Chapter Overview & Weightage
Heredity and Evolution is among the highest-weightage chapters in Class 10 Science. It covers inheritance (how traits pass from parents to offspring) and evolution (how species change over time). In CBSE board exams, this chapter consistently carries 8–12 marks.
| Year | Marks | Question Types |
|---|---|---|
| 2023 | 10 | 2 MCQ + 2 SA + 1 LA |
| 2022 | 12 | 2 MCQ + 2 SA + 1 LA (5 marks) |
| 2021 | 8 | 1 MCQ + 2 SA + 1 LA |
| 2020 | 10 | 2 SA + 1 LA |
Monohybrid and dihybrid crosses are guaranteed numerical questions. Diagrams of evolution evidence (vestigial organs, fossils) appear as 3-mark questions. Learn the definitions of key terms precisely — CBSE examiners check exact wording.
Key Concepts You Must Know
Heredity and Variation:
- Heredity: Transfer of traits from parents to offspring
- Variation: Differences between individuals of the same species
- Variations arise from DNA changes during reproduction (sexual reproduction → more variation; asexual → less)
Mendel’s Experiments:
Gregor Mendel worked with garden pea plants (Pisum sativum) and studied 7 pairs of contrasting traits. His key contributions:
- Monohybrid cross: Crossing plants differing in ONE trait
- Dihybrid cross: Crossing plants differing in TWO traits
- Law of Segregation (1st Law): Alleles separate during gamete formation; each gamete carries one allele
- Law of Independent Assortment (2nd Law): Alleles for different traits are inherited independently
Terminology:
- Allele: Alternative forms of a gene (e.g., T for tall, t for short)
- Dominant: Allele expressed even in heterozygous state (T)
- Recessive: Allele expressed only in homozygous state (tt)
- Genotype: Genetic makeup (TT, Tt, tt)
- Phenotype: Physical appearance (Tall, Tall, Short)
- Homozygous: Both alleles identical (TT or tt)
- Heterozygous: Different alleles (Tt)
Sex Determination:
- Human females: 46 chromosomes, sex chromosomes XX
- Human males: 46 chromosomes, sex chromosomes XY
- Sex is determined by the father’s sperm (X-bearing → girl, Y-bearing → boy)
Important Formulas and Ratios
F1 generation: All Tt (all tall, if T is dominant)
F2 generation phenotype ratio: 3 Tall : 1 Short (3:1)
F2 generation genotype ratio: 1 TT : 2 Tt : 1 tt (1:2:1)
Phenotype ratio: 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green
Evolution Key Terms:
- Speciation: Formation of a new species from an existing one
- Natural Selection: Process by which organisms with favourable traits survive and reproduce
- Acquired traits: Traits gained during lifetime — NOT inherited
- Inherited traits: Genetic traits passed to offspring — inherited
Solved Previous Year Questions
PYQ 1 — CBSE 2023
Q: In a monohybrid cross between tall (TT) and short (tt) plants, what is the F2 genotype ratio? Which of these plants are homozygous?
F1: All Tt (tall)
F2 cross: Tt × Tt
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
F2 genotype ratio: 1TT : 2Tt : 1tt
Homozygous plants: TT (tall homozygous) and tt (short homozygous) — 1/4 each = 50% of total.
PYQ 2 — CBSE 2022
Q: How does the sex of a child get determined? Explain with a cross diagram.
Sex is determined by the sex chromosomes at fertilisation.
Mothers produce only X-bearing eggs. Fathers produce 50% X-bearing and 50% Y-bearing sperm.
| X (egg) | X (egg) | |
|---|---|---|
| X (sperm) | XX (Girl) | XX (Girl) |
| Y (sperm) | XY (Boy) | XY (Boy) |
If X-sperm fertilises egg → XX → Girl
If Y-sperm fertilises egg → XY → Boy
Therefore, the sex of the child is determined by the father (which type of sperm fertilises the egg), NOT the mother.
PYQ 3 — CBSE 2021
Q: Give four evidences in favour of evolution.
-
Homologous organs: Organs with same basic structure but different functions (e.g., forelimb of humans, bat, dolphin, horse — same bone pattern, different purposes). Suggests common ancestry.
-
Analogous organs: Organs with different structures but similar functions (e.g., wings of insects vs birds). Suggests convergent evolution.
-
Fossil records: Preserved remains of organisms from past geological ages. Show progression from simpler to complex forms over time.
-
Vestigial organs: Reduced, non-functional organs inherited from ancestors (e.g., human appendix, coccyx/tail bone). Suggest evolution from ancestors that used these organs.
Difficulty Distribution
| Difficulty | % | Topics |
|---|---|---|
| Easy | 30% | Definitions, sex determination, basic terms |
| Medium | 45% | Monohybrid/dihybrid crosses, Mendel’s laws |
| Hard | 25% | Speciation, acquired vs inherited traits, evolution evidence evaluation |
Expert Strategy
Punnett squares are non-negotiable. Every heredity numerical requires drawing the Punnett square correctly — even if you can get the ratio mentally. CBSE examiners award 1 mark for the cross setup, 1 for the Punnett square, and 1 for the ratio/conclusion.
Learn Mendel’s contributions precisely. Not just “he found 3:1 ratio” — understand WHY: because alleles segregate during gamete formation, and gametes carry only one allele of each gene pair.
For dihybrid crosses, you don’t need to draw a 4×4 Punnett square every time. Use the FOIL-like multiplication: (3 round + 1 wrinkled) × (3 yellow + 1 green) = 9 round yellow + 3 round green + 3 wrinkled yellow + 1 wrinkled green = 9:3:3:1. This saves 3 minutes in the exam.
Evolution vs heredity distinction: Students mix up Lamarck (acquired characteristics are inherited — WRONG) with Darwin (natural selection acts on heritable variations — CORRECT). CBSE asks this explicitly.
Common Traps
Trap 1: Thinking “sex is determined by the mother.” The opposite is true — mothers always give X chromosomes. The father’s sperm determines sex. This is tested almost every year and students still get it wrong.
Trap 2: Writing “Acquired traits are inherited.” They are NOT. Giraffes don’t pass on stretched necks (Lamarck was wrong). Only traits encoded in DNA can be inherited. An example: a blacksmith’s muscular arms are an acquired trait — their children are not born more muscular.
Trap 3: Confusing homologous and analogous organs. Homologous = same structure, different function (evidence of common ancestry). Analogous = different structure, same function (convergent evolution, NOT common ancestry). Forelimbs of whale and bat are homologous; wings of bat and butterfly are analogous.
Trap 4: Writing the F2 ratio as 3:1 without specifying it’s the phenotype ratio. The genotype ratio is 1:2:1 (TT:Tt:tt). CBSE questions that ask “ratio of F2 offspring” and those that ask “genotypic ratio” want different answers.