Inherited traits (eye colour) — coded in DNA, passed to offspring.
Long Answer (5 marks)
Explain Mendel's dihybrid cross with checker board.
Use round yellow (RRYY) × wrinkled green (rryy). Show F1 (RrYy). Draw the 4×4 Punnett square. F2 ratio = 9:3:3:1. Mention Law of Independent Assortment.
What is speciation? List 4 factors that lead to speciation.
Define speciation as formation of new species. Factors: geographical isolation, genetic drift, natural selection, mutation. Explain each in 1-2 lines.
Explain Darwin's theory of natural selection with an example.
Use peppered moth or giraffe neck example. Steps: variation exists → struggle for existence → survival of the fittest → reproduction of favourable traits → gradual change over generations.
Diagram-Based Questions
Monohybrid cross diagram up to F2 — phenotypic and genotypic ratios
P: TT × tt → F1: Tt (all tall) → F1 × F1 → F2: TT, Tt, Tt, tt. Phenotypic 3:1. Genotypic 1:2:1. Label everything.
Sex determination diagram in human beings
Father (44+XY) × Mother (44+XX). Sperm types: 22+X and 22+Y. Egg: 22+X. Two outcomes: 44+XX (girl) or 44+XY (boy). Show with arrows.
Exam Tips for This Chapter
- Always draw diagrams. Punnett squares, sex determination crosses, and homologous vs analogous comparisons earn full marks
- Label everything. A diagram without labels gets zero marks
- Use correct terminology. Say "allele" not "type", say "homozygous" not "pure"
- Don't confuse homologous with analogous. Remember: Homo = same origin
- For evolution questions, always mention evidence (fossils, homologous organs, DNA similarity)
- "Mothers determine the sex of the child" — it's the father's sperm
- Confusing F1 and F2 generations in Mendel's cross
- "Humans evolved from monkeys" — we share a common ancestor
- Forgetting to write BOTH genotypic AND phenotypic ratios when asked for "ratios"
- Drawing Punnett squares without labelling gametes on the sides
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