Class 10 : Science (In English) – Lesson 8. Heredity
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.
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OTHER IMPORTANT QUESTIONS FOR EXAMS
Question 1
Which plant did Mendel choose for his experiments?
Wheat
Maize
Pea plant
Sunflower
Answer: 3 🌿
Question 2
Tallness in pea plants is dominant over dwarfness. Crossing Tt × Tt gives a genotypic ratio of:
1:1
2:1
1:2:1
3:1
Answer: 3 🟢 (1 TT : 2 Tt : 1 tt)
Question 3
The unit of heredity is:
Chromosome
Gene
DNA
Nucleus
Answer: 2 🧬
Question 4
Which of the following decides the sex of a child in humans?
Egg
Sperm
Uterus
Ovary
Answer: 2 🔵
Question 5
Which of the following is an example of recessive trait in pea plants?
Tall plant
Round seed
Green seed
Violet flower
Answer: 3 🌱
Question 6
The number of chromosome pairs in humans is:
23
46
44
22
Answer: 1 🟣 (23 pairs = 46 chromosomes)
Question 7
Which cross gives a ratio of 9:3:3:1 in F₂ generation?
Monohybrid cross
Test cross
Dihybrid cross
Back cross
Answer: 3 🟢
Question 8
In humans, sex chromosomes of female are:
XX
XY
YY
XO
Answer: 1 🌸
Question 9
A pure tall pea plant is crossed with a pure dwarf pea plant. The F₁ generation will be:
All tall
All dwarf
50% tall, 50% dwarf
3 tall : 1 dwarf
Answer: 1 ✔️
Question 10
The observable character of an organism is called:
Genotype
Phenotype
Allele
Trait
Answer: 2 🟡
Question 11
A: Alleles are alternative forms of the same gene.
R: Alleles are found on different loci of non-homologous chromosomes.
Both A and R true; R is correct explanation
Both A and R true; R is NOT correct explanation
A true; R false
A false; R true
Answer: 3 🟠
Question 12
A: Mendel’s law of segregation says that alleles separate during gamete formation.
R: Each gamete receives one allele of a gene pair.
Answer: 1 ✔️ Both true and correct explanation
Question 13
A: Traits are controlled by genes.
R: Genes are located on chromosomes.
Answer: 1 🧠
Question 14
A: Tallness is dominant in pea plants.
R: Crossing Tt × Tt gives 75% tall and 25% dwarf.
Answer: 1 🌿
Question 15
A: Dihybrid cross considers inheritance of two traits together.
R: Its F₂ ratio is 3:1.
Answer: 3 ❌ (F₂ ratio = 9:3:3:1)
Question 16
A: Sex in humans is determined genetically.
R: Male gametes decide the sex of child.
Answer: 1 🟢
Question 17
A: DNA copying introduces variations.
R: Variations are always harmful.
Answer: 3 🌍 (Variations may be useful)
Question 18
A: Phenotype shows external expression of traits.
R: Genotype represents genetic makeup.
Answer: 1 ✔️
Question 19
A: X chromosome is present in both males and females.
R: Y chromosome is found only in males.
Answer: 1 🟡
Question 20
A: Green colour in pea seeds is dominant over yellow.
R: Crossing Gg × Gg gives 100% green.
Answer: 3 ❌ (75% green : 25% yellow)
Question 21
Define heredity.
Answer:
🔵 Heredity = Transmission of traits from parents to offspring.
🟢 Ensures stability of species across generations.
Question 22
What is meant by variation?
Answer:
🔴 Variation = Differences among individuals of the same species.
🌍 Important for survival and evolution.
Question 23
State Mendel’s law of dominance.
Answer:
⚡ When two different alleles are present, one expresses (dominant) and other is suppressed (recessive).
Example: Tall (T) dominant over dwarf (t).
Question 24
What is a monohybrid cross?
Answer:
🔵 Cross considering inheritance of one trait.
🟢 Ratio in F₂ = 3:1 (phenotypic).
Question 25
Differentiate between genotype and phenotype.
Answer:
🧠 Genotype = Genetic constitution (TT, Tt).
🌸 Phenotype = Observable expression (Tall, Dwarf).
Question 26
Who is called the Father of Genetics?
Answer:
🌿 Gregor Johann Mendel.
⚡ He performed experiments on pea plants.
Question 27
Explain Mendel’s law of segregation with an example.
Answer:
🔵 Law: Alleles of a trait separate during gamete formation.
🟢 Example: Tt × Tt → Gametes: T, t.
🟡 F₂ ratio = 3 tall : 1 dwarf.
Question 28
Explain Mendel’s dihybrid cross.
Answer:
🌍 Cross: Round Yellow (RRYY) × Wrinkled Green (rryy).
🔴 F₁: All Round Yellow (RrYy).
🟣 F₂: 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green.
Question 29
Differentiate between homozygous and heterozygous.
Answer:
🟢 Homozygous: Same alleles (TT, tt).
🟡 Heterozygous: Different alleles (Tt).
✔️ Homozygous = pure line, Heterozygous = hybrid.
Question 30
What is sex determination in humans?
Answer:
🌸 Females → XX, Males → XY.
🟢 Egg always X; sperm may carry X or Y.
🔴 Fertilisation decides:
X sperm + X egg = Girl (XX).
Y sperm + X egg = Boy (XY).
Question 31
Why are traits acquired during lifetime not inherited?
Answer:
🧠 Acquired traits affect somatic cells, not germ cells.
🌍 Hence not passed to next generation.
Example: Muscle development in gym is not inherited.
Question 32
What is meant by dominant and recessive traits? Give examples.
Answer:
🔵 Dominant: Expressed in heterozygous (T = tall, W = violet).
🟡 Recessive: Hidden in heterozygous (t = dwarf, w = white).
✔️ TT or Tt → Tall, only tt → dwarf.
Question 33
Explain the importance of variations.
Answer:
🌍 Variations help survival in changing environment.
⚡ Provide raw material for evolution.
✔️ Example: Bacteria resistant to antibiotics survive.
Question 34
Describe Mendel’s monohybrid experiment and its conclusion.
Answer:
🌿 Mendel crossed pure tall (TT) pea plants with pure dwarf (tt).
🔵 F₁ generation: All tall (Tt).
🟢 F₂ generation (Tt × Tt): Ratio observed →
Phenotypic ratio = 3 Tall : 1 Dwarf.
Genotypic ratio = 1 TT : 2 Tt : 1 tt.
✔️ Conclusion:
Law of Dominance: One trait dominates over the other.
Law of Segregation: Alleles separate during gamete formation.
Question 35
Explain Mendel’s dihybrid cross with result.
Answer:
🌸 Mendel crossed Round Yellow (RRYY) × Wrinkled Green (rryy).
🔵 F₁ generation: All Round Yellow (RrYy).
🟢 F₂ generation: On selfing RrYy × RrYy →
Ratio = 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green.
🧠 Conclusion: Traits are inherited independently, proving the Law of Independent Assortment.
Question 36
Explain sex determination in humans with diagrammatic description.
Answer:
🌿 Humans have 23 pairs of chromosomes: 22 pairs autosomes + 1 pair sex chromosomes.
🔵 Female: XX; Male: XY.
🟢 Egg always contributes X; sperm contributes X or Y.
🟡 Fertilisation:
X sperm + X egg → XX → Girl child.
Y sperm + X egg → XY → Boy child.
✔️ Conclusion: Father’s sperm decides the sex of child.
Question 37
Case: A tall pea plant is crossed with a dwarf plant. In F₁, all plants are tall. In F₂, the ratio is 3 tall : 1 dwarf.
Questions:
(a) What is the genotype of parents?
(b) What is the genotype of F₁ plants?
(c) Give the genotypic ratio of F₂.
(d) State one conclusion of this experiment.
Answer:
(a) TT × tt.
(b) Tt.
(c) 1 TT : 2 Tt : 1 tt.
(d) Tall is dominant over dwarf → Law of Dominance.
Question 38
Case: A boy is born to a couple. The father has XY chromosomes, the mother has XX chromosomes.
Questions:
(a) Which gamete from father determines the sex of the child?
(b) What chromosome does the mother always contribute?
(c) If father contributes X, what will be the child’s sex?
(d) Why is father responsible for sex determination?
Answer:
(a) Sperm (X or Y).
(b) X chromosome.
(c) Girl child (XX).
(d) Because sperm can be X or Y, while egg always carries X.
Question 39
Case: A teacher explains that variation is important for survival of species.
Questions:
(a) What is variation?
(b) Why is variation significant?
(c) Give one example of variation helping survival.
(d) What is the ultimate result of accumulation of variations?
Answer:
(a) Differences in traits among individuals of the same species.
(b) Helps organisms adapt to changing environment.
(c) Bacteria becoming resistant to antibiotics.
(d) Evolution of new species.
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MIND MAPS
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