Class 12 Biology – Principles of Inheritance and Variation Notes

1. Mendelian Principles of Inheritance

• Gregor Mendel: Father of genetics; worked on pea plants.
• Mendel’s Laws:
  1. Law of Dominance – Dominant trait appears in F1; recessive trait hidden.
  2. Law of Segregation – Paired alleles separate during gamete formation.
  3. Law of Independent Assortment – Alleles of different genes segregate independently.
• Monohybrid cross: One trait (e.g., seed shape) → 3:1 ratio in F2.
• Dihybrid cross: Two traits (e.g., seed shape & color) → 9:3:3:1 ratio in F2.

---

2. Chromosomal Theory of Inheritance

• Genes are located on chromosomes.
• Alleles – alternative forms of a gene.
• Homologous chromosomes – carry the same gene loci.
• Sex determination:
  • Male: XY, Female: XX in humans.
  • Male determines sex of offspring.

---

3. Extensions of Mendelian Genetics

• Incomplete dominance: F1 shows intermediate phenotype (e.g., red × white → pink).
• Codominance: Both alleles expressed equally (e.g., AB blood group).
• Multiple alleles: More than 2 alleles for a gene (e.g., human blood groups – A, B, O).
• Pleiotropy: One gene affects multiple traits (e.g., sickle cell anemia).
• Polygenic inheritance: Many genes influence a trait (e.g., height, skin color).

---

4. Mutation

• Definition: Sudden, heritable changes in DNA.
• Types:
  • Gene mutation – change in a single gene.
  • Chromosomal mutation – structural or numerical changes.
• Importance: Source of genetic variation, evolution.

---

5. Evolution

• Definition: Gradual change in heritable traits in populations over generations.
• Mechanisms:
  • Variation – caused by mutation, recombination, sexual reproduction.
  • Natural selection – favorable traits survive (Darwin).
  • Speciation – formation of new species.
• Evidence for evolution: Fossils, comparative anatomy, embryology, molecular biology.

Principles of Inheritance and Variation | MCQs

1. Mendelian Genetics

1. Who is known as the Father of Genetics?
   a) Charles Darwin
   b) Gregor Mendel
   c) Watson and Crick
   d) Lamarck
   Answer: b) Gregor Mendel
2. The Law of Segregation states that:
   a) Genes are independent of each other
   b) Alleles of a gene separate during gamete formation
   c) Dominant allele always masks recessive
   d) None of the above
   Answer: b) Alleles of a gene separate during gamete formation
3. In a monohybrid cross of two heterozygotes (Aa × Aa), the phenotypic ratio in F2 is:
   a) 1:1
   b) 3:1
   c) 9:3:3:1
   d) 2:1
   Answer: b) 3:1
4. In a dihybrid cross (RrYy × RrYy), the F2 phenotypic ratio is:
   a) 3:1
   b) 9:3:3:1
   c) 1:1:1:1
   d) 2:1:1
   Answer: b) 9:3:3:1
5. The Law of Independent Assortment is valid for:
   a) Monohybrid crosses only
   b) Dihybrid crosses only
   c) All polygenic traits
   d) Linked genes
   Answer: b) Dihybrid crosses only

---

2. Chromosomal Basis of Inheritance

6. In humans, sex is determined by:
   a) X chromosome only
   b) Y chromosome only
   c) Male (XY) determining sex
   d) Female (XX) determining sex
   Answer: c) Male (XY) determining sex
7. The gene controlling a trait is located on:
   a) Ribosome
   b) Chromosome
   c) Cytoplasm
   d) Plasma membrane
   Answer: b) Chromosome
8. Hemophilia and color blindness are examples of:
   a) Autosomal recessive traits
   b) Sex-linked recessive traits
   c) Codominant traits
   d) Polygenic traits
   Answer: b) Sex-linked recessive traits

---

3. Extensions of Mendelian Genetics

9. In incomplete dominance, the F1 shows:
   a) Only dominant phenotype
   b) Only recessive phenotype
   c) Intermediate phenotype
   d) Both dominant and recessive phenotypes
   Answer: c) Intermediate phenotype
10. AB blood group is an example of:
    a) Incomplete dominance
    b) Codominance
    c) Polygenic inheritance
    d) Multiple alleles
    Answer: b) Codominance
11. Multiple alleles refer to:
    a) More than two alleles of a gene
    b) Only two alleles per gene
    c) Gene affecting multiple traits
    d) Interaction between two genes
    Answer: a) More than two alleles of a gene
12. Pleiotropy occurs when:
    a) One gene affects multiple traits
    b) Multiple genes affect one trait
    c) Two genes interact
    d) Alleles show incomplete dominance
    Answer: a) One gene affects multiple traits
13. Height in humans is an example of:
    a) Codominance
    b) Polygenic inheritance
    c) Multiple alleles
    d) Pleiotropy
    Answer: b) Polygenic inheritance

---

4. Mutation

14. A sudden heritable change in DNA is called:
    a) Recombination
    b) Variation
    c) Mutation
    d) Crossing over
    Answer: c) Mutation
15. Sickle cell anemia is caused due to:
    a) Chromosomal mutation
    b) Gene mutation
    c) Polygenic inheritance
    d) Codominance
    Answer: b) Gene mutation
16. Mutations are important because they:
    a) Cause diseases only
    b) Are source of genetic variation
    c) Prevent evolution
    d) Affect only sex chromosomes
    Answer: b) Are source of genetic variation

---

5. Evolution

17. Evolution is:
    a) Sudden change in a single organism
    b) Gradual change in heritable traits in a population over generations
    c) Adaptation of individuals
    d) None of the above
    Answer: b) Gradual change in heritable traits in a population over generations
18. Which is an example of evidence for evolution?
    a) Fossils
    b) Comparative anatomy
    c) Molecular biology
    d) All of the above
    Answer: d) All of the above
19. Darwin’s theory of natural selection states:
    a) Only the strongest survive
    b) Favorable traits increase survival and reproduction
    c) Evolution occurs by chance
    d) Traits acquired during life are inherited
    Answer: b) Favorable traits increase survival and reproduction
20. Speciation refers to:
    a) Formation of new traits
    b) Formation of new species
    c) Extinction of species
    d) Adaptation
    Answer: b) Formation of new species