CBSE Class 10 - Heredity Notes & MCQ

Heredity

1. Key Definitions

• Heredity: Transmission of characters/traits from parents to offspring.
• Variation: Differences among individuals of same species.
• Gene: Functional unit of heredity; segment of DNA that controls one character.
• Allele: Different forms of same gene (e.g., tall-T / short-t).
• Dominant trait: Expresses itself even in heterozygous condition.
• Recessive trait: Expresses only in homozygous condition.
• Genotype: Genetic constitution (TT, Tt, tt).
• Phenotype: Visible expression / physical appearance.
• Evolution: Gradual change in inherited characteristics over generations leading to formation of new species.

2. Mendel's Experiments (Father of Genetics)

Mendel performed experiments on garden pea (Pisum sativum) and gave laws of inheritance.

Monohybrid Cross (One trait)

Cross between tall (TT) and dwarf (tt) plants → F₁ generation: All tall (Tt)

Self-pollination of F₁ → F₂ generation: 3 Tall : 1 Dwarf (Phenotypic ratio 3:1)

Genotypic ratio: 1 TT : 2 Tt : 1 tt

Dihybrid Cross (Two traits)

Round-Yellow (RRYY) × Wrinkled-Green (rryy)

F₁ → All Round-Yellow (RrYy)

F₂ → 9 : 3 : 3 : 1 ratio (Round-Yellow : Round-Green : Wrinkled-Yellow : Wrinkled-Green)

Mendel's Laws

1. Law of Dominance: In F₁ hybrid, only dominant trait expresses.
2. Law of Segregation: Alleles separate during gamete formation (each gamete gets one allele).
3. Law of Independent Assortment: Inheritance of one trait is independent of another trait.

3. Sex Determination in Humans

Parent	Sex Chromosomes	Gametes
Male	XY	X or Y
Female	XX	X only

Child → XX = Girl, XY = Boy → 50% chance of either gender (Father determines sex of child).

4. Important Diagrams (Description)

1. Creation of Diversity (Figure usually shown in NCERT): Shows how one organism produces two slightly different offspring → each of them produces two more → finally 4 very different individuals (illustrates accumulation of variations).

2. Monohybrid Cross Punnett Square: 4 boxes showing Tt × Tt → TT, Tt, Tt, tt.

3. Dihybrid Cross Punnett Square: 16 boxes showing 9:3:3:1 ratio.

4. Homologous chromosomes & genes: Pair of chromosomes with same genes at same position (alleles may be same or different).

5. Evolution – Important Concepts

• Variation + Natural Selection → Basis of evolution.
• Speciation: Formation of new species due to accumulation of variations over long time + geographical isolation.
• Darwin's Theory: Natural Selection / Survival of the Fittest.
• Evidence of Evolution:
  • Fossils (e.g., Archaeopteryx – connecting link)
  • Homologous organs (same origin, different function) → e.g., Forelimbs of human, bat, whale
  • Analogous organs (different origin, same function) → e.g., Wings of bat & bird
  • Vestigial organs (e.g., Appendix, wisdom teeth in humans)

6. Important One-liners / Quick Revision

• Gene is the unit of heredity.
• Father of Genetics → Gregor Johann Mendel.
• Sex of child is determined by father.
• Acquired characters are not inherited (e.g., cut tail of mice → not passed).
• Most accepted theory of evolution → Darwin's theory of Natural Selection.
• Human beings have 23 pairs of chromosomes (46 total).
• Fossils tell us about evolution by showing intermediate forms.

MCQs: Heredity

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

a) Evolution
b) Variation
c) Heredity
d) Adaptation

Answer: c) Heredity

2. The study of heredity and variations is known as:

a) Ecology
b) Genetics
c) Evolution
d) Physiology

Answer: b) Genetics

3. Who is known as the Father of Genetics?

a) Darwin
b) Mendel
c) Lamarck
d) Morgan

Answer: b) Mendel

4. The alternative forms of a gene are called:

a) Chromosomes
b) Traits
c) Alleles
d) Gametes

Answer: c) Alleles

5. The physical appearance of an organism is called:

a) Genotype
b) Phenotype
c) Allele
d) Gene

Answer: b) Phenotype

6. A pair of alleles controlling the same character is located on:

a) Same chromosome
b) Different chromosomes
c) Ribosome
d) Cytoplasm

Answer: a) Same chromosome

7. When a trait is controlled by both alleles equally, it is called:

a) Dominance
b) Recessiveness
c) Co-dominance
d) Mutation

Answer: c) Co-dominance

8. Which of the following is a dominant trait in pea plants?

a) Wrinkled seeds
b) Green seeds
c) Tall plant
d) Constricted pod

Answer: c) Tall plant

9. A cross between two individuals differing in one pair of contrasting characters is called:

a) Dihybrid cross
b) Back cross
c) Monohybrid cross
d) Test cross

Answer: c) Monohybrid cross

10. The ratio obtained in F₂ generation of a monohybrid cross is:

a) 1:2:1
b) 2:1
c) 3:1
d) 9:3:3:1

Answer: c) 3:1

11. The genotypic ratio of a monohybrid cross is:

a) 3:1
b) 1:2:1
c) 9:3:3:1
d) 2:1

Answer: b) 1:2:1

12. The chromosomal basis of inheritance was given by:

a) Mendel
b) Darwin
c) Sutton and Boveri
d) Watson and Crick

Answer: c) Sutton and Boveri

13. Genes are located on:

a) Ribosomes
b) Mitochondria
c) Chromosomes
d) Cytoplasm

Answer: c) Chromosomes

14. Sex of a human child is determined by:

a) Mother
b) Father
c) Both parents equally
d) Environment

Answer: b) Father

15. Which chromosome combination represents a human female?

a) XY
b) YY
c) XX
d) XO

Answer: c) XX

16. Variation is important because it:

a) Causes diseases
b) Leads to extinction
c) Helps in evolution
d) Reduces survival

Answer: c) Helps in evolution

17. The trait which remains hidden in the presence of a dominant trait is called:

a) Dominant
b) Recessive
c) Co-dominant
d) Acquired

Answer: b) Recessive

18. Which of the following is an inherited trait?

a) Muscular body
b) Scars on skin
c) Blood group
d) Learned skills

Answer: c) Blood group

19. The passing of genetic information from one generation to the next is due to:

a) DNA
b) Proteins
c) Enzymes
d) Hormones

Answer: a) DNA

20. Which law of Mendel explains separation of alleles during gamete formation?

a) Law of Dominance
b) Law of Independent Assortment
c) Law of Segregation
d) Law of Evolution

Answer: c) Law of Segregation