Class 10th Science Chapter 8 – Heredity | Mendel’s Experiments and Laws of Inheritance

Heredity | Mendel’s Experiments and Laws of Inheritance

Class 10 Science | Chapter 9 | CBSE + Foundation + Competitive Level

🧬 Mendel’s Experiments and Laws of Inheritance

1. Introduction to Mendel

Gregor Johann Mendel was an Austrian monk and scientist who studied the inheritance of characteristics in pea plants.

Gregor Mendel is known as the “Father of Genetics”.

उन्होंने मटर के पौधों (garden pea – Pisum sativum) पर प्रयोग करके यह समझने का प्रयास किया कि माता-पिता से संतानों में लक्षण कैसे पहुँचते हैं।

CBSE Golden Point:
Mendel performed his experiments on garden pea plants because pea plants have several easily distinguishable contrasting traits and can undergo both self-pollination and cross-pollination.

2. Why Did Mendel Select Pea Plants?

Mendel selected pea plants because:

• Pea plants have several easily observable contrasting characters.
• They have a relatively short generation time.
• They produce many seeds.
• They normally undergo self-pollination.
• Cross-pollination can be performed artificially.
• Male and female reproductive structures are present in the same flower.

3. Seven Contrasting Characters Studied by Mendel

Character	Dominant Form	Recessive Form
Seed shape	Round	Wrinkled
Seed colour	Yellow	Green
Flower colour	Violet/Purple	White
Pod shape	Inflated	Constricted
Pod colour	Green	Yellow
Flower position	Axial	Terminal
Stem height	Tall	Dwarf

4. Important Terms in Mendel’s Experiments

Term	Meaning
Trait	A specific observable characteristic, such as tallness or seed colour.
Gene	A segment of DNA carrying information for a particular characteristic.
Alleles	Different forms of the same gene.
Dominant	An allele that can express its effect in a heterozygous condition.
Recessive	An allele whose effect is generally masked by a dominant allele in heterozygous condition.
Genotype	Genetic constitution of an organism.
Phenotype	Observable expression of a characteristic.
Homozygous	Both alleles are identical, e.g. TT or tt.
Heterozygous	Two alleles are different, e.g. Tt.

5. Mendel’s Monohybrid Cross

A monohybrid cross is a cross involving one pair of contrasting characteristics.

Mendel crossed a pure tall pea plant with a pure dwarf pea plant.

Let: T = Tall
t = Dwarf

TT
Tall

×

tt
Dwarf

Parental Generation (P)

TT → Gamete T      tt → Gamete t

T

+

t

→

Tt

F₁ Generation → All plants are Tt (Tall)

Observation: In the F₁ generation, all plants were tall even though one parent was dwarf.

This showed that the tall trait masked the dwarf trait in the heterozygous condition.

6. Self-Pollination of F₁ Plants

Tt × Tt

♀ / ♂	T	t
T	TT	Tt
t	Tt	tt

Genotypic Ratio:
1 TT : 2 Tt : 1 tt

Phenotypic Ratio:
3 Tall : 1 Dwarf

Out of four possible offspring combinations:

• 1 = TT → Tall
• 2 = Tt → Tall
• 1 = tt → Dwarf

Therefore: 75% Tall and 25% Dwarf under the simplified complete-dominance model.

7. Mendel’s First Law – Law of Dominance

The Law of Dominance states that when two contrasting alleles are present together in a heterozygous organism, one allele may express itself while the other remains masked.

For example: Tt → Tall Here T is dominant over t.

Important: The recessive allele is not destroyed. It remains present and can be expressed when present in the homozygous condition, such as tt.

8. Mendel’s Second Law – Law of Segregation

The Law of Segregation states that the two alleles of a gene separate from each other during the formation of gametes, so each gamete receives only one allele.

For: Tt the two alleles are: T and t

During gamete formation: Tt → T + t

One gamete receives T    |    Another gamete receives t

This law is also called the Law of Purity of Gametes in traditional textbook terminology.

9. Independent Assortment – Basic Idea

When Mendel studied inheritance of two pairs of contrasting characters together, he observed that the inheritance of one pair could occur independently of another pair under the conditions of his experiment. This led to the idea known as the Law of Independent Assortment.

For example, seed shape and seed colour can be considered as two different characters.

Class 10 Focus: Remember that Mendel's laws explain how characteristics are inherited through factors/genes. At higher levels, independent assortment is understood in relation to the behaviour of chromosome pairs during meiosis.

10. Monohybrid Cross – Complete Flow

11. Dominant and Recessive Traits

Feature	Dominant	Recessive
Expression	Can express in heterozygous condition	Usually expressed when homozygous
Example	T in Tt	t in tt
Phenotype in Tt	Expressed	Masked

12. Genotype and Phenotype

Genotype: Genetic constitution of an organism.

Examples: TT, Tt, tt

Phenotype: Observable characteristic.

Examples: Tall plant, dwarf plant

13. Homozygous and Heterozygous

Condition	Example	Meaning
Homozygous Dominant	TT	Both alleles dominant
Homozygous Recessive	tt	Both alleles recessive
Heterozygous	Tt	Two different alleles

14. Why Did the Dwarf Trait Reappear in F₂?

The dwarf trait did not disappear from the F₁ generation. In: Tt × Tt the recessive allele t segregates during gamete formation. When two t gametes combine: t + t → tt The recessive phenotype is therefore expressed.

15. Mendel’s Main Conclusions

Mendel's experiments helped establish that:

• Characteristics are controlled by hereditary factors, now understood as genes.
• Different forms of a gene are called alleles.
• Alleles may show dominance and recessiveness.
• The two alleles separate during gamete formation.
• Each gamete receives one allele from a pair.
• Fertilisation restores the pair of alleles.
• Sexual reproduction can generate combinations of hereditary information.

16. Mendelian Inheritance – Quick Table

Cross	Offspring	Result
TT × tt	All Tt	All Tall
Tt × Tt	TT, Tt, Tt, tt	3 Tall : 1 Dwarf
TT × Tt	TT, TT, Tt, Tt	All Tall
tt × Tt	tt, Tt	1 Tall : 1 Dwarf

17. 30 MCQs – Mendel’s Experiments and Laws of Inheritance

Q1. Who is known as the Father of Genetics?

A) Charles Darwin

B) Gregor Mendel

C) Lamarck

D) Watson

Answer: B

Q2. Mendel performed his famous experiments on:

A) Rose

B) Wheat

C) Garden pea

D) Maize

Answer: C

Q3. A monohybrid cross involves:

A) One pair of contrasting characters

B) Two species

C) Three characters

D) Only recessive characters

Answer: A

Q4. Which genotype represents a homozygous dominant condition?

A) Tt

B) tt

C) TT

D) T

Answer: C

Q5. Which genotype represents a heterozygous condition?

A) TT

B) tt

C) Tt

D) TTT

Answer: C

Q6. In a cross TT × tt, the F₁ generation will contain:

A) All TT

B) All tt

C) All Tt

D) TT and tt only

Answer: C

Q7. The phenotype of Tt when T is dominant is:

A) Dwarf

B) Tall

C) Intermediate

D) Cannot be determined

Answer: B

Q8. The phenotypic ratio obtained in Tt × Tt is:

A) 1:1

B) 1:2:1

C) 3:1

D) 9:3:3:1

Answer: C

Q9. The genotypic ratio in Tt × Tt is:

A) 3:1

B) 1:2:1

C) 9:3:3:1

D) 1:1

Answer: B

Q10. The Law of Segregation describes:

A) Fusion of gametes

B) Separation of alleles during gamete formation

C) Mutation

D) DNA replication only

Answer: B

Q11. A recessive trait appears in:

A) TT

B) Tt

C) tt

D) T only

Answer: C

Q12. Which represents phenotype?

A) Tt

B) TT

C) Tall plant

D) tt

Answer: C

Q13. Which represents genotype?

A) Tall

B) Dwarf

C) Green seed

D) Tt

Answer: D

Q14. In a gamete produced by Tt, how many alleles of the pair are present?

A) Zero

B) One

C) Two

D) Four

Answer: B

Q15. Mendel selected pea plants mainly because:

A) They had no flowers

B) Their traits were easily distinguishable

C) They were very large

D) They could not self-pollinate

Answer: B

Q16. In Tt × Tt, the probability of tt is:

A) 100%

B) 75%

C) 50%

D) 25%

Answer: D

Q17. In Tt × Tt, the probability of a tall phenotype is:

A) 25%

B) 50%

C) 75%

D) 100%

Answer: C

Q18. The allele pair in TT is:

A) Heterozygous

B) Homozygous

C) Hybrid only

D) Haploid

Answer: B

Q19. The Law of Dominance explains:

A) Why one allele can mask another in a heterozygote

B) DNA mutation

C) Cell division only

D) Protein digestion

Answer: A

Q20. Which cross gives all heterozygous offspring?

A) TT × tt

B) Tt × Tt

C) TT × Tt

D) tt × tt

Answer: A

Q21. In Tt × Tt, how many offspring are expected to be heterozygous in a four-box Punnett square?

A) 1

B) 2

C) 3

D) 4

Answer: B

Q22. Which is a recessive phenotype in Mendel’s tallness experiment?

A) Tall

B) Dwarf

C) Both

D) Neither

Answer: B

Q23. The alternative forms of a gene are called:

A) Chromosomes

B) Alleles

C) Ribosomes

D) Enzymes

Answer: B

Q24. Mendel's experiments provided evidence that:

A) Traits are inherited through hereditary factors

B) Traits are never inherited

C) Only acquired traits are inherited

D) Environment controls every trait completely

Answer: A

Q25. The term “hybrid” in a simple Mendelian cross refers to an organism:

A) With identical alleles

B) With different alleles for a trait

C) Without genes

D) Without chromosomes

Answer: B

Q26. Which ratio is characteristic of the F₂ generation in a simple monohybrid cross with complete dominance?

A) 1:1

B) 2:1

C) 3:1

D) 4:1

Answer: C

Q27. If a tall plant has genotype tt, then:

A) The statement is inconsistent with T as dominant

B) It must be tall

C) It must be Tt

D) It must be TT

Answer: A

Q28. Which of the following is NOT a genotype?

A) TT

B) Tt

C) tt

D) Tall

Answer: D

Q29. The segregation of alleles occurs during:

A) Gamete formation

B) Digestion

C) Respiration

D) Photosynthesis

Answer: A

Q30. Which combination correctly represents a heterozygous tall plant?

A) TT

B) tt

C) Tt

D) t

Answer: C

18. 30 Subjective Questions with Answers

2 Marks Q1. Who was Gregor Mendel? Why is he important in genetics?

Answer: Gregor Mendel was an Austrian scientist who studied inheritance in pea plants. His experiments established basic principles of inheritance, so he is called the Father of Genetics.

2 Marks Q2. What is a monohybrid cross?

Answer: A cross involving one pair of contrasting characters is called a monohybrid cross. Example: a cross between tall and dwarf pea plants.

2 Marks Q3. Define genotype and phenotype.

Answer: Genotype is the genetic constitution of an organism, while phenotype is its observable characteristic.

2 Marks Q4. What is meant by a dominant allele?

Answer: A dominant allele is an allele whose effect can be expressed in a heterozygous condition.

2 Marks Q5. What is the Law of Segregation?

Answer: The two alleles of a gene separate during gamete formation, so each gamete receives only one allele.

3 Marks Q6. Give three reasons why Mendel selected pea plants.

Answer:

1. They had easily distinguishable contrasting traits.
2. They could self-pollinate.
3. Cross-pollination could be performed artificially and they produced many seeds.

3 Marks Q7. Explain TT × tt cross.

Answer: TT produces only T gametes and tt produces only t gametes. Therefore all F₁ offspring are Tt. If T is dominant, all F₁ plants are tall.

3 Marks Q8. Differentiate between homozygous and heterozygous conditions.

Answer: Homozygous condition has identical alleles, e.g. TT or tt. Heterozygous condition has different alleles, e.g. Tt.

3 Marks Q9. What is the phenotypic ratio in a monohybrid cross? Explain.

Answer: The phenotypic ratio is 3:1. Three offspring show the dominant phenotype and one shows the recessive phenotype in the F₂ generation.

3 Marks Q10. What is meant by alleles?

Answer: Alleles are alternative forms of the same gene. For example, T and t can represent alternative alleles for plant height in a simple Mendelian model.

4 Marks Q11. Explain Mendel’s monohybrid experiment.

Answer: Mendel crossed a pure tall plant (TT) with a pure dwarf plant (tt). All F₁ plants were Tt and tall. On self-pollination of F₁ plants, Tt × Tt produced TT, Tt, Tt and tt. Thus the F₂ phenotypic ratio was 3 tall : 1 dwarf.

4 Marks Q12. Explain the Law of Dominance with an example.

Answer: When two contrasting alleles are present together, one may express itself while the other remains masked. In Tt, T is dominant and the plant is tall.

4 Marks Q13. Explain the Law of Segregation using Tt.

Answer: Tt contains two alleles, T and t. During gamete formation, these alleles separate. Half the gametes receive T and half receive t in the idealised Mendelian model.

4 Marks Q14. Distinguish between genotype and phenotype with examples.

Answer: Genotype refers to genetic constitution, e.g. TT, Tt or tt. Phenotype refers to the observable characteristic, e.g. tall or dwarf plant.

4 Marks Q15. Why does the recessive trait reappear in F₂?

Answer: The recessive allele is not lost in F₁. It remains in Tt and separates during gamete formation. When two recessive alleles combine as tt in F₂, the recessive phenotype appears.

5 Marks Q16. Explain the complete Tt × Tt cross using a Punnett square.

Answer: Gametes from each Tt parent are T and t.

×	T	t
T	TT	Tt
t	Tt	tt

Genotypic ratio = 1 TT : 2 Tt : 1 tt. Phenotypic ratio = 3 Tall : 1 Dwarf.

5 Marks Q17. Explain why Mendel's pea experiments were successful.

Answer: Pea plants provided many contrasting traits, short generations, many seeds and easy self-pollination. Mendel could also perform controlled cross-pollination, making inheritance patterns easier to study.

5 Marks Q18. Explain dominant and recessive traits.

Answer: A dominant allele can express in a heterozygous condition. A recessive allele is generally masked in heterozygous condition and is expressed when present in homozygous condition. Example: Tt is tall, while tt is dwarf when T is dominant.

5 Marks Q19. Explain the relationship between genes and alleles.

Answer: A gene is a segment of DNA carrying hereditary information. Different forms of the same gene are called alleles. For a diploid organism, two alleles of a gene are generally inherited, one from each parent.

5 Marks Q20. Explain the significance of Mendel’s experiments.

Answer: Mendel's experiments showed that traits are inherited through discrete hereditary factors. They helped establish concepts such as dominant and recessive alleles and segregation of alleles during gamete formation.

6 Marks Q21. Describe Mendel’s experiment from parental generation to F₂ generation.

Answer: P generation: TT × tt → all F₁ Tt. F₁ self-pollination: Tt × Tt. F₂: TT, Tt, Tt, tt. Genotypic ratio = 1:2:1. Phenotypic ratio = 3:1. The experiment demonstrated dominance and segregation of alleles.

6 Marks Q22. Explain Mendel’s Law of Dominance and Law of Segregation.

Answer: Law of Dominance states that one allele may express itself in a heterozygous condition while the other remains masked. Law of Segregation states that the two alleles separate during gamete formation and each gamete receives one allele.

6 Marks Q23. Explain how a 3:1 phenotypic ratio is obtained.

Answer: In Tt × Tt, the four combinations are TT, Tt, Tt and tt. TT and both Tt individuals are tall, while tt is dwarf. Therefore three are tall and one is dwarf, giving 3:1.

6 Marks Q24. Explain the importance of Punnett square in inheritance.

Answer: A Punnett square provides a systematic way to represent possible combinations of parental alleles. It helps predict possible genotypes and phenotypes and their expected ratios.

6 Marks Q25. Explain genotype, phenotype, homozygous and heterozygous conditions with examples.

Answer: Genotype = genetic constitution, e.g. TT. Phenotype = observable trait, e.g. tall. Homozygous = identical alleles, e.g. TT or tt. Heterozygous = different alleles, e.g. Tt.

6 Marks Q26. A tall pea plant is crossed with a dwarf pea plant and all F₁ plants are tall. Explain the result.

Answer: Assuming the tall parent is pure TT and the dwarf parent is tt, the cross TT × tt gives all Tt offspring. Since T is dominant, all F₁ plants are tall.

6 Marks Q27. A heterozygous tall plant is crossed with another heterozygous tall plant. Find the expected genotypes and phenotypes.

Answer: Tt × Tt gives TT, Tt, Tt and tt. Genotype ratio = 1:2:1. Phenotype ratio = 3 Tall : 1 Dwarf.

6 Marks Q28. Why is the recessive allele not considered destroyed in a heterozygous plant?

Answer: In Tt, both T and t are still present. The dominant T masks the expression of t, but t can be passed to gametes. If two t alleles combine as tt, the recessive phenotype appears.

6 Marks Q29. Explain the importance of segregation during gamete formation.

Answer: Segregation ensures that each gamete receives one allele of a gene pair. At fertilisation, alleles from two parents can combine again, restoring the pair in the offspring and producing different genetic combinations.

6 Marks Q30. Explain how Mendel’s work forms the basis of modern genetics.

Answer: Mendel showed that hereditary characteristics are transmitted through discrete factors. His observations of dominance, segregation and independent assortment provided the foundation for understanding gene-based inheritance. Modern genetics later connected these hereditary factors with genes and DNA.

19. HOTS / Competency-Based Questions

HOTS 1. A plant with genotype Tt is tall. Can we conclude that it is homozygous?

Answer: No. Tt is heterozygous. It is tall because T is dominant.

HOTS 2. If two tall pea plants produce a dwarf offspring, what can be inferred about their genotypes?

Answer: Both parents must carry the recessive allele. In the simple Mendelian model, both are likely Tt.

HOTS 3. Why does TT × tt not produce dwarf offspring in F₁?

Answer: All F₁ offspring receive T from the first parent and t from the second, giving Tt. Since T is dominant, the phenotype is tall.

HOTS 4. If Tt × Tt produces 100 offspring, approximately how many would be expected to be dwarf under the ideal Mendelian model?

Answer: Approximately 25, because the expected probability of tt is 1/4.

HOTS 5. Why is phenotype alone sometimes insufficient to determine genotype?

Answer: A dominant phenotype can be produced by either a homozygous dominant genotype or a heterozygous genotype. For example, both TT and Tt are tall.

20. Assertion–Reason

Q1. Assertion: All F₁ plants in TT × tt are tall.
Reason: T is dominant over t.

Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion.

Q2. Assertion: The genotype of a dwarf plant is tt in the simple Mendelian model.
Reason: The recessive phenotype is expressed when both alleles are recessive.

Answer: Both statements are true, and the Reason correctly explains the Assertion.

Q3. Assertion: Tt is homozygous.
Reason: T and t are identical alleles.

Answer: Both statements are false. Tt is heterozygous and T and t are different alleles.

Q4. Assertion: The F₂ phenotypic ratio in a simple monohybrid cross is 3:1.
Reason: The F₂ genotypes are TT, Tt, Tt and tt.

Answer: Both statements are true, and the Reason correctly explains the Assertion.

Q5. Assertion: A recessive allele disappears permanently in F₁.
Reason: A dominant allele masks the expression of a recessive allele in a heterozygote.

Answer: Assertion is false, but Reason is true.

21. Important Diagram Labelling

Mendelian Monohybrid Cross

22. CBSE Golden Points

• Mendel is called the Father of Genetics.
• Mendel used garden pea plants.
• Monohybrid cross studies one pair of contrasting traits.
• TT = homozygous dominant.
• tt = homozygous recessive.
• Tt = heterozygous.
• TT × tt → all Tt in F₁.
• Tt × Tt → 1 TT : 2 Tt : 1 tt.
• F₂ phenotypic ratio = 3:1.
• F₂ genotypic ratio = 1:2:1.
• Law of Dominance explains masking of recessive expression in a heterozygote.
• Law of Segregation explains separation of alleles during gamete formation.
• Recessive alleles are not destroyed when masked.
• Each gamete receives one allele of a gene pair.

23. One-Line Revision

Mendel → Pea Plant → Contrasting Traits → Monohybrid Cross → Dominance → Segregation → 3:1 Phenotypic Ratio → 1:2:1 Genotypic Ratio

24. Memory Trick

“D-S-P-R”

D → Dominance – Dominant allele can express itself.
S → Segregation – Alleles separate during gamete formation.
P → Punnett Square – Predicts possible combinations.
R → Ratio – Monohybrid F₂ phenotype = 3:1.

🧬 Final Revision:

Mendel's pea plant experiments demonstrated that hereditary characteristics are transmitted through discrete hereditary factors. His observations formed the foundation of genetics and helped explain dominance, segregation and inheritance of traits.

TT × tt → All Tt → Tall
Tt × Tt → 1 TT : 2 Tt : 1 tt
Phenotypic Ratio → 3 Tall : 1 Dwarf

Class 10th Science Chapter 8 – Heredity | Inheritance of Traits

Heredity | Inheritance of Traits

Class 10 Science | Heredity and Evolution | CBSE + Foundation + Competitive

🧬 Heredity | Inheritance of Traits

1. Inheritance of Traits | लक्षणों का वंशागति द्वारा संचरण

Inheritance is the process by which genetic information and characteristics are passed from parents to offspring.

Inheritance (वंशागति) वह प्रक्रिया है जिसमें माता-पिता से आनुवंशिक जानकारी और लक्षण संतानों तक पहुँचते हैं।

Examples:

• Eye colour – आँखों का रंग
• Hair characteristics – बालों के लक्षण
• Blood group – रक्त समूह
• Some aspects of height – ऊँचाई के कुछ पहलू
• Genetic disorders – कुछ आनुवंशिक विकार

2. Role of Genes in Inheritance | आनुवंशिकता में जीन की भूमिका

Genes are specific segments of DNA that carry hereditary information. A gene can influence a particular characteristic or a functional product of the organism.

Gene = Unit of hereditary information

Parent → Gametes → Genetic Information → Offspring

3. How Do Traits Pass from Parents to Offspring?

During sexual reproduction, parents produce gametes. Gametes contain genetic material. During fertilisation, male and female gametes fuse and form a zygote. The zygote receives genetic information from both parents.

Father's Gamete + Mother's Gamete
↓
Zygote
↓
Offspring

4. Alleles – Different Forms of a Gene

Different forms of the same gene are called alleles. For example, for a simplified Mendelian trait:

T = allele for tallness
t = allele for dwarfness

An individual generally receives one allele from each parent for a given gene.

5. Dominant Trait | प्रभावी लक्षण

A dominant allele can express its trait even when paired with a different recessive allele. Example:

TT → Tall
Tt → Tall

The capital letter T represents the dominant allele in this simplified example.

6. Recessive Trait | अप्रभावी लक्षण

A recessive trait is generally expressed when the individual possesses two recessive alleles. Example:

tt → Dwarf

7. Genotype and Phenotype

Genotype	Phenotype
TT	Tall
Tt	Tall
tt	Dwarf

Genotype: Genetic constitution of an organism.

Phenotype: Observable characteristic of an organism.

8. Homozygous and Heterozygous

Homozygous | समयुग्मजी

Both alleles are identical.

TT
or
tt

Heterozygous | विषमयुग्मजी

The two alleles are different.

Tt

9. Mendel's Contribution to Inheritance

Gregor Johann Mendel studied inheritance using garden pea plants. He selected contrasting characteristics and performed controlled crosses. His experiments helped establish basic principles of inheritance.

Mendel = Father of Genetics

10. Monohybrid Cross | एकल संकरण

A cross involving one pair of contrasting traits is called a monohybrid cross. Consider:

Pure Tall Plant = TT
Pure Dwarf Plant = tt

TT × tt

11. Punnett Square – TT × tt

🧬 Animated Mendelian Cross

Gametes	T	T
t	Tt	Tt
t	Tt	Tt

F₁ Generation:
All offspring = Tt
All offspring are phenotypically Tall.

Genotypic ratio: 100% Tt
Phenotypic ratio: 100% Tall

12. Formation of Gametes

A diploid organism has two alleles for a gene in the simplified Mendelian model. During gamete formation, the two alleles separate so that each gamete receives one allele. For genotype Tt:

Tt → T + t

This principle is represented by Mendel's Law of Segregation: the two alleles of a gene separate during gamete formation.

13. Law of Segregation

The two alleles of a gene pair separate during gamete formation, so each gamete receives only one allele.

Example:

Tt
↓
T | t

14. F₂ Generation – Tt × Tt

When two F₁ heterozygous tall plants are crossed:

Tt × Tt

	T	t
T	TT	Tt
t	Tt	tt

Genotypic Ratio:
1 TT : 2 Tt : 1 tt

Phenotypic Ratio:
3 Tall : 1 Dwarf

15. Why is F₂ Ratio 3:1?

From the cross: Tt × Tt The four possible combinations are:

• TT – Tall
• Tt – Tall
• Tt – Tall
• tt – Dwarf

Therefore: 3 Tall : 1 Dwarf

16. Inheritance of Traits – Flow Diagram

17. Independent Assortment – Basic Idea

When more than one pair of genes is studied, different combinations of alleles can occur in gametes. This contributes to genetic variation.

For Class 10 understanding, remember: More combinations of genetic material → More possible variations.

18. Inheritance and Variation

Inheritance explains why offspring resemble their parents. Variation explains why offspring are not exactly identical to their parents or siblings. Both processes occur together.

Heredity → Similarities
Variation → Differences

19. Inherited vs Acquired Characteristics

Inherited Characteristic	Acquired Characteristic
Passed through genetic information.	Develops during lifetime.
Can be transmitted to offspring.	Generally not transmitted to offspring.
Natural eye colour.	Muscle development due to exercise.
Blood group.	Scar due to injury.

20. Role of DNA Copying in Inheritance

For reproduction, genetic material must be copied and passed to the next generation. DNA copying is generally accurate but not absolutely error-free. Small differences can arise during copying, contributing to variation.

DNA Copying → Genetic Continuity + Possible Variation

21. Why Do Children Resemble Their Parents?

Children inherit genetic material from their parents. Genes influence many characteristics. Therefore, offspring show similarities with their parents. However, because genetic combinations are not necessarily identical, offspring can also show variations.

22. Important Mendelian Terms

Term	Meaning
Gene	Specific segment of DNA carrying hereditary information.
Allele	Different form of a gene.
Dominant	Allele whose trait can express in heterozygous condition.
Recessive	Trait generally expressed in homozygous condition.
Genotype	Genetic constitution.
Phenotype	Observable characteristic.
Homozygous	Identical alleles.
Heterozygous	Different alleles.
Hybrid	Offspring produced by crossing genetically different parental forms; in Mendelian context often heterozygous.

23. CBSE Golden Points ⭐

1. Heredity is the transmission of traits from parents to offspring.
2. Genes are specific segments of DNA.
3. Different forms of a gene are called alleles.
4. Dominant traits can express in heterozygous condition.
5. Recessive traits generally express in homozygous condition.
6. TT is homozygous dominant.
7. tt is homozygous recessive.
8. Tt is heterozygous.
9. Genotype means genetic constitution.
10. Phenotype means observable characteristic.
11. Mendel worked mainly with pea plants.
12. TT × tt produces all Tt in F₁.
13. Tt × Tt gives 1:2:1 genotypic ratio.
14. Tt × Tt gives 3:1 phenotypic ratio in a complete dominance model.
15. Variation is important for natural selection and evolution.

🧠 MEMORY TRICK

G-A-D-R-G-P

G = Gene A = Allele D = Dominant R = Recessive G = Genotype P = Phenotype

TT = Pure Tall | Tt = Hybrid Tall | tt = Pure Dwarf

24. 30 CBSE + Foundation + Competitive MCQs

Q1. The transmission of traits from parents to offspring is called:

A) Heredity

B) Respiration

C) Nutrition

D) Excretion

Answer: A) Heredity

Q2. A specific segment of DNA is called:

A) Tissue

B) Gene

C) Organ

D) Cell wall

Answer: B) Gene

Q3. Different forms of a gene are called:

A) Gametes

B) Chromosomes

C) Alleles

D) Tissues

Answer: C) Alleles

Q4. Which genotype is heterozygous?

A) TT

B) tt

C) XX

D) Tt

Answer: D) Tt

Q5. Which genotype represents homozygous dominant?

A) TT

B) Tt

C) tt

D) tT

Answer: A) TT

Q6. Which genotype represents homozygous recessive?

A) TT

B) tt

C) Tt

D) tT

Answer: B) tt

Q7. Mendel performed his major inheritance experiments on:

A) Rose

B) Mango

C) Garden pea

D) Wheat

Answer: C) Garden pea

Q8. In the cross TT × tt, the F₁ genotype is:

A) TT

B) tt

C) TT and tt

D) Tt

Answer: D) Tt

Q9. In TT × tt, the F₁ phenotype is:

A) All tall

B) All dwarf

C) 50% tall

D) 25% tall

Answer: A) All tall

Q10. The genetic constitution of an organism is called:

A) Phenotype

B) Genotype

C) Variation

D) Species

Answer: B) Genotype

Q11. Observable characteristics are called:

A) Genes

B) Alleles

C) Phenotype

D) Genotype

Answer: C) Phenotype

Q12. Which of the following represents a dominant allele in the given example?

A) tt

B) t

C) Tt

D) T

Answer: D) T

Q13. A monohybrid cross studies:

A) One pair of contrasting traits

B) Three species

C) Only acquired traits

D) No traits

Answer: A)

Q14. The phenotypic ratio in Tt × Tt is:

A) 1:1

B) 3:1

C) 1:2:1

D) 9:3:3:1

Answer: B) 3:1

Q15. The genotypic ratio in Tt × Tt is:

A) 3:1

B) 1:1

C) 1:2:1

D) 9:3:3:1

Answer: C) 1:2:1

Q16. The principle that alleles separate during gamete formation is known as:

A) Law of dominance

B) Law of variation

C) Law of evolution

D) Law of segregation

Answer: D) Law of segregation

Q17. A recessive phenotype in the given pea plant example is produced by:

A) tt

B) TT

C) Tt

D) TTT

Answer: A) tt

Q18. Which condition contains two identical alleles?

A) Heterozygous

B) Homozygous

C) Hybrid only

D) Phenotypic

Answer: B) Homozygous

Q19. Which condition contains two different alleles?

A) Homozygous

B) Pure

C) Heterozygous

D) Recessive

Answer: C) Heterozygous

Q20. The observable expression of genes is called:

A) Allele

B) Gene

C) Genotype

D) Phenotype

Answer: D) Phenotype

Q21. Which is generally an inherited trait?

A) Natural blood group

B) Scar

C) Learned skill

D) Exercise-developed muscle

Answer: A)

Q22. Which of the following is generally an acquired characteristic?

A) Blood group

B) Scar due to injury

C) Natural eye colour

D) Genetic disorder

Answer: B)

Q23. If T is dominant over t, a Tt plant will be:

A) Dwarf

B) Neither tall nor dwarf

C) Tall

D) Always sterile

Answer: C) Tall

Q24. Which cross produces all heterozygous offspring?

A) Tt × Tt

B) TT × TT

C) tt × tt

D) TT × tt

Answer: D)

Q25. In Tt × Tt, the probability of tt offspring is:

A) 25%

B) 50%

C) 75%

D) 100%

Answer: A) 25%

Q26. In Tt × Tt, the probability of tall phenotype is:

A) 25%

B) 75%

C) 50%

D) 100%

Answer: B) 75%

Q27. A gamete from Tt can contain:

A) T only

B) t only

C) T or t

D) TT

Answer: C) T or t

Q28. Which process contributes to new combinations of genetic material?

A) Sexual reproduction

B) Only digestion

C) Only excretion

D) Only respiration

Answer: A)

Q29. Mendelian inheritance primarily deals with:

A) Food digestion

B) Patterns of inheritance of traits

C) Respiration

D) Excretion

Answer: B)

Q30. Variation is important because it:

A) Makes every organism identical

B) Stops reproduction

C) Can provide differences on which natural selection may act

D) Removes genetic information

Answer: C)

25. 30 Subjective Questions with Answers

2 Marks Q1. What is inheritance?

Answer: Inheritance is the transmission of genetic information and traits from parents to offspring.

2 Marks Q2. What is an allele?

Answer: An allele is one of the different forms of the same gene.

2 Marks Q3. Define genotype.

Answer: Genotype is the genetic constitution of an organism for a particular trait or set of traits.

2 Marks Q4. Define phenotype.

Answer: Phenotype is the observable expression of a characteristic.

2 Marks Q5. What is a dominant trait?

Answer: A dominant trait is expressed when its corresponding allele is present in a heterozygous condition.

3 Marks Q6. Differentiate between genotype and phenotype.

Answer:
1. Genotype is the genetic constitution.
2. Phenotype is the observable characteristic.
3. TT, Tt and tt are genotypes; tall and dwarf are phenotypes in the given example.

3 Marks Q7. Differentiate between homozygous and heterozygous.

Answer:
Homozygous: identical alleles, e.g. TT or tt.
Heterozygous: different alleles, e.g. Tt.

3 Marks Q8. What are dominant and recessive alleles?

Answer: A dominant allele can express its trait in heterozygous condition. A recessive allele generally expresses its trait when present in homozygous condition.

3 Marks Q9. Why did Mendel select pea plants?

Answer:
1. Pea plants have many contrasting traits.
2. Their generation time is relatively short.
3. Self-pollination and controlled cross-pollination are possible.

3 Marks Q10. What is a monohybrid cross?

Answer: A monohybrid cross is a genetic cross involving one pair of contrasting characteristics.

4 Marks Q11. Explain the inheritance of tallness in pea plants using TT and tt.

Answer:
Pure tall = TT
Pure dwarf = tt
TT × tt
All F₁ offspring = Tt
Since T is dominant, all F₁ plants are tall.

4 Marks Q12. Explain the Law of Segregation.

Answer: The two alleles of a gene separate during gamete formation. Thus, each gamete receives only one allele of the pair. For Tt, gametes contain either T or t.

4 Marks Q13. What is the importance of a Punnett square?

Answer: A Punnett square provides a systematic way to represent possible combinations of parental alleles and helps predict the possible genotypes and phenotypes of offspring.

4 Marks Q14. Explain the terms gene and allele.

Answer: A gene is a specific segment of DNA carrying hereditary information. An allele is a particular form of that gene. For example, T and t can represent two alleles of a gene in a simplified Mendelian example.

4 Marks Q15. Why do offspring resemble their parents?

Answer: Offspring receive genetic material from their parents through gametes. Genes influence many characteristics, producing similarities between parents and offspring.

5 Marks Q16. Explain Tt × Tt with a Punnett square.

Answer:
Tt × Tt

	T	t
T	TT	Tt
t	Tt	tt

Genotypic ratio = 1 TT : 2 Tt : 1 tt
Phenotypic ratio = 3 Tall : 1 Dwarf

5 Marks Q17. Explain dominant and recessive inheritance using an example.

Answer: Let T represent tallness and t dwarfness. T is dominant. Therefore TT and Tt are tall, while tt is dwarf. The dominant allele masks the expression of the recessive allele in the heterozygous condition.

5 Marks Q18. Explain how gametes receive alleles.

Answer: During gamete formation, the two alleles of a gene pair separate. Therefore a gamete receives only one allele. A Tt individual produces two types of gametes: T and t.

5 Marks Q19. Explain why the F₁ generation of TT × tt is all tall.

Answer: TT produces only T gametes and tt produces only t gametes. Therefore every offspring receives T from one parent and t from the other, giving Tt. Since T is dominant, all offspring are tall.

5 Marks Q20. Explain the relationship between heredity and variation.

Answer: Heredity transfers genetic information and creates similarities between generations. Variation produces differences among individuals. Genetic variation can arise through DNA-copying changes and sexual reproduction. Together they explain both continuity and diversity of life.

6 Marks Q21. Describe Mendel's monohybrid experiment.

Answer:
1. Mendel selected pea plants with contrasting traits.
2. He crossed pure tall and pure dwarf plants.
3. Parental genotypes were TT and tt.
4. All F₁ plants were Tt and tall.
5. He allowed F₁ plants to self-pollinate.
6. F₂ showed approximately 3 tall : 1 dwarf phenotype under the simplified complete-dominance model.

6 Marks Q22. Explain the inheritance of traits from DNA to offspring.

Answer: DNA contains hereditary information. Genes are specific DNA segments. Chromosomes organise DNA. During gamete formation, genetic material is distributed into gametes. During fertilisation, gametes from two parents fuse and form a zygote. The offspring therefore receives genetic information from both parents.

6 Marks Q23. Explain genotype, phenotype, homozygous and heterozygous conditions with examples.

Answer:
Genotype = genetic constitution.
Phenotype = observable trait.
Homozygous = identical alleles, TT or tt.
Heterozygous = different alleles, Tt.
TT → Tall.
Tt → Tall.
tt → Dwarf.

6 Marks Q24. Explain the importance of variation in inheritance.

Answer: Inheritance maintains genetic continuity, while variation produces differences. Variations can arise through DNA-copying errors and sexual reproduction. Some variations may be advantageous in particular environments. Natural selection can favour such variations, contributing to evolutionary change.

6 Marks Q25. Explain the difference between inherited and acquired traits.

Answer: Inherited traits are transmitted through genetic information from parents to offspring. Acquired traits develop during an individual's lifetime due to environment, behaviour, learning, injury or other factors. Acquired traits generally do not produce a heritable change in germ-line DNA and therefore are usually not passed to offspring.

6 Marks Q26. Why are siblings similar but not identical?

Answer: Siblings receive genetic material from the same parents, producing similarities. However, each child receives a different combination of parental genetic material through gamete formation and fertilisation. Genetic variation therefore makes siblings different in many characteristics.

6 Marks Q27. Explain the significance of Mendel's experiments.

Answer: Mendel's controlled pea plant experiments revealed predictable patterns of inheritance. They established basic concepts related to dominant and recessive traits and segregation of alleles. His work became the foundation of classical genetics.

6 Marks Q28. Explain how sexual reproduction contributes to variation.

Answer: Sexual reproduction involves two parents. During gamete formation, different combinations of genetic material arise. Fusion of gametes creates another combination. Consequently, offspring can differ genetically from their parents and siblings.

6 Marks Q29. Explain the importance of the Law of Segregation.

Answer: The Law of Segregation explains that the two alleles of a gene separate during gamete formation. Each gamete receives one allele. During fertilisation, alleles from two gametes combine again. This helps explain inheritance ratios observed in Mendelian crosses.

6 Marks Q30. Explain why heredity alone cannot explain every difference between individuals.

Answer: Heredity explains the transmission of genetic information, but differences among individuals can arise from genetic variation and environmental influences. Nutrition, lifestyle and environmental conditions can affect the phenotype of many traits. Therefore, observable differences may result from both genetic and environmental factors.

26. HOTS / Competency-Based Questions

HOTS 1: A tall pea plant has genotype Tt. What type of gametes can it produce?

Answer: It can produce two types of gametes: T and t.

HOTS 2: Two tall pea plants produce a dwarf offspring. How is this possible?

Answer: Both tall parents can be heterozygous Tt. Their cross Tt × Tt can produce tt offspring, which is dwarf.

HOTS 3: Why does a recessive trait disappear in F₁ but reappear in F₂?

Answer: In F₁, the recessive allele is present with a dominant allele and is masked. In F₂, two recessive alleles can come together, producing the recessive phenotype.

HOTS 4: If a population has no genetic variation, what could be its disadvantage?

Answer: The population may have reduced ability to respond to environmental changes because there are fewer genetic differences from which natural selection can favour advantageous characteristics.

HOTS 5: Can an acquired characteristic become automatically inherited by children?

Answer: No. An acquired characteristic generally does not alter the germ-line DNA in a heritable manner, so it is usually not passed genetically to offspring.

27. Assertion – Reason

Q1. Assertion: Tt is a heterozygous genotype.
Reason: Tt contains two different alleles.

Answer: Both Assertion and Reason are true, and Reason correctly explains the Assertion.

Q2. Assertion: TT × tt produces all Tt offspring.
Reason: TT produces only T gametes and tt produces only t gametes.

Answer: Both statements are true and the Reason correctly explains the Assertion.

Q3. Assertion: In Tt × Tt, dwarf offspring may appear.
Reason: Each heterozygous parent can produce a t gamete.

Answer: Both statements are true and the Reason correctly explains the Assertion.

28. Quick Revision Chart

Genotype	Condition	Phenotype
TT	Homozygous dominant	Tall
Tt	Heterozygous	Tall
tt	Homozygous recessive	Dwarf

TT × tt → 100% Tt → 100% Tall

Tt × Tt → 1 TT : 2 Tt : 1 tt
Phenotype → 3 Tall : 1 Dwarf

29. Important Diagram / Cross Labelling

🧬 Mendelian Cross – Important Labels

30. One-Line Revision

Inheritance transfers genetic information from parents to offspring through genes, while different combinations of alleles produce variations in traits.

🧬 FINAL FORMULA

GENE → ALLELE → GAMETE → FERTILISATION → OFFSPRING

Heredity = Similarities    |    Variation = Differences

Class 10 Science – Heredity
Inheritance of Traits | Mendelian Genetics

CBSE + Foundation + Competitive Preparation

Class 10th Science Chapter 8 – Heredity | Introduction to Heredity and Variation

Heredity | Introduction to Heredity and Variation

Class 10 Science | Chapter – Heredity and Evolution | CBSE + Foundation + Competitive

🧬 Heredity | Introduction to Heredity and Variation

1. Heredity – Introduction | आनुवंशिकता का परिचय

Heredity is the transmission of characteristics or traits from parents to their offspring.

आनुवंशिकता (Heredity) वह प्रक्रिया है जिसके द्वारा माता-पिता के लक्षण उनकी संतानों में स्थानांतरित होते हैं।

Examples of Hereditary Traits

• Eye colour – आँखों का रंग
• Hair type – बालों का प्रकार
• Blood group – रक्त समूह
• Height tendency – ऊँचाई की प्रवृत्ति
• Skin pigmentation – त्वचा का वर्णकता स्तर

Examples of Variations

• Different height among siblings
• Different hair texture
• Differences in fingerprints
• Differences in facial features
• Differences in body characteristics

2. Variation – विविधता

Variation means differences in characteristics among individuals of the same species.

Variation (विविधता) का अर्थ है एक ही species के अलग-अलग individuals में पाए जाने वाले लक्षणों में अंतर।

Important: No two individuals of a species are exactly identical in all characteristics.
किसी species के दो individuals अपने सभी लक्षणों में बिल्कुल समान नहीं होते।

3. Heredity and Variation – Relationship

Heredity | आनुवंशिकता	Variation | विविधता
Transfers traits from parents to offspring.	Produces differences among individuals.
Helps maintain characteristics of a species.	Provides differences within a population.
Associated with genetic information.	Can arise due to genetic differences and, for some traits, environmental effects.
Provides continuity between generations.	Provides raw material for natural selection and evolution.

4. DNA, Genes and Chromosomes

The genetic information responsible for inherited traits is stored in DNA (Deoxyribonucleic Acid).

DNA is organised into structures called chromosomes. A specific segment of DNA that carries information for a particular trait or functional product is called a gene.

DNA → Genes → Chromosomes → Hereditary Information

🧬 Animated Diagram – DNA, Gene and Chromosome

5. Where is Genetic Material Present? | आनुवंशिक पदार्थ कहाँ होता है?

In most eukaryotic cells, chromosomes are present inside the nucleus.

Chromosomes contain DNA, and genes are specific segments of DNA.

Cell → Nucleus → Chromosome → DNA → Gene → Trait

6. What is a Trait? | लक्षण क्या है?

A trait is a particular characteristic of an organism.

Trait = लक्षण / विशेषता

Examples:

• Height
• Eye colour
• Hair type
• Seed colour in pea plants
• Seed shape in pea plants

7. Inherited Traits and Acquired Traits

Inherited Traits	Acquired Traits
Passed genetically from parents to offspring.	Develop during lifetime due to environment, learning, use/disuse etc.
Associated with genetic information.	Generally do not alter the DNA sequence of germ cells in a way that is inherited.
Example: natural eye colour.	Example: muscle development due to exercise.
Can be inherited by offspring.	Normally not inherited by offspring.

CBSE Golden Point:
Acquired characteristics generally do not bring a change in the DNA of germ cells and therefore are not inherited in the usual Mendelian sense.

8. Why Does Sexual Reproduction Produce Variation?

During sexual reproduction, genetic material comes from two parents. The offspring receives genetic information from both parents. During formation of gametes and their fusion, different combinations of genetic information can arise. Therefore, offspring may differ from their parents and from one another.

Parent 1 Genetic Material + Parent 2 Genetic Material
↓
New Combination
↓
Variation in Offspring

9. Asexual vs Sexual Reproduction

Feature	Asexual Reproduction	Sexual Reproduction
Number of parents	Usually one	Usually two
Gamete fusion	Absent	Present
Genetic variation	Usually lower, though mutations can occur	Usually greater
Offspring	Usually genetically similar to parent	Genetically variable

10. Gregor Johann Mendel – Father of Genetics

Gregor Johann Mendel studied inheritance using garden pea plants (Pisum sativum). He selected pea plants because they had several contrasting traits and could be easily cross-pollinated.

Why Pea Plant?

• Short generation time
• Many easily distinguishable traits
• Self-pollination normally occurs
• Cross-pollination can be controlled
• Produces many seeds

Mendel's Approach

• Selected contrasting traits
• Performed controlled crosses
• Observed offspring
• Counted different phenotypes
• Studied inheritance patterns

11. Seven Pairs of Contrasting Traits in Pea Plant

Character	Dominant Form	Recessive Form
Seed shape	Round	Wrinkled
Seed colour	Yellow	Green
Flower colour	Violet/Purple	White
Pod shape	Inflated	Constricted
Pod colour	Green	Yellow
Flower position	Axial	Terminal
Stem length	Tall	Dwarf

12. Dominant and Recessive Traits

A dominant trait is expressed in the phenotype when its corresponding allele is present in a heterozygous condition.

A recessive trait is expressed when the individual has two recessive alleles for that trait.

TT = Tall
Tt = Tall
tt = Dwarf

Here T represents the dominant allele for tallness and t represents the recessive allele for dwarfness.

13. Alleles – वैकल्पिक रूप

Different forms of the same gene are called alleles.

Example: T = allele for tallness
t = allele for dwarfness

The pair of alleles is inherited, one from each parent.

14. Genotype and Phenotype

Term	Meaning	Example
Genotype	Genetic constitution of an organism for a trait.	TT, Tt, tt
Phenotype	Observable characteristic.	Tall or dwarf

15. Homozygous and Heterozygous

Homozygous

Both alleles are identical.

TT or tt

Examples: Pure tall or pure dwarf.

Heterozygous

Both alleles are different.

Tt

Example: Hybrid tall plant.

16. Simple Mendelian Cross – Tall × Dwarf

Suppose a pure tall plant is crossed with a pure dwarf plant.

TT × tt
↓
Gametes: T,T × t,t
↓
F₁: Tt, Tt, Tt, Tt

	T	T
t	Tt	Tt
t	Tt	Tt

F₁ Result: All offspring are Tt and phenotypically tall.
Phenotypic ratio: 100% Tall
Genotypic ratio: 100% Tt

17. Sources of Variation

Genetic Sources

• DNA copying errors / mutations
• Recombination
• Independent assortment of chromosomes
• Random fertilisation

Environmental Influence

• Nutrition
• Temperature
• Light
• Physical activity
• Other environmental factors

18. Importance of Variation

Variation is important because it can help populations survive environmental changes. If environmental conditions change, individuals with advantageous variations may have better chances of survival and reproduction. Over many generations, such differences can contribute to evolution.

Variation → Natural Selection → Differential Survival/Reproduction → Evolutionary Change

🌱 Animated Diagram – Heredity and Variation

19. CBSE Golden Points ⭐

1. Heredity is the transmission of traits from parents to offspring.
2. Variation means differences among individuals of the same species.
3. Genes are specific segments of DNA.
4. Chromosomes contain DNA.
5. DNA carries hereditary information.
6. Mendel is known as the father of genetics.
7. Mendel performed experiments mainly on pea plants.
8. Different forms of a gene are called alleles.
9. Dominant traits can express in heterozygous condition.
10. Recessive traits generally express in homozygous condition.
11. Genotype represents genetic constitution.
12. Phenotype represents observable characteristics.
13. TT and tt are homozygous.
14. Tt is heterozygous.
15. Variation provides the raw material for natural selection.

🧠 MEMORY TRICK

H → G → D → V → E

H = Heredity G = Gene D = DNA V = Variation E = Evolution

“Genes carry DNA information → Heredity transfers traits → Variation creates differences → Evolution may occur over generations.”

20. 30 CBSE + Foundation + Competitive MCQs

Q1. The transmission of traits from parents to offspring is called:

A) Heredity

B) Respiration

C) Nutrition

D) Excretion

Answer: A) Heredity

Q2. Differences among individuals of the same species are called:

A) Evolution

B) Variation

C) Fertilisation

D) Respiration

Answer: B) Variation

Q3. The hereditary material in most organisms is:

A) Protein

B) Lipid

C) DNA

D) Starch

Answer: C) DNA

Q4. A specific segment of DNA that carries information is called:

A) Cell

B) Tissue

C) Chromosome

D) Gene

Answer: D) Gene

Q5. Genes are located on:

A) Chromosomes

B) Ribosomes

C) Cell wall

D) Vacuoles

Answer: A) Chromosomes

Q6. Gregor Mendel conducted his famous experiments on:

A) Rose

B) Garden pea

C) Mango

D) Wheat

Answer: B) Garden pea

Q7. Which genotype represents a homozygous dominant condition?

A) Tt

B) tt

C) TT

D) tT

Answer: C) TT

Q8. Which genotype represents a heterozygous condition?

A) TT

B) tt

C) XX

D) Tt

Answer: D) Tt

Q9. A trait that is expressed in a heterozygous condition is generally:

A) Dominant

B) Recessive

C) Acquired

D) Environmental only

Answer: A) Dominant

Q10. Which is an example of an inherited characteristic?

A) Scar caused by injury

B) Natural eye colour

C) Increased muscle size due to exercise

D) Learned handwriting

Answer: B) Natural eye colour

Q11. The genetic constitution of an organism is its:

A) Phenotype

B) Variation

C) Genotype

D) Species

Answer: C) Genotype

Q12. The observable expression of a trait is called:

A) Genotype

B) Allele

C) Gene

D) Phenotype

Answer: D) Phenotype

Q13. Different forms of the same gene are known as:

A) Alleles

B) Tissues

C) Gametes

D) Enzymes

Answer: A) Alleles

Q14. Which pair is homozygous?

A) Tt

B) TT

C) Tt and tT

D) XY

Answer: B) TT

Q15. Which of the following is generally not inherited?

A) Blood group

B) Natural eye colour

C) Scar acquired after an accident

D) Genetic condition

Answer: C) Scar acquired after an accident

Q16. In a cross TT × tt, the genotype of all F₁ offspring will be:

A) TT

B) tt

C) Tt

D) TTT

Answer: C) Tt

Q17. In TT × tt, the phenotype of F₁ generation is:

A) All tall

B) All dwarf

C) 50% tall and 50% dwarf

D) None

Answer: A) All tall

Q18. Variation is important for:

A) Digestion only

B) Evolution

C) Respiration only

D) Excretion only

Answer: B) Evolution

Q19. Which process combines genetic material from two parents?

A) Binary fission

B) Budding

C) Sexual reproduction

D) Fragmentation

Answer: C) Sexual reproduction

Q20. The term “genetics” is most closely associated with the study of:

A) Inheritance and variation

B) Digestion

C) Respiration

D) Photosynthesis

Answer: A) Inheritance and variation

Q21. Which statement about acquired traits is correct?

A) They are always inherited.

B) They generally do not alter germ-line DNA in a heritable way.

C) They are always dominant.

D) They are always recessive.

Answer: B)

Q22. A pair of identical alleles is called:

A) Heterozygous

B) Hybrid

C) Homozygous

D) Dominant

Answer: C) Homozygous

Q23. If T is dominant over t, which genotype gives the recessive phenotype?

A) TT

B) Tt

C) tT

D) tt

Answer: D) tt

Q24. Which of the following contributes to genetic variation?

A) Genetic recombination

B) Only digestion

C) Only respiration

D) Only excretion

Answer: A) Genetic recombination

Q25. A phenotype is influenced by:

A) Genes only in every case

B) Environment only in every case

C) Genes and, for many traits, environmental factors

D) Neither genes nor environment

Answer: C)

Q26. Which one is a recessive phenotype in Mendel's tall/dwarf example?

A) Tall

B) Dwarf

C) Hybrid

D) Heterozygous

Answer: B) Dwarf

Q27. The transmission of DNA information from parents to offspring provides:

A) Heredity

B) Digestion

C) Respiration

D) Excretion

Answer: A) Heredity

Q28. Why can siblings show differences even though they have the same parents?

A) They always have identical DNA.

B) Different combinations of genetic material can arise.

C) Heredity does not occur.

D) Genes are absent.

Answer: B)

Q29. Which sequence is correct?

A) Gene → DNA → Chromosome

B) DNA → Gene → Cell wall

C) Chromosome → DNA → Gene

D) Tissue → Gene → DNA

Answer: C) Chromosome → DNA → Gene

Q30. Which statement best explains the importance of variation?

A) It guarantees survival of every individual.

B) It prevents reproduction.

C) It removes all differences.

D) It may help populations adapt to changing environments.

Answer: D)

21. 30 Subjective Questions with Answers

2 Marks Q1. Define heredity.

Answer: Heredity is the transmission of characteristics or traits from parents to offspring through genetic information.

2 Marks Q2. What is variation?

Answer: Variation refers to differences in characteristics among individuals of the same species.

2 Marks Q3. What is a gene?

Answer: A gene is a specific segment of DNA that carries information for a particular trait or functional product.

2 Marks Q4. Differentiate between genotype and phenotype.

Answer: Genotype is the genetic constitution of an organism, whereas phenotype is the observable expression of its characteristics.

2 Marks Q5. What are alleles?

Answer: Alleles are different forms of the same gene.

3 Marks Q6. Differentiate between inherited and acquired traits.

Answer:
1. Inherited traits are transmitted genetically from parents to offspring.
2. Acquired traits develop during an individual's lifetime.
3. Acquired traits generally are not inherited because they do not cause a heritable change in germ-line DNA.

3 Marks Q7. Why did Mendel select pea plants?

Answer:
1. Pea plants have several contrasting traits.
2. They have a short generation time.
3. Self-pollination and controlled cross-pollination are possible.

3 Marks Q8. What is the difference between homozygous and heterozygous conditions?

Answer: Homozygous condition contains identical alleles, such as TT or tt. Heterozygous condition contains different alleles, such as Tt.

3 Marks Q9. What is meant by a dominant trait?

Answer: A dominant trait is expressed when its allele is present in a heterozygous condition. For example, Tt is tall when T is dominant over t.

3 Marks Q10. Why is variation important?

Answer: Variation creates differences among individuals. Some variations may provide advantages in changing environments and can contribute to natural selection and evolution.

4 Marks Q11. Explain the relationship between DNA, genes and chromosomes.

Answer:
1. DNA is the main hereditary material.
2. DNA is organised into chromosomes.
3. Genes are specific segments of DNA.
4. Genes carry hereditary information for traits or functional products.

4 Marks Q12. Explain genotype and phenotype with an example.

Answer:
Genotype is the genetic constitution, e.g. TT, Tt or tt.
Phenotype is the observable characteristic, e.g. tall or dwarf.
TT → Tall
Tt → Tall
tt → Dwarf

4 Marks Q13. Explain the importance of Mendel's pea plant experiments.

Answer: Mendel studied contrasting traits, performed controlled crosses, observed offspring and established basic principles of inheritance. His work formed the foundation of genetics.

4 Marks Q14. Explain how sexual reproduction generates variation.

Answer:
1. Genetic material comes from two parents.
2. Gamete formation involves different combinations of chromosomes.
3. Fertilisation combines genetic material from two gametes.
4. Therefore, offspring may have new combinations of traits.

4 Marks Q15. Differentiate between heredity and variation.

Answer: Heredity maintains continuity of traits between generations, while variation produces differences among individuals. Heredity transfers genetic information; variation provides differences that may contribute to evolutionary change.

5 Marks Q16. Explain Mendel's monohybrid cross between tall and dwarf pea plants.

Answer:
Pure tall = TT
Pure dwarf = tt
TT × tt
All F₁ offspring = Tt
Since T is dominant, all F₁ plants are tall.
Phenotypic result: 100% Tall
Genotypic result: 100% Tt

5 Marks Q17. Explain the terms allele, dominant, recessive, homozygous and heterozygous.

Answer:
• Alleles – different forms of a gene.
• Dominant – allele expressed in heterozygous condition.
• Recessive – allele generally expressed when homozygous.
• Homozygous – identical alleles, e.g. TT.
• Heterozygous – different alleles, e.g. Tt.

5 Marks Q18. Explain why acquired characteristics are generally not inherited.

Answer: Acquired characteristics develop during an individual's lifetime due to environmental effects, use, learning or injury. They generally do not change the DNA sequence of germ cells in a heritable manner. Therefore, they are usually not transmitted to offspring.

5 Marks Q19. Explain the role of variation in evolution.

Answer: Variations create differences within a population. Environmental changes may favour some variations over others. Individuals with advantageous variations may survive and reproduce more successfully. Over many generations, the frequency of such traits can increase, contributing to evolutionary change.

5 Marks Q20. Describe the seven contrasting traits studied by Mendel.

Answer:
1. Round / wrinkled seeds
2. Yellow / green seeds
3. Violet / white flowers
4. Inflated / constricted pods
5. Green / yellow pods
6. Axial / terminal flowers
7. Tall / dwarf stem

6 Marks Q21. Explain heredity, variation and their importance in evolution.

Answer: Heredity transfers genetic characteristics from parents to offspring. Variation produces differences among individuals. Genetic variation can arise through mutation, recombination and different combinations of genetic material. Some variations may help organisms survive changing conditions. Natural selection can increase favourable variations over generations, contributing to evolution.

6 Marks Q22. Explain DNA, gene, chromosome and trait with a suitable sequence.

Answer:
DNA is the hereditary material.
↓
DNA is organised into chromosomes.
↓
Specific segments of DNA are genes.
↓
Genes influence characteristics or traits.
Example: A gene can contribute to variation in a particular characteristic.

6 Marks Q23. Explain the difference between genotype and phenotype with suitable examples.

Answer:
Genotype = genetic constitution.
Phenotype = observable expression.
TT → Tall
Tt → Tall
tt → Dwarf
TT and tt are homozygous, while Tt is heterozygous.

6 Marks Q24. Explain how variations can arise in organisms.

Answer:
1. Errors during DNA copying can introduce changes.
2. Mutations can produce new genetic variants.
3. Sexual reproduction produces new combinations of genetic material.
4. Recombination during meiosis contributes to genetic diversity.
5. Independent assortment and random fertilisation also increase combinations.
6. Environmental factors can influence the phenotype of some traits.

6 Marks Q25. Explain Mendel's contribution to the study of heredity.

Answer: Mendel used pea plants with contrasting traits and performed controlled crosses. He observed the characteristics of parents and offspring and counted their occurrence. His experiments established fundamental patterns of inheritance and introduced ideas corresponding to dominant and recessive traits. His work became the foundation of classical genetics.

6 Marks Q26. Explain why offspring are similar to parents but not identical to them.

Answer: Offspring receive genetic material from their parents, producing similarities through heredity. However, sexual reproduction combines genetic material from two parents and generates new combinations. DNA-copying variations and recombination can also create differences. Therefore, offspring resemble parents but are usually not identical.

6 Marks Q27. Explain the significance of dominant and recessive alleles.

Answer: A dominant allele can express its trait in heterozygous condition. A recessive allele generally expresses its trait only when both alleles are recessive. For example, Tt is tall if T is dominant, while tt is dwarf.

6 Marks Q28. Explain why variation is more significant in sexual reproduction.

Answer: Sexual reproduction involves two parents and the formation and fusion of gametes. Each gamete carries a different combination of genetic material. Fusion creates another new combination. Therefore, sexual reproduction generally produces more genetic variation than asexual reproduction.

6 Marks Q29. Explain the role of heredity in maintaining the identity of a species.

Answer: Heredity transmits genetic information from one generation to the next. Important species characteristics are therefore maintained across generations. At the same time, variations introduce differences without completely disrupting the basic characteristics of the species.

6 Marks Q30. Explain the statement: “Variation is the raw material for evolution.”

Answer: Variation creates differences among organisms. When environmental conditions change, certain variations may provide a survival or reproductive advantage. Such individuals may leave more offspring. Over many generations, advantageous characteristics can become more common in a population. Thus variation provides the basis on which natural selection can act, contributing to evolution.

22. HOTS / Competency Based Questions

HOTS 1: Two children of the same parents have different heights. Does this prove that heredity is absent?

Answer: No. Both children inherit genetic information from their parents, but they can receive different combinations of alleles. Height can also be influenced by environmental factors such as nutrition and health. Therefore, heredity and variation can occur together.

HOTS 2: A person develops strong muscles after regular exercise. Will the increased muscle size necessarily be inherited by the person's children?

Answer: No. Increased muscle size due to exercise is an acquired characteristic and does not normally alter the germ-line DNA in a heritable manner.

HOTS 3: If all organisms in a population were genetically identical, what could happen if the environment changed drastically?

Answer: The population could be highly vulnerable because there would be little or no genetic variation from which advantageous characteristics could arise or be selected.

HOTS 4: A tall pea plant has genotype Tt. Is it pure or hybrid?

Answer: It is hybrid/heterozygous because it contains two different alleles, T and t.

HOTS 5: Why can a recessive trait remain hidden for generations?

Answer: A recessive allele can remain present in heterozygous individuals without being expressed when a dominant allele is present. It becomes phenotypically visible when an individual receives two recessive alleles.

23. Assertion – Reason

Q1. Assertion: Genes are involved in the inheritance of traits.
Reason: Genes are specific segments of DNA.

Answer: Both Assertion and Reason are true, and Reason correctly explains the Assertion.

Q2. Assertion: Acquired traits are generally not inherited.
Reason: Acquired traits generally do not produce a heritable change in germ-line DNA.

Answer: Both Assertion and Reason are true, and Reason correctly explains the Assertion.

Q3. Assertion: Tt is heterozygous.
Reason: Tt contains two different alleles.

Answer: Both statements are true and the Reason correctly explains the Assertion.

24. Important Diagram Labelling Points

🧬 Chromosome – Important Labels

Important Labels

• Chromosome
• Chromatid
• Centromere
• DNA
• Gene

Exam Diagram Sequence

Cell → Nucleus → Chromosome → DNA → Gene

25. One-Line Revision

Heredity transfers traits from parents to offspring, while variation creates differences among individuals; genes are segments of DNA located on chromosomes, and inherited variations can contribute to natural selection and evolution.

🧬 FINAL REVISION FORMULA

DNA → Gene → Chromosome → Heredity
↓
Variation → Natural Selection → Evolution

26. Exam-Focused Quick Points

Remember for CBSE:

1. Heredity = transmission of traits.
2. Variation = differences among individuals.
3. Gene = specific segment of DNA.
4. Chromosome contains DNA.
5. Mendel = Father of Genetics.
6. Mendel used pea plants.
7. Alleles = alternative forms of a gene.
8. TT = homozygous dominant.
9. Tt = heterozygous.
10. tt = homozygous recessive.
11. Genotype = genetic constitution.
12. Phenotype = observable character.
13. Dominant trait can express in heterozygous condition.
14. Recessive trait generally expresses in homozygous condition.
15. Variation is important for evolution.

Class 10 Science – Heredity
Introduction to Heredity and Variation

CBSE + Foundation + Competitive Preparation