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