Class 10 : Science (In English) – Lesson 8. Heredity

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On June 20, 2025

EXPLANATION & SUMMARY

🔵 Detailed Explanation
Introduction: The Concept of Heredity

🌍 Heredity is the transmission of characters (traits) from one generation to the next.
Offspring resemble parents but also show differences due to variations.
The study of heredity and variation is called Genetics.

💡 Concept: Heredity ensures stability of species, while variation ensures adaptability and evolution.

🟢 Mendel’s Experiments – The Father of Genetics
Gregor Johann Mendel (1822–1884), an Austrian monk, experimented on pea plants (Pisum sativum).
Selected traits like flower colour, seed shape, seed colour, plant height.
Chose pea plants because: short life cycle, bisexual flowers, easy to cross-pollinate.

⚡ Mendel’s experiments laid the foundation of modern genetics.

🟡 Mendel’s Laws of Inheritance
Law of Dominance ✨
When two different alleles (forms of a gene) are present, only one expresses itself (dominant), while the other is suppressed (recessive).
Example: Tall (T) × Dwarf (t) → All F₁ progeny tall.

Law of Segregation 💧
Allele pairs separate during gamete formation; each gamete carries only one allele.
Example: Crossing Tt × Tt → F₂ generation shows ratio 3 Tall : 1 Dwarf.

Law of Independent Assortment 🌿
Alleles of different traits separate independently during gamete formation.
Example: Dihybrid cross (Seed shape × Seed colour) gives ratio 9:3:3:1 in F₂.
✔️ Mendel’s ratios: Monohybrid (3:1), Dihybrid (9:3:3:1).

🔴 Important Genetic Terms
Gene: Unit of heredity controlling a character.
Allele: Alternate forms of a gene (e.g. Tall T, Dwarf t).
Homozygous: Both alleles same (TT, tt).
Heterozygous: Two different alleles (Tt).
Phenotype: Observable character (tall, round).
Genotype: Genetic makeup (TT, Tt).
F₁ generation: First filial generation.
F₂ generation: Second filial generation from selfing of F₁.

🟠 Monohybrid Cross (One Trait)
Example: Tall (TT) × Dwarf (tt)
F₁: All tall (Tt).
F₂: Ratio = 3 Tall : 1 Dwarf.
Genotypic ratio = 1 TT : 2 Tt : 1 tt.
🌟 Shows dominance and segregation.

🟣 Dihybrid Cross (Two Traits)
Example: Round Yellow (RRYY) × Wrinkled Green (rryy)
F₁: All Round Yellow (RrYy).
F₂: Ratio = 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green.
💡 Concept: Traits are inherited independently.

🟤 Sex Determination in Humans

Humans have 23 pairs of chromosomes → 22 pairs autosomes + 1 pair sex chromosomes.
Female: XX, Male: XY.
Egg always carries X; sperm carries either X or Y.
If X sperm fertilises → XX (girl); if Y sperm fertilises → XY (boy).
✔️ Sex of child is determined by father’s sperm, not mother.

🌿 Variation and Evolution
Variation: Differences in characters among individuals of same species.
Causes: DNA replication errors, sexual reproduction, crossing over.
Importance: Helps organisms adapt, ensures survival in changing environment.
Example: Some bacteria develop antibiotic resistance due to variation.

🔵 Heredity and Evolution Link
Heredity provides stability → traits pass on.
Variation introduces diversity → drives evolution by natural selection.
Example: Peppered moth during industrial revolution.

🟢 Summary
Heredity: Transmission of traits from parents to offspring.
Mendel’s Work: Pea plant experiments → Laws of Dominance, Segregation, Independent Assortment.
Monohybrid & Dihybrid Crosses: Ratios 3:1 and 9:3:3:1 respectively.
Genetic Terms: Gene, allele, homozygous, heterozygous, phenotype, genotype.
Sex Determination: Male decides child’s sex (XY system).
Variation: Errors in DNA, sexual reproduction → basis of evolution.

📝 Quick Recap
🌱 Mendel proved inheritance patterns.
⚡ Monohybrid = 3:1, Dihybrid = 9:3:3:1.
🧬 Genes = units of heredity; alleles = variations.
🧠 Sex of baby determined by father’s sperm.
🌍 Variation → survival + evolution.

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QUESTIONS FROM TEXTBOOK

Question 1
A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic make-up of the tall parent can be depicted as
(a) TTWW
(b) TTWw
(c) TtWW
(d) TtWw
Answer
🔵 All progeny are violet → Tall parent must carry at least one dominant W allele.
🟢 Almost half progeny are short → Tall parent must be heterozygous for height (Tt).
🟡 Genotype of tall parent = TtWW.
✔️ Hence, correct option is (c) TtWW.

Question 2
A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not?
Answer
🔵 Observation: Children with light eyes generally have parents with light eyes.
🟢 Possibility: Light eye colour may be recessive trait.
🟡 But conclusion cannot be confirmed because:
✨ We do not know if dark-eyed parents can produce light-eyed children.
✨ Proper experiments (like Mendel’s crosses) are needed.
✔️ Therefore, we cannot firmly conclude dominance/recessiveness from this observation alone.

Question 3
Outline a project which aims to find the dominant coat colour in dogs.
Answer
🌿 Steps of project:
🔵 Select two pure-bred dogs of different coat colours (e.g. black × brown).
🟢 Cross them and observe the F₁ generation:
➡️ If all pups black → black is dominant.
🟡 Cross the F₁ dogs (selfing) → observe F₂ generation.
➡️ Ratio 3:1 (black:brown) confirms black is dominant.
✔️ Conclusion: Dominant coat colour identified through offspring analysis.

Question 4
How is the equal genetic contribution of male and female parents ensured in the progeny?
Answer
🧠 Explanation:
🔵 Humans have 23 pairs of chromosomes.
🟢 Male gamete (sperm) carries 22 autosomes + X/Y chromosome.
🟡 Female gamete (egg) carries 22 autosomes + X chromosome.
🔴 During fertilisation:
➡️ Sperm (22 + X or Y) + Egg (22 + X) = Zygote (46 chromosomes).
✔️ Thus, half chromosomes come from father, half from mother → equal genetic contribution ensured.

(CBSE MODEL QUESTION PAPER)

ESPECIALLY MADE FROM THIS LESSON ONLY

⚙️ Section A: Q1–6 (1 Mark Each – MCQ Type)

🔵 Question 1: The number of pairs of chromosomes in humans is —
🔵 (A) 23
🟢 (B) 46
🔴 (C) 22
🟡 (D) 24
✔️ Answer: (A) 23
💡 Humans have 23 pairs (46 chromosomes), where one set is inherited from each parent.

🔵 Question 2: Which of the following determines the sex of a child in humans?
🔵 (A) Mother
🟢 (B) Father
🔴 (C) Both equally
🟡 (D) Hormones
✔️ Answer: (B) Father
💡 The sperm from the father carries either an X or Y chromosome, determining the child’s sex (XX = female, XY = male).

🔵 Question 3: The gene responsible for dwarfness in pea plant is —
🔵 (A) Recessive
🟢 (B) Dominant
🔴 (C) Co-dominant
🟡 (D) Mutant
✔️ Answer: (A) Recessive
💡 According to Mendel, tallness (T) is dominant over dwarfness (t).

🔵 Question 4: Which of the following is the unit of heredity?
🔵 (A) Gene
🟢 (B) Chromosome
🔴 (C) DNA
🟡 (D) Trait
✔️ Answer: (A) Gene
💡 Gene is a segment of DNA that determines a specific character or trait.

🔵 Question 5: Which of these will show variation when reproduced?
🔵 (A) Asexual reproduction
🟢 (B) Binary fission
🔴 (C) Sexual reproduction
🟡 (D) Budding
✔️ Answer: (C) Sexual reproduction
💡 Variation arises when genes from two parents combine during sexual reproduction.

🔵 Question 6: The physical appearance of an individual is called —
🔵 (A) Genotype
🟢 (B) Phenotype
🔴 (C) Chromosome
🟡 (D) Gene
✔️ Answer: (B) Phenotype
💡 Phenotype expresses the visible characteristics determined by the genotype.

⚡ Section B: Q7–12 (2 Marks Each – Short Answers)

🔵 Question 7: Define heredity and variation.
✔️ Answer:
➡️ Heredity is the transmission of traits from parents to offspring through genes.
➡️ Variation is the difference in characteristics among individuals of the same species.

🟢 Question 8: What is a gene? What is its function?
✔️ Answer:
A gene is a unit of heredity present on DNA that controls specific traits.
💡 It carries information for protein synthesis and determines features like eye colour or height.

🔴 Question 9: What is meant by dominant and recessive traits?
✔️ Answer:
➡️ A dominant trait is expressed even when only one copy of the gene is present (e.g., tallness in pea plants).
➡️ A recessive trait is expressed only when both alleles are recessive (e.g., dwarfness).

🟡 Question 10: What is sex determination? Explain briefly.
✔️ Answer:
Sex determination is the process by which the sex of an organism is decided at fertilization.
In humans:
➡️ Female has XX chromosomes.
➡️ Male has XY chromosomes.
The sperm determines the sex of the child.

🔵 Question 11: What are inherited traits? Give one example.
✔️ Answer:
Traits that are passed from parents to offspring through genes are called inherited traits.
💡 Example: Eye colour, height, blood group.

🟢 Question 12: What is a monohybrid cross?
✔️ Answer:
A cross involving inheritance of a single contrasting character is called a monohybrid cross.
💡 Example: Cross between tall (TT) and dwarf (tt) pea plants.

⚙️ Section C: Q13–22 (3 Marks Each – Mid-Length Answers)

🔵 Question 13: State Mendel’s laws of inheritance.
✔️ Answer:
1️⃣ Law of Dominance: One factor (gene) dominates over the other.
2️⃣ Law of Segregation: Gene pairs separate during gamete formation.
3️⃣ Law of Independent Assortment: Different traits are inherited independently of each other.

🟢 Question 14: Describe Mendel’s monohybrid cross with pea plants.
✔️ Answer:
Mendel crossed tall (TT) and dwarf (tt) pea plants.
➡️ F₁ generation: All tall (Tt).
➡️ F₂ generation: Tall and dwarf in 3:1 ratio.
💡 Conclusion: The tall trait is dominant; dwarf is recessive.

🔴 Question 15: Explain Mendel’s dihybrid cross and the law derived from it.
✔️ Answer:
Crossed plants with two traits — round yellow (RRYY) × wrinkled green (rryy).
F₁: All round yellow (RrYy).
F₂: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.
💡 Law of Independent Assortment derived.

🟡 Question 16: Differentiate between inherited and acquired traits.
✔️ Answer:
➡️ Inherited traits: Passed through genes (e.g., height).
➡️ Acquired traits: Developed during lifetime due to environment (e.g., muscle growth).
Inherited traits are genetic; acquired are not.

🔵 Question 17: Explain the role of DNA in inheritance.
✔️ Answer:
DNA carries genetic information from parents to offspring.
During cell division, DNA replicates, ensuring the transfer of hereditary material to new cells.

🟢 Question 18: What is evolution? How is it related to heredity?
✔️ Answer:
Evolution is gradual change in organisms over generations.
Small variations during heredity accumulate over time, leading to evolution of new species.

🔴 Question 19: Explain the mechanism of sex determination in humans with diagrammatic support.
✔️ Answer:
Female gamete: X
Male gamete: X or Y
➡️ XX → Female child
➡️ XY → Male child
💡 Thus, the father determines the sex of the child.

🟡 Question 20: What are homologous organs? Give examples.
✔️ Answer:
Organs with similar structure but different functions are homologous organs.
Examples: Forelimbs of humans, wings of birds, and flippers of whales.

🔵 Question 21: What are fossils and how do they provide evidence for evolution?
✔️ Answer:
Fossils are preserved remains of ancient organisms.
They show structural similarities between extinct and living organisms, supporting evolution.

🟢 Question 22: How does variation help species to survive in changing environment?
✔️ Answer:
Variation allows individuals to adapt to environmental changes.
Organisms with beneficial variations survive and reproduce, ensuring species continuity.

🧠 Section D: Q23–30 (4 Marks Each – Long and Case-Based Answers)

🔵 Question 23: Describe Mendel’s experiment on inheritance of two traits in pea plants.
✔️ Answer:
Mendel crossed pure round yellow (RRYY) and wrinkled green (rryy) pea plants.
F₁: All round yellow (RrYy).
F₂: 9:3:3:1 ratio.
💡 Conclusion: Traits assort independently; inheritance of one does not affect another.

🟢 Question 24: Explain the mechanism of evolution with an example.
✔️ Answer:
Evolution occurs through accumulation of variations.
Example: Giraffes with longer necks survived better during food scarcity; over time, long-necked giraffes dominated the population.

🔴 Question 25: What are chromosomes? Describe their role in inheritance.
✔️ Answer:
Chromosomes are thread-like structures in the nucleus carrying genes made of DNA.
They control hereditary traits and transfer them to offspring during reproduction.

🟡 Question 26: Explain Darwin’s theory of natural selection.
✔️ Answer:
Darwin proposed that:
1️⃣ Variations exist among individuals.
2️⃣ Organisms compete for survival.
3️⃣ Only the fittest survive and reproduce.
💡 Gradual accumulation of beneficial traits leads to evolution.

🔵 Question 27: How do fossils help in understanding evolutionary relationships?
✔️ Answer:
Fossils show the existence of intermediate forms linking present-day species with ancestors.
Example: Archaeopteryx shows features of both reptiles and birds.

🟢 Question 28: Case Study – A family has a history of colour blindness in males. Explain the reason and inheritance pattern.
✔️ Answer:
Colour blindness is a sex-linked recessive trait carried on the X chromosome.
Since males have one X chromosome, inheritance of the defective gene leads to colour blindness.
Females usually remain carriers (XX’).

🔴 Question 29: Explain how variations in DNA lead to formation of new species.
✔️ Answer:
Mutations and recombination during sexual reproduction introduce variations in DNA.
Accumulation of such variations over many generations leads to evolution and speciation.

🟡 Question 30: Discuss how evolution and classification are interlinked.
✔️ Answer:
Classification groups organisms based on similarities in structure, function, and genetic makeup.
These similarities arise due to common ancestry, proving evolutionary relationships.

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