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