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

CBSE Class 12 Biology Very 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: Very Short Answer Type (VSA)
Answer Length: 20–30 words each

Section A: Mendel’s Laws of Inheritance

Q1. Who is known as the father of genetics?

Answer: Gregor Mendel is known as the father of genetics for discovering the fundamental laws of inheritance through pea plant experiments.

Q2. What is meant by a gene?

Answer: A gene is the functional unit of inheritance located on chromosomes, responsible for controlling a specific trait in an organism.

Q3. Define allele.

Answer: Alleles are alternative forms of the same gene that occupy the same position on homologous chromosomes and control contrasting expressions of a trait.

Q4. State the Law of Dominance.

Answer: The Law of Dominance states that in a heterozygous condition, only one allele expresses itself, while the other remains masked.

Q5. What is the Law of Segregation?

Answer: The Law of Segregation states that alleles of a gene separate during gamete formation so that each gamete carries only one allele.

Section B: Inheritance of One Gene (Monohybrid Cross)

Q6. What is a monohybrid cross?

Answer: A monohybrid cross is a genetic cross involving only one pair of contrasting characters to study the inheritance pattern of a single trait.

Q7. What is the phenotypic ratio of a monohybrid cross?

Answer: The phenotypic ratio in the F₂ generation of a monohybrid cross is 3:1, showing dominant and recessive traits.

Q8. What is the genotypic ratio in a monohybrid cross?

Answer: The genotypic ratio in the F₂ generation of a monohybrid cross is 1:2:1.

Q9. Define homozygous condition.

Answer: Homozygous condition refers to the presence of identical alleles for a particular gene, either dominant or recessive.

Q10. Define heterozygous condition.

Answer: Heterozygous condition refers to the presence of two different alleles of a gene in an individual.

Section C: Inheritance of Two Genes (Dihybrid Cross)

Q11. What is a dihybrid cross?

Answer: A dihybrid cross involves two pairs of contrasting traits and is used to study the inheritance of two genes simultaneously.

Q12. What is the phenotypic ratio of a dihybrid cross?

Answer: The phenotypic ratio in the F₂ generation of a dihybrid cross is 9:3:3:1.

Q13. Which Mendelian law is proved by a dihybrid cross?

Answer: A dihybrid cross proves the Law of Independent Assortment.

Q14. What is independent assortment?

Answer: Independent assortment refers to the random and independent separation of alleles of different gene pairs during gamete formation.

Q15. When does independent assortment not apply?

Answer: Independent assortment does not apply when genes are located close together on the same chromosome.

Section D: Deviations from Mendelian Inheritance

Q16. What is incomplete dominance?

Answer: In incomplete dominance, neither allele is completely dominant, and the heterozygous individual shows an intermediate phenotype.

Q17. Give one example of incomplete dominance.

Answer: Flower colour inheritance in Mirabilis jalapa, where red and white flowers produce pink offspring, is an example of incomplete dominance.

Q18. Define codominance.

Answer: Codominance is a condition where both alleles of a gene express themselves equally in a heterozygous individual.

Q19. Give one example of codominance.

Answer: The ABO blood group system in humans shows codominance between IA and IB alleles.

Q20. Why is the phenotypic ratio altered in incomplete dominance?

Answer: The phenotypic ratio changes because the heterozygote shows a new intermediate phenotype instead of a dominant-recessive pattern.

Section E: Multiple Alleles and Pleiotropy

Q21. What are multiple alleles?

Answer: Multiple alleles are more than two alternative forms of a gene present in a population but only two occur in an individual.

Q22. Name a trait controlled by multiple alleles.

Answer: The ABO blood group system in humans is controlled by multiple alleles.

Q23. What is pleiotropy?

Answer: Pleiotropy is a phenomenon in which a single gene affects multiple phenotypic traits in an organism.

Q24. Give one example of pleiotropy.

Answer: Phenylketonuria is a pleiotropic disorder affecting mental development, skin pigmentation, and metabolism.

Q25. Why is pleiotropy significant?

Answer: Pleiotropy explains how one gene mutation can cause multiple physiological and structural abnormalities.

Section F: Chromosomal Theory, Linkage and Recombination

Q26. What is the chromosomal theory of inheritance?

Answer: The chromosomal theory states that genes are located on chromosomes and their behavior during meiosis explains Mendelian inheritance.

Q27. Who proposed the chromosomal theory of inheritance?

Answer: The chromosomal theory of inheritance was proposed by Sutton and Boveri.

Q28. Define linkage.

Answer: Linkage refers to the tendency of genes located close together on the same chromosome to be inherited together.

Q29. What is recombination?

Answer: Recombination is the formation of new gene combinations due to crossing over during prophase I of meiosis.

Q30. How does recombination create variation?

Answer: Recombination produces new allele combinations, increasing genetic variation among offspring.

Section G: Sex Determination Mechanisms

Q31. What is the sex determination system in humans?

Answer: Humans follow the XX–XY sex determination system, where females are XX and males are XY.

Q32. Who is heterogametic in humans?

Answer: Males are heterogametic in humans as they produce two types of gametes, X-bearing and Y-bearing sperms.

Q33. What is the XX–XO mechanism?

Answer: In the XX–XO mechanism, females are XX while males possess only one X chromosome and lack the second sex chromosome.

Q34. Which sex is heterogametic in birds?

Answer: In birds, females are heterogametic with ZW chromosomes, while males are homogametic with ZZ chromosomes.

Q35. What is haplodiploid sex determination?

Answer: Haplodiploidy is a system where males develop from unfertilised haploid eggs and females from fertilised diploid eggs.

Section H: Mutation

Q36. What is a mutation?

Answer: A mutation is a sudden, heritable change in the DNA sequence or chromosome structure that leads to genetic variation.

Q37. Name two causes of mutation.

Answer: Mutations can be caused by radiation, chemical mutagens, or errors during DNA replication.

Q38. What is a gene mutation?

Answer: A gene mutation is a change in the nucleotide sequence of a gene affecting its normal function.

Q39. What is a chromosomal mutation?

Answer: A chromosomal mutation involves changes in chromosome number or structure, such as deletions, duplications, or nondisjunction.

Q40. Why are mutations important in evolution?

Answer: Mutations introduce new genetic variations, which serve as raw material for natural selection and evolution.

Section I: Genetic Disorders

Q41. What are Mendelian disorders?

Answer: Mendelian disorders are genetic diseases caused by mutations in a single gene and follow Mendelian inheritance patterns.

Q42. Name one X-linked recessive disorder.

Answer: Haemophilia is an X-linked recessive disorder affecting the blood clotting mechanism.

Q43. What causes sickle cell anaemia?

Answer: Sickle cell anaemia is caused by a mutation in the gene coding for haemoglobin, resulting in abnormal sickle-shaped red blood cells.

Q44. What are chromosomal disorders?

Answer: Chromosomal disorders result from abnormalities in chromosome number or structure due to nondisjunction during meiosis.

Q45. Name the chromosomal disorder caused by trisomy 21.

Answer: Down’s syndrome is caused by trisomy of chromosome 21.

Section J: Application-Based Concepts

Q46. What is nondisjunction?

Answer: Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis.

Q47. What is Turner’s syndrome?

Answer: Turner’s syndrome is a chromosomal disorder in females characterised by the presence of only one X chromosome.

Q48. What is Klinefelter’s syndrome?

Answer: Klinefelter’s syndrome is a chromosomal disorder in males caused by the presence of an extra X chromosome.

Q49. Why are linked genes inherited together?

Answer: Linked genes are inherited together because they are located close to each other on the same chromosome and rarely separate.

Q50. Why is genetics important in medicine?

Answer: Genetics helps in understanding hereditary diseases, genetic counselling, early diagnosis, and prevention of inherited disorders.