Class 10th Science Chapter 8 – Heredity | Mendel’s Monohybrid Cross

🧬 Heredity | Mendel’s Monohybrid Cross

Class 10 Science | Chapter – Heredity | CBSE + Foundation + Competitive Level

🧬 Mendel’s Monohybrid Cross

1. What is a Monohybrid Cross?

A monohybrid cross is a genetic cross in which the inheritance of one pair of contrasting traits is studied at a time.

For example, Mendel studied the inheritance of:

• Tallness of pea plant
• Dwarfness of pea plant

Thus, plant height was one character and tall/dwarf were its contrasting forms.

Mono = One
Hybrid = Offspring produced from genetically different parents.

Therefore, a monohybrid cross studies inheritance of one character/pair of contrasting traits.

2. Gregor Johann Mendel

Gregor Johann Mendel was an Austrian monk and scientist who performed experiments on pea plants.

He is widely known as the:

Father of Genetics

Mendel selected the garden pea plant: Pisum sativum for his experiments.

3. Why Did Mendel Choose Pea Plants?

Mendel selected pea plants because:

• They have many easily observable contrasting traits.
• They have a relatively short generation time.
• They can self-pollinate.
• Artificial cross-pollination can be performed easily.
• Large numbers of offspring can be obtained.
• Pure-breeding varieties were available.

4. Contrasting Traits in Pea Plants

Character	Contrasting Forms
Plant height	Tall / Dwarf
Seed shape	Round / Wrinkled
Seed colour	Yellow / Green
Flower colour	Violet / White
Pod shape	Inflated / Constricted
Pod colour	Green / Yellow
Flower position	Axial / Terminal
Stem length	Long / Short

5. Symbols Used in Monohybrid Cross

Let:

T

= Dominant allele for tallness

t

= Recessive allele for dwarfness

Therefore: TT = Homozygous tall
Tt = Heterozygous tall
tt = Homozygous dwarf

6. Parental Generation – P Generation

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

Pure tall plant: TT
Pure dwarf plant: tt

P Generation:

TT × tt

7. Formation of Gametes

The alleles separate during gamete formation. A TT plant can produce only T gametes. A tt plant can produce only t gametes.

TT

→

T

T

tt

→

t

t

8. F₁ Generation

When the gametes fuse:

T × t → Tt

All F₁ offspring have genotype: Tt Since T is dominant, all F₁ plants are: Tall

TT × tt

↓

All F₁ = Tt

Phenotype = All Tall

9. Mendel’s F₁ Observation

Mendel observed that only one of the two contrasting traits appeared in the F₁ generation.

The tall characteristic appeared, while dwarfness was not visible.

This led to the concept of: Dominant and Recessive traits.

The recessive trait did not disappear genetically. Its allele remained present in the F₁ plants as part of the heterozygous genotype Tt.

10. Self-Pollination of F₁ Plants

Mendel allowed F₁ plants to self-pollinate. Each F₁ plant had genotype: Tt Therefore:

Tt × Tt

Each parent can produce two types of gametes: T and t

11. Punnett Square – Tt × Tt

Gametes	T	t
T	TT Tall	Tt Tall
t	Tt Tall	tt Dwarf

12. Genotypic Ratio

The four possible genotypes are:

• TT
• Tt
• Tt
• tt

Therefore:

Genotypic Ratio = 1 TT : 2 Tt : 1 tt

13. Phenotypic Ratio

TT = Tall
Tt = Tall
Tt = Tall
tt = Dwarf

Therefore:

Phenotypic Ratio = 3 Tall : 1 Dwarf

14. Complete Monohybrid Cross

P

→

TT × tt

↓

F₁

→

All Tt

→

Tall

↓ Selfing

Tt × Tt

↓

TT

Tt

Tt

tt

3 Tall : 1 Dwarf

15. Law of Segregation

Mendel proposed that the two factors/alleles for a character separate during gamete formation.

Thus a heterozygous plant: Tt produces: T and t gametes.

This principle is called the: Law of Segregation

Tt → T + t

16. Law of Dominance – Basic Idea

When two contrasting alleles occur together in a heterozygote, one may express itself while the other remains masked.

Example: Tt → Tall Here T is dominant and t is recessive.

17. Pure-Breeding Plants

A pure-breeding organism produces offspring with the same form of a trait when self-pollinated under the relevant conditions. For the simple pea-height example: TT → pure tall
tt → pure dwarf

18. Importance of Monohybrid Cross

A monohybrid cross helps us understand:

• Inheritance of a single characteristic.
• Dominant and recessive traits.
• Genotype and phenotype.
• Homozygous and heterozygous conditions.
• Segregation of alleles.
• Probability of different offspring genotypes.
• Why recessive traits can reappear in later generations.

19. Probability in Monohybrid Cross

For: Tt × Tt The four equally likely allele combinations in the simple Mendelian model are:

TT, Tt, Tt, tt

Therefore: Probability of TT = 1/4 = 25%
Probability of Tt = 2/4 = 50%
Probability of tt = 1/4 = 25%

Thus the probability of tall phenotype is: 3/4 = 75% and dwarf phenotype: 1/4 = 25%

20. Complete Result Table

Offspring	Genotype	Phenotype	Probability
1	TT	Tall	25%
2	Tt	Tall	25%
3	Tt	Tall	25%
4	tt	Dwarf	25%

21. Genotypic Ratio vs Phenotypic Ratio

Ratio	Meaning	Monohybrid F₂
Genotypic Ratio	Ratio of genetic combinations	1 : 2 : 1
Phenotypic Ratio	Ratio of observable traits	3 : 1

22. 30 MCQs – Mendel’s Monohybrid Cross

Q1. A monohybrid cross studies:

A) One pair of contrasting traits

B) Two pairs of contrasting traits

C) Three traits

D) All traits simultaneously

Answer: A

Q2. Mendel performed his famous experiments mainly on:

A) Rose

B) Garden pea

C) Wheat

D) Maize

Answer: B

Q3. The scientific name of garden pea is:

A) Pisum sativum

B) Oryza sativa

C) Zea mays

D) Triticum aestivum

Answer: A

Q4. If T is dominant for tallness, the genotype of a pure tall plant is:

A) Tt

B) tt

C) TT

D) T

Answer: C

Q5. The genotype of a pure dwarf plant is:

A) TT

B) Tt

C) tt

D) T

Answer: C

Q6. TT × tt produces:

A) All TT

B) All Tt

C) All tt

D) TT and tt only

Answer: B

Q7. The phenotype of Tt is:

A) Dwarf

B) Tall

C) Both tall and dwarf

D) Cannot be determined

Answer: B

Q8. F₁ generation obtained from TT × tt is:

A) All tall

B) All dwarf

C) 3 tall : 1 dwarf

D) 1 tall : 1 dwarf

Answer: A

Q9. The genotype of an F₁ plant in TT × tt is:

A) TT

B) tt

C) Tt

D) TTT

Answer: C

Q10. F₁ self-cross in a monohybrid experiment is:

A) TT × TT

B) tt × tt

C) Tt × Tt

D) TT × tt

Answer: C

Q11. The F₂ genotypic ratio is:

A) 3:1

B) 1:1

C) 1:2:1

D) 9:3:3:1

Answer: C

Q12. The F₂ phenotypic ratio under complete dominance is:

A) 1:2:1

B) 3:1

C) 1:1

D) 2:1

Answer: B

Q13. In Tt × Tt, the probability of tt is:

A) 25%

B) 50%

C) 75%

D) 100%

Answer: A

Q14. In Tt × Tt, the probability of TT is:

A) 50%

B) 25%

C) 75%

D) 100%

Answer: B

Q15. In Tt × Tt, the probability of heterozygous offspring is:

A) 25%

B) 75%

C) 50%

D) 100%

Answer: C

Q16. Which law explains the separation of alleles during gamete formation?

A) Law of dominance

B) Law of segregation

C) Law of independent assortment

D) Law of conservation

Answer: B

Q17. A Tt plant produces which gametes?

A) Only T

B) Only t

C) T and t

D) TT and tt

Answer: C

Q18. A TT plant produces:

A) Only T gametes

B) Only t gametes

C) T and t gametes

D) No gametes

Answer: A

Q19. A tt plant produces:

A) Only T

B) Only t

C) T and t

D) TT

Answer: B

Q20. Which generation is obtained immediately after crossing P generation?

A) F₂

B) F₁

C) F₃

D) F₄

Answer: B

Q21. In the cross TT × tt, the phenotype of F₁ is:

A) All dwarf

B) All tall

C) 50% tall

D) 25% tall

Answer: B

Q22. Which genotype represents heterozygous condition?

A) TT

B) tt

C) Tt

D) T

Answer: C

Q23. Which genotype represents homozygous recessive?

A) TT

B) Tt

C) tt

D) T

Answer: C

Q24. The 3:1 ratio is observed in:

A) F₁ genotype

B) F₂ phenotype

C) P genotype

D) Gametes

Answer: B

Q25. The 1:2:1 ratio represents:

A) F₂ genotypes

B) F₁ phenotypes

C) P phenotypes

D) Gamete ratio of TT

Answer: A

Q26. Which is the correct sequence?

A) P → F₂ → F₁

B) F₁ → P → F₂

C) P → F₁ → F₂

D) F₂ → F₁ → P

Answer: C

Q27. Why does dwarfness reappear in F₂?

A) Dwarf allele is newly created

B) Recessive alleles combine as tt

C) Tall allele disappears

D) Mutation is necessary

Answer: B

Q28. Mendel is known as:

A) Father of Evolution

B) Father of Genetics

C) Father of Ecology

D) Father of Anatomy

Answer: B

Q29. Which ratio is expected when Tt is crossed with tt?

A) 3 Tall : 1 Dwarf

B) All Tall

C) 1 Tall : 1 Dwarf

D) All Dwarf

Answer: C

Q30. The main purpose of a Punnett square is to:

A) Measure plant height

B) Predict possible genetic combinations in offspring

C) Identify cell organelles

D) Measure DNA length

Answer: B

23. 30 Subjective Questions with Answers

2 Marks Q1. What is a monohybrid cross?

Answer: A monohybrid cross is a cross in which inheritance of one pair of contrasting traits is studied.

2 Marks Q2. Who was Gregor Mendel?

Answer: Gregor Mendel was a scientist who studied inheritance using garden pea plants. He is known as the Father of Genetics.

2 Marks Q3. What is the genotype of a pure tall pea plant?

Answer: TT, assuming T represents the dominant allele for tallness.

2 Marks Q4. What is the genotype of a pure dwarf pea plant?

Answer: tt, assuming t represents the recessive allele for dwarfness.

2 Marks Q5. What is the F₁ genotype in TT × tt?

Answer: All F₁ offspring are Tt.

3 Marks Q6. Why did Mendel select pea plants?

Answer: Pea plants have clear contrasting traits, short generation time, easy self-pollination and controlled cross-pollination, and produce many offspring.

3 Marks Q7. Define P, F₁ and F₂ generations.

Answer: P generation is the parental generation. F₁ is the first filial generation produced from P. F₂ is the second filial generation generally obtained by selfing/crossing F₁ individuals in Mendel's experiment.

3 Marks Q8. What happens when TT is crossed with tt?

Answer: TT produces only T gametes and tt produces only t gametes. Therefore all offspring are Tt and show the dominant tall phenotype.

3 Marks Q9. What are the gametes produced by Tt?

Answer: A Tt plant produces two types of gametes: T and t.

3 Marks Q10. State the genotypic and phenotypic ratios of Tt × Tt.

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

4 Marks Q11. Explain the F₁ generation of Mendel's monohybrid cross.

Answer: Mendel crossed pure tall TT with pure dwarf tt. TT produces T gametes and tt produces t gametes. All offspring receive T from one parent and t from the other, giving Tt. Since T is dominant, all F₁ plants are tall.

4 Marks Q12. Explain the F₂ generation of a monohybrid cross.

Answer: F₁ plants are Tt. On self-pollination, Tt × Tt gives TT, Tt, Tt and tt. Therefore the genotypic ratio is 1:2:1 and the phenotypic ratio is 3 Tall:1 Dwarf.

4 Marks Q13. What is the significance of the 3:1 ratio?

Answer: The 3:1 ratio represents the expected phenotypic ratio in the F₂ generation of a simple monohybrid cross showing complete dominance.

4 Marks Q14. What is the significance of the 1:2:1 ratio?

Answer: The 1:2:1 ratio is the expected genotypic ratio of the F₂ generation in a simple monohybrid cross.

4 Marks Q15. Explain the Law of Segregation.

Answer: The two alleles of a gene separate during gamete formation. Thus a heterozygous Tt organism forms T and t gametes.

5 Marks Q16. Draw and explain the Punnett square for Tt × Tt.

Answer: The parents produce T and t gametes.

×	T	t
T	TT	Tt
t	Tt	tt

Genotypic ratio = 1:2:1. Phenotypic ratio = 3:1.

5 Marks Q17. Explain why all F₁ plants are tall in TT × tt.

Answer: TT produces only T gametes and tt produces only t gametes. All offspring therefore have Tt genotype. Since T is dominant over t, all F₁ plants are tall.

5 Marks Q18. Explain why dwarf plants appear in F₂.

Answer: F₁ plants are Tt. During gamete formation T and t separate. When two t gametes fuse, the offspring becomes tt. This homozygous recessive genotype expresses dwarfness.

5 Marks Q19. Differentiate between genotypic and phenotypic ratios.

Answer: Genotypic ratio compares genetic combinations, such as 1 TT : 2 Tt : 1 tt. Phenotypic ratio compares visible characteristics, such as 3 Tall : 1 Dwarf.

5 Marks Q20. Explain the role of dominance in a monohybrid cross.

Answer: When T and t occur together in Tt, T is expressed and t is masked. Thus Tt plants are tall. This dominance relationship results in the 3:1 phenotypic ratio in the F₂ generation.

6 Marks Q21. Describe Mendel's complete monohybrid cross using plant height.

Answer: Mendel crossed pure tall TT with pure dwarf tt. P: TT × tt All F₁: Tt → Tall The F₁ plants were self-pollinated: Tt × Tt F₂: TT, Tt, Tt, tt Genotypic ratio: 1 TT : 2 Tt : 1 tt Phenotypic ratio: 3 Tall : 1 Dwarf.

6 Marks Q22. Explain Mendel's observation in F₁ and F₂ generations.

Answer: In F₁, only tall plants appeared because the dominant allele T masked t. In F₂, dwarf plants reappeared because the recessive alleles separated in F₁ gamete formation and two t alleles could combine. Thus F₂ showed 3 tall and 1 dwarf phenotype.

6 Marks Q23. Explain the inheritance of tallness using a Punnett square.

Answer: For Tt × Tt:

Gametes	T	t
T	TT	Tt
t	Tt	tt

TT and Tt are tall; tt is dwarf. Therefore phenotypic ratio = 3:1.

6 Marks Q24. Explain why the recessive trait is not visible in F₁ but appears in F₂.

Answer: The recessive allele is present in F₁ as Tt but is masked by T. During gamete formation the alleles segregate. In F₂, two recessive t alleles may combine to form tt. Hence the recessive phenotype becomes visible.

6 Marks Q25. Explain the difference between homozygous and heterozygous individuals in a monohybrid cross.

Answer: TT and tt contain identical alleles and are homozygous. Tt contains different alleles and is heterozygous. TT is tall, tt is dwarf and Tt is tall when T is dominant.

6 Marks Q26. Explain how the Law of Segregation is demonstrated by a monohybrid cross.

Answer: F₁ plants are Tt. During gamete formation, T and t separate. Therefore the gametes carry either T or t. Their random combination during fertilisation produces TT, Tt and tt offspring in F₂. This demonstrates segregation of alleles.

6 Marks Q27. A tall plant is crossed with a dwarf plant and all offspring are tall. What could be the genotype of the tall parent?

Answer: If all offspring are tall and the dwarf parent is tt, the tall parent could be TT in the standard monohybrid example. TT × tt → all Tt → all tall.

6 Marks Q28. A tall plant crossed with a dwarf plant produces both tall and dwarf offspring. Explain.

Answer: The tall parent must be heterozygous Tt. Tt × tt Gametes: T, t and t. Offspring: 50% Tt → Tall 50% tt → Dwarf Therefore both phenotypes occur.

6 Marks Q29. If 400 offspring are produced in Tt × Tt, how many dwarf plants are expected theoretically?

Answer: Probability of tt = 1/4. Therefore: 400 × 1/4 = 100 Approximately 100 dwarf plants are expected in the ideal theoretical ratio.

6 Marks Q30. Explain why actual offspring numbers may not exactly follow a 3:1 ratio.

Answer: The 3:1 ratio is a theoretical expectation based on probability. Actual offspring numbers can deviate due to chance, sample size and biological factors. With larger sample sizes, observed ratios may tend to approach the expected ratio more closely.

24. HOTS / Competency-Based Questions

HOTS 1. Two tall plants produce a dwarf offspring. What can you conclude about the parents?

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

HOTS 2. Why can two tall plants have different genotypes?

Answer: A tall phenotype can result from TT or Tt when T is dominant.

HOTS 3. Why is the F₂ phenotypic ratio 3:1 but genotypic ratio 1:2:1?

Answer: TT and Tt have the same dominant phenotype, so three genotypes correspond to tallness while only tt is dwarf. Hence phenotype 3:1, but genotype remains 1:2:1.

HOTS 4. If a Tt plant produces 100 gametes theoretically, what proportion would carry T and t?

Answer: Approximately 50% T and 50% t in the simple Mendelian model.

HOTS 5. A student says, “The dwarf allele disappears in F₁.” Is this correct?

Answer: No. The allele remains present in Tt plants but its expression is masked by the dominant allele.

25. Assertion–Reason

Q1. Assertion: All F₁ plants in TT × tt are tall.
Reason: All F₁ plants have genotype Tt and T is dominant.

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

Q2. Assertion: The F₂ phenotypic ratio is 3:1 in a simple monohybrid cross.
Reason: TT and Tt show the same dominant phenotype.

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

Q3. Assertion: Tt produces only T gametes.
Reason: The two alleles segregate during gamete formation.

Answer: Assertion is false, but Reason is true.

Q4. Assertion: The F₂ genotypic ratio is 1:2:1.
Reason: Tt × Tt can produce TT, Tt, Tt and tt combinations.

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

Q5. Assertion: A recessive allele is destroyed in F₁.
Reason: The recessive phenotype is masked in a heterozygous individual.

Answer: Assertion is false, but Reason is true.

26. Important Monohybrid Cross Diagram

27. CBSE Golden Points

• Monohybrid cross studies one pair of contrasting traits.
• Mendel used garden pea plants.
• TT = homozygous dominant.
• tt = homozygous recessive.
• Tt = heterozygous.
• TT × tt → all Tt.
• All F₁ plants are tall in this example.
• F₁ self-cross = Tt × Tt.
• F₂ genotypic ratio = 1:2:1.
• F₂ phenotypic ratio = 3:1.
• Tt produces T and t gametes.
• Law of Segregation explains separation of alleles.
• Recessive allele is not destroyed in F₁.
• Dominant phenotype may have TT or Tt genotype.
• Probability of tt in Tt × Tt = 25%.
• Probability of tall phenotype = 75%.

28. Quick Revision Table

Stage	Cross / Result	Important Point
P	TT × tt	Pure parents
Gametes	T and t	Alleles segregate
F₁	All Tt	All Tall
F₁ Selfing	Tt × Tt	Both T and t gametes
F₂ Genotype	TT, Tt, Tt, tt	1:2:1
F₂ Phenotype	3 Tall : 1 Dwarf	3:1

29. Memory Trick

“TT → Tt → 3:1” Trick

TT × tt ↓ All Tt ↓ Tt × Tt ↓ 1 TT : 2 Tt : 1 tt ↓ 3 Tall : 1 Dwarf

Remember: Genotype = 1:2:1
Phenotype = 3:1

30. One-Line Revision

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

🧬 Final Summary – Mendel’s Monohybrid Cross

Mendel's monohybrid cross demonstrates the inheritance of one pair of contrasting traits.

P: TT × tt
F₁: All Tt → Tall
F₂: TT, Tt, Tt, tt

Genotypic Ratio = 1 : 2 : 1
Phenotypic Ratio = 3 : 1

Law of Segregation → Alleles separate during gamete formation.

Class 10th Science Chapter 8 – Heredity | Dominant and Recessive Traits

Heredity | Dominant and Recessive Traits

Class 10 Science | Heredity | CBSE + Foundation + Competitive Level

🧬 Dominant and Recessive Traits

1. Introduction – Dominant and Recessive Traits

In organisms, a particular characteristic may be controlled by different forms of a gene. These alternative forms of a gene are called alleles.

When two different alleles are present together, one may express itself while the effect of the other is masked.

The allele that expresses its effect in the heterozygous condition is called the: Dominant allele

The allele whose expression is masked in the presence of a dominant allele is called the: Recessive allele

Example:

Let T represent the dominant allele for tallness and t represent the recessive allele for dwarfness.

TT → Tall
Tt → Tall
tt → Dwarf

2. What is a Trait?

A trait is a specific characteristic of an organism.

Examples:

• Height of a plant
• Seed colour
• Seed shape
• Flower colour
• Pod shape

A trait may have different forms. For example: Tallness and dwarfness are contrasting forms of the plant-height trait.

3. What is an Allele?

An allele is an alternative form of a gene.

For a simple Mendelian example:

T = Tall allele

t = Dwarf allele

Thus T and t are two alleles associated with the same characteristic – plant height.

4. Dominant Trait

A dominant trait is the trait that is expressed when the organism has at least one dominant allele.

For example: Tt → Tall

Here:

• T = dominant allele
• t = recessive allele
• Tt = heterozygous genotype
• Tall = phenotype

The dominant allele masks the expression of the recessive allele in this simple Mendelian model.

5. Recessive Trait

A recessive trait is generally expressed only when both alleles are recessive.

Example: tt → Dwarf

The recessive allele is not destroyed in a heterozygous organism. It is simply not expressed in the phenotype when the dominant allele is present.

Very Important:

Dominant does NOT mean “better”, “stronger”, “more common” or “more powerful”.

Recessive does NOT mean “weak” or “inferior”.

Dominance describes the pattern of expression of alleles in a heterozygote.

6. Genotype and Phenotype

Term	Meaning	Examples
Genotype	Genetic constitution of an organism	TT, Tt, tt
Phenotype	Observable characteristic	Tall, Dwarf

7. Homozygous Condition

When both alleles of a gene are identical, the organism is said to be homozygous for that gene.

Two possibilities:

TT

Homozygous dominant

tt

Homozygous recessive

8. Heterozygous Condition

When the two alleles of a gene are different, the organism is said to be heterozygous.

Example:

Tt

If T is dominant, the phenotype is Tall.

9. Comparison – Dominant vs Recessive

Feature	Dominant	Recessive
Symbol	Usually represented by capital letter	Usually represented by small letter
Expression in heterozygote	Expressed	Masked
Expression in homozygote	Expressed	Expressed
Example	T	t
Example genotype	TT or Tt	tt

10. Visual Representation

TT

→

Tall

Tt

→

Tall

tt

→

Dwarf

11. Why Does Dominant Trait Appear in Tt?

In the genotype Tt, two different alleles are present. The allele T is dominant. Therefore, the phenotype expresses the dominant characteristic.

Thus: Tt → Tall The recessive allele t remains present and can be passed to offspring.

12. Why Does Recessive Trait Reappear?

Suppose two heterozygous plants are crossed:

Tt × Tt

During gamete formation: Tt → T + t Both parents can therefore produce T and t gametes. When two t gametes combine:

t + t → tt

The recessive phenotype appears.

13. Punnett Square – Tt × Tt

♀ / ♂	T	t
T	TT Tall	Tt Tall
t	Tt Tall	tt Dwarf

Genotypic Ratio
1 TT : 2 Tt : 1 tt

Phenotypic Ratio
3 Tall : 1 Dwarf

14. Dominant Trait Does Not Mean Common Trait

A common misconception is: Dominant = common This is incorrect.

Dominance describes how an allele behaves in a heterozygous condition. It does not automatically tell us how frequent the allele is in a population.

15. Dominant Trait Does Not Mean Stronger Trait

Dominant and recessive are genetic terms. They do not mean:

• strong vs weak
• good vs bad
• healthy vs unhealthy
• common vs rare
• important vs unimportant

They describe the expression of alleles in a particular genetic context.

16. Relationship Between Gene, Allele and Trait

DNA

→

Gene

→

Alleles

→

Trait

DNA contains hereditary information.

A gene is a segment of DNA associated with a particular characteristic.

Different forms of a gene are called alleles.

The expression of genetic information contributes to the organism's observable trait, along with environmental influences where applicable.

17. Mendel’s Example of Plant Height

Mendel studied plant height using contrasting forms:

• Tall plant
• Dwarf plant

He found that when pure tall and pure dwarf plants were crossed:

TT × tt → All Tt

All F₁ plants were tall. When F₁ plants were self-pollinated:

Tt × Tt

the dwarf phenotype appeared again in F₂.

18. Important Crosses

Cross	Genotypes	Phenotype
TT × TT	All TT	All Tall
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
tt × tt	All tt	All Dwarf

19. Test Cross – Basic Concept

A cross between an individual showing a dominant phenotype and a homozygous recessive individual can help determine whether the dominant individual is homozygous or heterozygous.

For example: TT × tt → All offspring Tt → Tall.

But: Tt × tt → Tt and tt.

The appearance of recessive offspring indicates that the dominant-looking parent carried the recessive allele.

20. Key Differences – Genotype and Phenotype

Genotype	Phenotype
Genetic constitution	Observable characteristic
Written using symbols	Usually described using words
TT, Tt, tt	Tall, Dwarf
May be same for individuals with same genetic constitution	Can also be influenced by environment

21. 30 MCQs – Dominant and Recessive Traits

Q1. Which allele is expressed in a heterozygous condition in complete dominance?

A) Dominant allele

B) Recessive allele

C) Both equally

D) Neither

Answer: A

Q2. Which genotype represents homozygous dominant condition?

A) Tt

B) tt

C) TT

D) T

Answer: C

Q3. Which genotype represents heterozygous condition?

A) TT

B) tt

C) Tt

D) TTT

Answer: C

Q4. If T is dominant over t, the phenotype of Tt is:

A) Dwarf

B) Tall

C) Both

D) Cannot be determined

Answer: B

Q5. Which genotype expresses the recessive phenotype?

A) TT

B) Tt

C) tt

D) T

Answer: C

Q6. Alternative forms of a gene are called:

A) Proteins

B) Alleles

C) Enzymes

D) Tissues

Answer: B

Q7. Which represents phenotype?

A) TT

B) Tt

C) Tall

D) tt

Answer: C

Q8. Which represents genotype?

A) Tall

B) Dwarf

C) Tt

D) Height

Answer: C

Q9. In Tt × Tt, the probability of tt is:

A) 25%

B) 50%

C) 75%

D) 100%

Answer: A

Q10. In Tt × Tt, the probability of tall phenotype is:

A) 25%

B) 50%

C) 75%

D) 100%

Answer: C

Q11. The genotypic ratio of Tt × Tt is:

A) 3:1

B) 1:2:1

C) 1:1

D) 9:3:3:1

Answer: B

Q12. The phenotypic ratio of a simple monohybrid cross is:

A) 1:2:1

B) 1:1

C) 3:1

D) 2:1

Answer: C

Q13. Which is homozygous recessive?

A) TT

B) Tt

C) tt

D) T

Answer: C

Q14. Dominance refers to:

A) Strength of an organism

B) Expression of one allele over another in a heterozygote

C) Frequency of an allele

D) Size of a gene

Answer: B

Q15. Which statement is correct?

A) Recessive allele is destroyed

B) Recessive allele is always absent in heterozygotes

C) Recessive allele can remain present but masked

D) Dominant allele is always more common

Answer: C

Q16. A tall plant having genotype Tt is:

A) Homozygous

B) Heterozygous

C) Haploid

D) Recessive

Answer: B

Q17. A dwarf plant with genotype tt produces which type of allele in its gametes?

A) Only T

B) Only t

C) T and t

D) Neither

Answer: B

Q18. A Tt individual produces:

A) Only T gametes

B) Only t gametes

C) T and t gametes

D) TT gametes

Answer: C

Q19. Which genotype can produce only dominant T gametes?

A) TT

B) Tt

C) tt

D) Tt and tt

Answer: A

Q20. In TT × tt, all offspring are:

A) TT

B) tt

C) Tt

D) TTT

Answer: C

Q21. If T is dominant, which genotype can have the same phenotype as TT?

A) tt

B) Tt

C) Only tt

D) None

Answer: B

Q22. Which cross gives a 1:1 tall:dwarf ratio?

A) TT × TT

B) TT × tt

C) Tt × tt

D) Tt × Tt

Answer: C

Q23. If a dominant-looking plant produces dwarf offspring when crossed with tt, the plant is likely:

A) TT

B) Tt

C) tt

D) TTT

Answer: B

Q24. Which statement about a dominant trait is correct?

A) It is always more common

B) It is always stronger

C) It can be expressed in a heterozygote

D) It cannot occur in homozygous condition

Answer: C

Q25. Which is NOT a correct meaning of recessive?

A) Its expression may be masked in a heterozygote

B) It may be expressed in homozygous condition

C) It is genetically weaker

D) It can be inherited

Answer: C

Q26. Which genotype has two identical alleles?

A) Tt

B) TT

C) Both TT and tt

D) Neither

Answer: C

Q27. The observable expression of a genotype is called:

A) Allele

B) Phenotype

C) Genotype

D) Chromosome

Answer: B

Q28. Which combination is correct?

A) TT – heterozygous

B) Tt – homozygous

C) tt – homozygous recessive

D) Tt – homozygous recessive

Answer: C

Q29. In a heterozygous individual, the recessive allele:

A) Is destroyed

B) Is absent

C) Remains present but may be masked

D) Changes into a dominant allele

Answer: C

Q30. Which ratio represents the expected phenotype in Tt × Tt?

A) 1 Tall : 1 Dwarf

B) 2 Tall : 2 Dwarf

C) 3 Tall : 1 Dwarf

D) 1 Tall : 3 Dwarf

Answer: C

22. 30 Subjective Questions with Answers

2 Marks Q1. What is a dominant trait?

Answer: A dominant trait is expressed when at least one dominant allele is present in a heterozygous condition.

2 Marks Q2. What is a recessive trait?

Answer: A recessive trait is generally expressed when both alleles are recessive.

2 Marks Q3. Define homozygous condition.

Answer: A condition in which both alleles of a gene are identical is called homozygous condition, e.g. TT or tt.

2 Marks Q4. Define heterozygous condition.

Answer: A condition in which the two alleles of a gene are different is called heterozygous condition, e.g. Tt.

2 Marks Q5. What is an allele?

Answer: An allele is an alternative form of a gene.

3 Marks Q6. Differentiate between dominant and recessive traits.

Answer: Dominant trait can be expressed in heterozygous condition, while recessive trait is generally expressed in homozygous recessive condition. For example, Tt is tall and tt is dwarf when T is dominant.

3 Marks Q7. Differentiate between genotype and phenotype.

Answer: Genotype is the genetic constitution, such as TT, Tt or tt. Phenotype is the observable characteristic, such as tall or dwarf.

3 Marks Q8. Explain TT, Tt and tt.

Answer: TT is homozygous dominant, Tt is heterozygous and tt is homozygous recessive. If T represents tallness, TT and Tt are tall while tt is dwarf.

3 Marks Q9. Why does a recessive trait reappear in F₂ generation?

Answer: The recessive allele remains present in F₁ heterozygotes. During gamete formation the alleles segregate, and two recessive alleles may combine to form tt in F₂.

3 Marks Q10. Does dominant mean stronger or better? Explain.

Answer: No. Dominance describes the expression of one allele in a heterozygous condition. It does not mean stronger, better, healthier or more common.

4 Marks Q11. Explain the expression of dominant and recessive alleles using Tt.

Answer: Tt contains one dominant allele T and one recessive allele t. The dominant allele T is expressed, so the plant is tall. The recessive allele remains present but its effect is masked.

4 Marks Q12. Explain homozygous dominant and homozygous recessive conditions.

Answer: TT is homozygous dominant because both alleles are dominant. tt is homozygous recessive because both alleles are recessive.

4 Marks Q13. What is the difference between Tt and tt?

Answer: Tt is heterozygous and has one dominant and one recessive allele. tt is homozygous recessive and has two recessive alleles. Therefore Tt is tall while tt is dwarf if T is dominant.

4 Marks Q14. Explain why TT and Tt can have the same phenotype.

Answer: TT and Tt both contain at least one dominant T allele. Therefore, when T is completely dominant over t, both genotypes produce the same tall phenotype.

4 Marks Q15. Explain the relationship between gene, allele and trait.

Answer: A gene is a segment of DNA associated with hereditary information. Alternative forms of a gene are alleles. Different alleles can contribute to different forms of a characteristic or trait.

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

Answer: Each Tt parent produces T and t gametes.

×	T	t
T	TT	Tt
t	Tt	tt

Genotypic ratio = 1:2:1. Phenotypic ratio = 3:1.

5 Marks Q17. Why is the recessive allele not destroyed in a heterozygous organism?

Answer: In Tt, both T and t alleles are present. T masks the expression of t, but t remains part of the genetic constitution and can pass to gametes.

5 Marks Q18. Explain the significance of dominant and recessive alleles in Mendelian inheritance.

Answer: Dominant and recessive relationships help explain why certain phenotypes appear in heterozygous organisms and why recessive phenotypes can reappear in later generations.

5 Marks Q19. Explain why phenotype alone may not reveal genotype.

Answer: A dominant phenotype may result from either TT or Tt. Therefore, observing a tall plant does not always tell us whether its genotype is TT or Tt.

5 Marks Q20. Explain the statement: “Dominant does not mean common.”

Answer: Dominance describes allele expression in a heterozygote. It does not indicate the frequency of the allele in a population. A dominant allele may be less common than a recessive allele.

6 Marks Q21. Explain dominant and recessive traits with suitable examples.

Answer: A dominant trait is expressed in a heterozygous condition. A recessive trait is generally expressed when both alleles are recessive. For example, if T is dominant for tallness and t is recessive: TT → Tall Tt → Tall tt → Dwarf Thus T is dominant and t is recessive in this model.

6 Marks Q22. Explain how a recessive trait can remain hidden for one generation and appear in the next.

Answer: A recessive allele may be present in a heterozygous individual but its expression is masked by the dominant allele. When two heterozygous individuals reproduce, their recessive alleles can combine to form a homozygous recessive offspring. Thus the recessive phenotype can reappear.

6 Marks Q23. Explain the genotypic and phenotypic ratios of Tt × Tt.

Answer: The offspring are TT, Tt, Tt and tt. Genotypic ratio: 1 TT : 2 Tt : 1 tt Phenotypic ratio: 3 Tall : 1 Dwarf This occurs because TT and Tt are tall while tt is dwarf.

6 Marks Q24. Explain why two tall plants can produce a dwarf plant.

Answer: Two tall plants may both be heterozygous Tt. When crossed: Tt × Tt one possible offspring is tt. Since tt is homozygous recessive, the dwarf phenotype appears.

6 Marks Q25. Explain homozygous and heterozygous conditions with examples.

Answer: Homozygous means identical alleles, such as TT or tt. Heterozygous means different alleles, such as Tt. TT is homozygous dominant, tt is homozygous recessive and Tt is heterozygous.

6 Marks Q26. A tall plant is crossed with a dwarf plant and all offspring are tall. Explain.

Answer: If the tall parent is TT and dwarf parent is tt: TT × tt → all Tt. Since T is dominant, all offspring are tall. The recessive allele t is present but masked.

6 Marks Q27. A tall plant is crossed with tt and half the offspring are dwarf. What is the genotype of the tall parent?

Answer: The tall parent must be Tt. Tt × tt produces: 50% Tt → Tall 50% tt → Dwarf Therefore the tall parent is heterozygous.

6 Marks Q28. Explain why a dominant phenotype can have two different genotypes.

Answer: When complete dominance occurs, both TT and Tt express the dominant phenotype. Therefore a tall phenotype can have genotype TT or Tt.

6 Marks Q29. Explain the role of segregation in the inheritance of dominant and recessive traits.

Answer: In a heterozygous organism such as Tt, the two alleles separate during gamete formation. Thus some gametes receive T and others receive t. During fertilisation, different combinations can form, including tt, allowing the recessive phenotype to reappear.

6 Marks Q30. Explain how Mendel's observations helped distinguish dominant and recessive traits.

Answer: Mendel crossed pure-breeding contrasting pea plants. In the F₁ generation only one phenotype appeared. When F₁ plants were self-pollinated, the masked phenotype reappeared in F₂. This showed that one hereditary factor could mask another in the heterozygous condition and established the basis for the concepts of dominance and recessiveness.

23. HOTS / Competency-Based Questions

HOTS 1. A plant is tall. Can you always say that its genotype is TT?

Answer: No. Its genotype could be TT or Tt if T is dominant.

HOTS 2. Two tall plants produce one dwarf offspring. What does this tell you?

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

HOTS 3. Why does the recessive phenotype not appear in the F₁ generation of TT × tt?

Answer: All offspring are Tt, and the dominant T masks the recessive t.

HOTS 4. If 200 offspring result from Tt × Tt, approximately how many are expected to show the recessive phenotype?

Answer: Approximately 50, because the expected probability of tt is 1/4.

HOTS 5. A student says, “A recessive allele disappears in F₁.” Is this correct?

Answer: No. The recessive allele remains present in heterozygous offspring but its expression is masked.

24. Assertion–Reason

Q1. Assertion: Tt is a heterozygous genotype.
Reason: T and t are different alleles.

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

Q2. Assertion: tt expresses the recessive phenotype.
Reason: Both alleles in tt are recessive.

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

Q3. Assertion: A dominant trait is always more common in a population.
Reason: Dominance means an allele is expressed in a heterozygote.

Answer: Assertion is false, but Reason is true.

Q4. Assertion: TT and Tt can have the same phenotype.
Reason: Both contain at least one dominant allele.

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

Q5. Assertion: The recessive allele is destroyed in Tt.
Reason: The recessive allele is not expressed in the heterozygous condition under complete dominance.

Answer: Assertion is false, but Reason is true.

25. Important Diagram – Dominance

26. CBSE Golden Points

• Dominant and recessive describe the expression of alleles.
• Dominant allele can express in a heterozygous condition.
• Recessive phenotype is generally expressed in homozygous recessive condition.
• TT = homozygous dominant.
• Tt = heterozygous.
• tt = homozygous recessive.
• TT and Tt can have the same phenotype under complete dominance.
• A recessive allele is not destroyed when masked.
• Dominant does not mean stronger.
• Dominant does not necessarily mean common.
• Genotype = genetic constitution.
• Phenotype = observable characteristic.
• Tt × Tt gives 1:2:1 genotypic ratio.
• Tt × Tt gives 3:1 phenotypic ratio.
• Alleles segregate during gamete formation.

27. Quick Revision Table

Concept	Remember
Dominant	Expressed in heterozygote
Recessive	Usually expressed in homozygous condition
Homozygous	Same alleles
Heterozygous	Different alleles
Genotype	TT, Tt, tt
Phenotype	Tall, Dwarf
Monohybrid F₂ Genotype	1:2:1
Monohybrid F₂ Phenotype	3:1

28. Memory Trick

“D-H-R” Trick

D → Dominant
Expressed in heterozygous condition.

H → Heterozygous
Different alleles → Tt.

R → Recessive
Usually expressed when two recessive alleles combine → tt.

29. One-Line Revision

Dominant = Expressed in Heterozygote

Recessive = Masked in Heterozygote

TT / Tt → Dominant Phenotype
tt → Recessive Phenotype

🧬 Final Summary – Dominant and Recessive Traits

Dominant and recessive traits explain how different alleles of a gene are expressed.

TT → Tall
Tt → Tall
tt → Dwarf

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

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