Chapter 5: Principles of Inheritance and Variation – Short Answer Type Questions

CBSE Class 12 Biology Short Answer Questions (NCERT): Principles of Inheritance and Variation

Course & Examination Details

Course: CBSE Class 12 Biology
Unit: Unit II – Genetics and Evolution
Chapter: Chapter 5 – Principles of Inheritance and Variation
Prescribed Textbook: NCERT Biology Class XII
Examination: CBSE Class 12 Board Examination
Question Type: Short Answer Type
Answer Length: 60–80 words each

Section A: Mendel’s Laws of Inheritance

Q1. Explain why Gregor Mendel chose pea plants for his experiments.

Answer: Mendel selected pea plants because they showed several clear contrasting traits, had a short life cycle, and were easy to cultivate. Pea flowers are bisexual, allowing self-pollination as well as controlled cross-pollination. Large sample size and true-breeding varieties helped Mendel obtain reliable statistical results, leading to the formulation of inheritance laws.

Q2. Describe the Law of Dominance with an example.

Answer: The Law of Dominance states that in a heterozygous condition, one allele expresses itself and masks the other. For example, in pea plants, tallness (T) is dominant over dwarfness (t). When a tall plant is crossed with a dwarf plant, all F₁ offspring are tall, though they carry both alleles.

Q3. Explain the Law of Segregation.

Answer: The Law of Segregation states that the two alleles of a gene separate from each other during gamete formation. As a result, each gamete carries only one allele. This law ensures that alleles do not blend and retain their individual identity across generations, forming the basis of Mendelian inheritance.

Q4. Why is the Law of Segregation also called the Law of Purity of Gametes?

Answer: It is called the Law of Purity of Gametes because alleles separate during meiosis and gametes receive only one allele for each trait. There is no mixing or blending of alleles, ensuring genetic purity of gametes and stable inheritance patterns across generations.

Q5. Explain the Law of Independent Assortment.

Answer: The Law of Independent Assortment states that alleles of different gene pairs segregate independently during gamete formation. This means inheritance of one trait does not affect another, provided the genes are located on different chromosomes or far apart on the same chromosome.

Section B: Inheritance of One Gene (Monohybrid Cross)

Q6. Describe a monohybrid cross and its significance.

Answer: A monohybrid cross involves a single pair of contrasting traits. In pea plants, a cross between tall and dwarf plants results in all tall plants in F₁ and a 3:1 ratio in F₂. This cross demonstrates the Law of Dominance and Law of Segregation clearly.

Q7. Explain the genotypic and phenotypic ratios obtained in a monohybrid cross.

Answer: In the F₂ generation of a monohybrid cross, the genotypic ratio is 1 TT : 2 Tt : 1 tt, while the phenotypic ratio is 3 Tall : 1 Dwarf. This difference arises because both TT and Tt show the dominant phenotype.

Q8. What is meant by homozygous and heterozygous conditions?

Answer: Homozygous condition refers to the presence of identical alleles of a gene, such as TT or tt. Heterozygous condition refers to the presence of two different alleles, such as Tt, where one allele may be dominant over the other.

Q9. Why does the F₁ generation show uniformity in a monohybrid cross?

Answer: The F₁ generation shows uniformity because all offspring receive one dominant allele from one parent and one recessive allele from the other. Due to dominance, only the dominant trait is expressed, resulting in phenotypic uniformity.

Q10. Define test cross and state its importance.

Answer: A test cross is a cross between an individual showing a dominant phenotype and a homozygous recessive individual. It helps determine the genotype of the dominant individual and is useful in plant breeding and genetic analysis.

Section C: Inheritance of Two Genes (Dihybrid Cross)

Q11. Explain a dihybrid cross with its phenotypic ratio.

Answer: A dihybrid cross studies the inheritance of two pairs of contrasting traits. In pea plants, seed shape and seed colour produce a phenotypic ratio of 9:3:3:1 in the F₂ generation, demonstrating independent assortment of gene pairs.

Q12. Why does a dihybrid cross produce more variations than a monohybrid cross?

Answer: A dihybrid cross involves two gene pairs, resulting in four types of gametes instead of two. Independent assortment of alleles increases the number of possible combinations, leading to greater phenotypic variation among offspring.

Q13. Under what conditions does the Law of Independent Assortment fail?

Answer: The Law of Independent Assortment fails when genes are located close together on the same chromosome. Such genes are linked and tend to be inherited together, reducing the expected Mendelian ratio.

Q14. What is a Punnett square and why is it used?

Answer: A Punnett square is a graphical tool used to predict the possible genotypes and phenotypes of offspring in a genetic cross. It helps in understanding inheritance patterns and calculating expected ratios.

Q15. Explain why Mendelian ratios are not always observed in nature.

Answer: Mendelian ratios are altered due to factors like incomplete dominance, codominance, linkage, multiple alleles, lethal genes, and environmental influences. These deviations show that inheritance patterns can be more complex than simple Mendelian laws.

Section D: Deviations from Mendelian Inheritance

Q16. Explain incomplete dominance with an example.

Answer: In incomplete dominance, neither allele is completely dominant, and the heterozygote shows an intermediate phenotype. For example, in Mirabilis jalapa, red and white flowers produce pink flowers in the F₁ generation, with a 1:2:1 phenotypic ratio.

Q17. How is codominance different from incomplete dominance?

Answer: In codominance, both alleles express themselves fully in the heterozygote, while in incomplete dominance, an intermediate phenotype appears. ABO blood group shows codominance, whereas flower colour in Mirabilis jalapa shows incomplete dominance.

Q18. Explain the ABO blood group system.

Answer: The ABO blood group system is controlled by three alleles—IA, IB, and i. IA and IB are codominant, while i is recessive. Different combinations of these alleles produce four blood groups: A, B, AB, and O.

Q19. Why does incomplete dominance produce a 1:2:1 phenotypic ratio?

Answer: In incomplete dominance, the heterozygous genotype has a distinct phenotype. Since each genotype produces a unique phenotype, the phenotypic ratio becomes identical to the genotypic ratio of 1:2:1.

Q20. State the importance of studying non-Mendelian inheritance.

Answer: Non-Mendelian inheritance explains variations that cannot be accounted for by Mendel’s laws. It helps understand complex genetic interactions, human blood groups, genetic disorders, and traits influenced by multiple factors.

Section E: Multiple Alleles and Pleiotropy

Q21. Explain the concept of multiple alleles.

Answer: Multiple alleles occur when more than two allelic forms of a gene exist in a population. However, an individual carries only two alleles. The ABO blood group system is a classic example involving three alleles.

Q22. Why are multiple alleles common at the population level?

Answer: Multiple alleles arise due to mutations over long evolutionary periods. These variations persist in populations and increase genetic diversity, although only two alleles are inherited by an individual.

Q23. What is pleiotropy? Explain with an example.

Answer: Pleiotropy is the phenomenon where a single gene influences multiple traits. Phenylketonuria is an example, where a single gene mutation affects brain development, skin colour, and metabolism.

Q24. Why does pleiotropy complicate genetic analysis?

Answer: Pleiotropy complicates genetic analysis because a single gene affects multiple traits, making it difficult to associate a particular phenotype with a specific genetic change.

Q25. Distinguish between multiple alleles and pleiotropy.

Answer: Multiple alleles involve more than two forms of a gene affecting one trait, while pleiotropy involves one gene affecting multiple traits. Both increase genetic complexity.

Section F: Chromosomal Theory, Linkage and Recombination

Q26. Explain the chromosomal theory of inheritance.

Answer: The chromosomal theory states that genes are located on chromosomes and their behavior during meiosis explains Mendel’s laws. Segregation and independent assortment correspond to chromosomal separation and alignment.

Q27. How does linkage affect inheritance patterns?

Answer: Linkage causes genes located close together on the same chromosome to be inherited together. This reduces recombination frequency and alters expected Mendelian ratios.

Q28. Differentiate between complete and incomplete linkage.

Answer: In complete linkage, genes are inherited together without recombination. In incomplete linkage, crossing over occurs, producing some recombinant offspring.

Q29. Explain recombination and its genetic significance.

Answer: Recombination occurs due to crossing over during meiosis. It creates new gene combinations, increasing genetic variation and playing a crucial role in evolution.

Q30. Why is recombination frequency used in gene mapping?

Answer: Recombination frequency reflects the distance between genes on a chromosome. Lower frequency indicates closer proximity, helping construct genetic maps.

Section G: Sex Determination Mechanisms

Q31. Explain sex determination in humans.

Answer: Humans follow the XX–XY sex determination system. Females are XX and males are XY. The sex of the child depends on whether an X-bearing or Y-bearing sperm fertilizes the ovum.

Q32. Why are males called heterogametic in humans?

Answer: Males are heterogametic because they produce two types of gametes—X-bearing and Y-bearing—while females produce only X-bearing gametes.

Q33. Describe the XX–XO sex determination mechanism.

Answer: In the XX–XO mechanism, females have two X chromosomes, while males have only one X chromosome. The absence of the second sex chromosome determines maleness.

Q34. Explain sex determination in birds.

Answer: Birds show the ZW–ZZ mechanism. Females are heterogametic with ZW chromosomes, while males are homogametic with ZZ chromosomes.

Q35. What is haplodiploid sex determination?

Answer: In haplodiploidy, males develop from unfertilised haploid eggs and females from fertilised diploid eggs, as seen in honeybees.

Section H: Mutation

Q36. Define mutation and its significance.

Answer: Mutation is a sudden heritable change in DNA or chromosome structure. It creates genetic variation, which is essential for evolution and adaptation.

Q37. Explain gene mutation with an example.

Answer: Gene mutation involves changes in nucleotide sequence of a gene. Sickle cell anaemia is caused by a point mutation affecting haemoglobin structure.

Q38. What are chromosomal mutations?

Answer: Chromosomal mutations involve changes in chromosome number or structure, such as deletions, duplications, inversions, and nondisjunction.

Q39. Name the agents that cause mutations.

Answer: Mutations are caused by physical agents like radiation, chemical mutagens, and biological agents, or errors during DNA replication.

Q40. Why are mutations considered random events?

Answer: Mutations occur spontaneously without specific direction or purpose and are not influenced by the needs of the organism.

Section I: Genetic Disorders

Q41. Explain Mendelian disorders with an example.

Answer: Mendelian disorders result from mutations in single genes and follow Mendelian inheritance patterns. Haemophilia is an X-linked recessive disorder affecting blood clotting.

Q42. Describe sickle cell anaemia.

Answer: Sickle cell anaemia is an autosomal recessive disorder caused by a mutation in haemoglobin gene, leading to sickle-shaped red blood cells and reduced oxygen transport.

Q43. What are chromosomal disorders?

Answer: Chromosomal disorders occur due to abnormal number or structure of chromosomes, often caused by nondisjunction during meiosis.

Q44. Explain Down’s syndrome.

Answer: Down’s syndrome is caused by trisomy of chromosome 21. Affected individuals show intellectual disability, short stature, and characteristic facial features.

Q45. Describe Turner’s syndrome.

Answer: Turner’s syndrome is caused by monosomy X in females. Affected individuals have underdeveloped ovaries, short stature, and absence of secondary sexual characteristics.

Section J: Application-Based Concepts

Q46. Explain Klinefelter’s syndrome.

Answer: Klinefelter’s syndrome is caused by an extra X chromosome in males (XXY). Affected individuals show underdeveloped testes, infertility, and feminine characteristics.

Q47. What is nondisjunction?

Answer: Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate during meiosis, leading to abnormal chromosome numbers.

Q48. Why is genetic counselling important?

Answer: Genetic counselling helps identify inherited disorders, assess risk, and guide families in prevention, diagnosis, and management of genetic diseases.

Q49. How does genetics help in disease prevention?

Answer: Genetics enables early detection of hereditary diseases, carrier identification, prenatal diagnosis, and informed medical decisions to reduce disease incidence.

Q50. Why is the study of inheritance important in biology?

Answer: Understanding inheritance explains trait transmission, variation, evolution, and genetic disorders, forming the foundation of medicine, agriculture, and biotechnology.