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

CBSE Class 12 Biology Long 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: Long Answer Type
Answer Length: 120–150 words each

Section A: Mendel’s Experiments and Laws of Inheritance

Q1. Describe the experimental approach of Gregor Mendel and explain why his work was scientifically significant.

Answer:
Gregor Mendel conducted systematic hybridisation experiments on pea plants to study inheritance patterns. He selected true-breeding varieties showing clear contrasting traits and performed controlled cross-pollination. Mendel carefully recorded observations over several generations and analysed the results mathematically. His scientific significance lies in introducing quantitative analysis in biology, identifying discrete hereditary units called genes, and proposing laws of inheritance. Mendel’s work explained how traits are transmitted without blending, maintaining their identity across generations. Although ignored initially, his findings later became the foundation of classical genetics. The clarity of traits, large sample size, and logical interpretation made his conclusions universally applicable and reproducible, distinguishing his work from earlier inheritance theories.

Q2. Explain the Law of Dominance with reference to a monohybrid cross.

Answer:
The Law of Dominance states that when two contrasting alleles are present together in a heterozygous individual, only one allele expresses itself phenotypically, while the other remains masked. In a monohybrid cross between tall (TT) and dwarf (tt) pea plants, all F₁ progeny are tall (Tt), indicating that tallness is dominant over dwarfness. The recessive allele does not disappear but remains present and reappears in the F₂ generation. This law explains the uniformity of the F₁ generation and establishes the concept of dominant and recessive traits. It highlights that expression of a trait depends on allele interaction, not allele presence alone.

Q3. Describe the Law of Segregation and its cytological basis.

Answer:
The Law of Segregation states that the two alleles of a gene separate during gamete formation so that each gamete receives only one allele. This ensures that alleles retain their individuality and do not blend. The cytological basis of this law lies in meiosis, particularly during anaphase I, when homologous chromosomes separate and move to opposite poles. Since alleles are located on homologous chromosomes, their separation results in segregation of alleles. Upon fertilisation, the alleles recombine randomly, restoring the diploid condition. This law explains the reappearance of recessive traits in the F₂ generation and ensures genetic purity of gametes.

Q4. Explain the Law of Independent Assortment with suitable examples.

Answer:
The Law of Independent Assortment states that alleles of different gene pairs segregate independently during gamete formation. This law is best demonstrated using a dihybrid cross involving two traits, such as seed shape and seed colour in pea plants. Mendel observed that inheritance of seed shape did not influence inheritance of seed colour, resulting in a 9:3:3:1 phenotypic ratio in the F₂ generation. This independent distribution occurs because genes controlling different traits are located on separate chromosomes or are far apart on the same chromosome. The law contributes significantly to genetic variation by producing new trait combinations in offspring.

Q5. Why are Mendel’s laws not universally applicable?

Answer:
Mendel’s laws are not universally applicable because they are based on traits controlled by single genes with complete dominance. In nature, many traits show incomplete dominance, codominance, polygenic inheritance, or gene linkage. Genes located close together on the same chromosome do not assort independently, violating the Law of Independent Assortment. Additionally, environmental factors influence gene expression, altering expected ratios. Presence of multiple alleles and pleiotropic effects further complicate inheritance patterns. Therefore, while Mendel’s laws form the foundation of genetics, they represent simplified models that do not account for the full complexity of inheritance observed in organisms.

Section B: Monohybrid and Dihybrid Crosses

Q6. Describe a monohybrid cross and explain its importance in genetics.

Answer:
A monohybrid cross is a genetic cross involving one pair of contrasting traits. In pea plants, crossing tall and dwarf plants produces tall offspring in the F₁ generation. When F₁ plants are self-pollinated, the F₂ generation shows a phenotypic ratio of 3 tall to 1 dwarf. This cross is important because it demonstrates the Law of Dominance and Law of Segregation. It provides a simple and clear understanding of how traits are inherited and helps in predicting offspring characteristics. Monohybrid crosses are widely used in breeding programmes and genetic analysis.

Q7. Explain a dihybrid cross and the reason for obtaining a 9:3:3:1 ratio.

Answer:
A dihybrid cross involves two pairs of contrasting traits studied simultaneously. In pea plants, Mendel crossed plants differing in seed shape and seed colour. The F₁ generation showed only dominant traits. On self-pollination, the F₂ generation exhibited four phenotypes in the ratio 9:3:3:1. This ratio occurs because alleles of different gene pairs assort independently during gamete formation, producing four types of gametes in equal proportions. Random fertilisation leads to multiple combinations, increasing variation. This cross provides experimental proof of the Law of Independent Assortment.

Q8. Explain the concept and significance of a test cross.

Answer:
A test cross involves crossing an individual showing a dominant phenotype with a homozygous recessive individual. It is used to determine whether the dominant individual is homozygous or heterozygous. If all offspring show the dominant trait, the individual is homozygous; if both dominant and recessive phenotypes appear, it is heterozygous. The test cross is significant in genetic analysis, plant breeding, and research, as it helps identify genotypes accurately. It also provides evidence for segregation of alleles and helps in studying linkage relationships between genes.

Q9. Distinguish between genotype and phenotype with examples.

Answer:
Genotype refers to the genetic makeup of an organism, represented by allele combinations such as TT, Tt, or tt. Phenotype refers to the observable traits resulting from genotype expression, such as tall or dwarf plants. Different genotypes can sometimes produce the same phenotype due to dominance, as seen in TT and Tt plants, both appearing tall. Phenotype is influenced by both genotype and environmental factors, while genotype remains constant throughout life. Understanding this distinction is essential for interpreting inheritance patterns and predicting trait transmission.

Q10. Explain why F₁ hybrids show uniformity in Mendelian crosses.

Answer:
F₁ hybrids show uniformity because they inherit one allele from each parent, and the dominant allele expresses itself in all individuals. Since both parents are true-breeding and produce identical gametes, all F₁ offspring have the same genotype and phenotype. The recessive allele remains masked but is not eliminated. This uniformity confirms the Law of Dominance and demonstrates predictable inheritance patterns. Uniformity in F₁ generation helps establish consistency in experimental results and supports Mendel’s conclusions about allele behaviour.

Section C: Deviations from Mendelian Inheritance

Q11. Explain incomplete dominance with reference to phenotypic ratios.

Answer:
Incomplete dominance occurs when neither allele is completely dominant, and the heterozygote exhibits an intermediate phenotype. In Mirabilis jalapa, crossing red and white flowers produces pink flowers in the F₁ generation. When F₁ plants self-pollinate, the F₂ generation shows red, pink, and white flowers in a 1:2:1 ratio. Here, each genotype corresponds to a distinct phenotype, making the phenotypic ratio identical to the genotypic ratio. Incomplete dominance highlights that dominance is not always absolute and gene interaction can modify trait expression.

Q12. Describe codominance with the ABO blood group system.

Answer:
Codominance occurs when both alleles of a gene express themselves equally in a heterozygous individual. In the ABO blood group system, three alleles—IA, IB, and i—control blood groups. IA and IB are codominant and express simultaneously in individuals with AB blood group. Neither allele masks the other. The i allele is recessive. This system demonstrates multiple alleles and codominance, showing how inheritance can deviate from simple dominant-recessive patterns. It has practical importance in blood transfusion and genetic studies.

Q13. Explain multiple alleles and their genetic significance.

Answer:
Multiple alleles refer to the presence of more than two alternative forms of a gene in a population. Although an individual carries only two alleles, multiple alleles increase genetic diversity within the population. The ABO blood group system is a classic example involving three alleles. Multiple alleles arise due to mutations accumulated over generations. Their significance lies in producing varied phenotypes, enhancing adaptability, and contributing to population-level variation. They also complicate inheritance patterns beyond simple Mendelian ratios.

Q14. What is pleiotropy? Explain its effects with an example.

Answer:
Pleiotropy is a phenomenon in which a single gene influences multiple phenotypic traits. In phenylketonuria, a mutation in a gene involved in amino acid metabolism affects brain development, skin pigmentation, and mental ability. This occurs because the gene product participates in multiple physiological pathways. Pleiotropy complicates genetic analysis, as one mutation results in several effects. It highlights the interconnectedness of biological systems and explains why certain genetic disorders show multiple symptoms.

Q15. Explain why Mendelian ratios change in non-Mendelian inheritance.

Answer:
Mendelian ratios change due to variations in allele interactions, gene location, and expression patterns. In incomplete dominance and codominance, heterozygotes produce distinct phenotypes. Linkage prevents independent assortment of genes on the same chromosome. Multiple alleles increase phenotypic combinations, and pleiotropy causes multiple trait effects. Environmental factors and gene interactions further modify outcomes. These factors show that inheritance is influenced by complex biological processes, leading to deviations from classic Mendelian ratios.

Section D: Chromosomal Theory, Linkage and Recombination

Q16. Explain the chromosomal theory of inheritance and its importance.

Answer:
The chromosomal theory of inheritance states that genes are located on chromosomes and their behaviour during meiosis explains Mendelian inheritance. Proposed by Sutton and Boveri, it connects genetic principles with cytology. Chromosome pairing, segregation, and independent assortment correspond to Mendel’s laws. This theory provided physical evidence for abstract genetic concepts and established chromosomes as carriers of hereditary information. It forms the basis of modern genetics and explains inheritance at the cellular level.

Q17. Describe linkage and its effect on inheritance patterns.

Answer:
Linkage refers to the tendency of genes located close together on the same chromosome to be inherited together. Linked genes do not assort independently, resulting in altered Mendelian ratios. The closer the genes, the stronger the linkage and lower the recombination frequency. Linkage reduces genetic variation for those traits but helps in maintaining favourable gene combinations. It plays a key role in gene mapping and understanding chromosome structure.

Q18. Explain recombination and its evolutionary significance.

Answer:
Recombination is the exchange of genetic material between homologous chromosomes during crossing over in prophase I of meiosis. It produces new gene combinations in gametes, increasing genetic variation among offspring. Recombination is significant in evolution because variation is essential for natural selection. It also helps in gene mapping by indicating distances between genes based on recombination frequency.

Q19. Distinguish between complete and incomplete linkage.

Answer:
In complete linkage, genes are inherited together without any recombination because they are very close on the chromosome. In incomplete linkage, genes are close but crossing over occurs occasionally, producing some recombinant offspring. Incomplete linkage is more common and results in modified Mendelian ratios. These concepts help explain inheritance patterns and gene arrangement.

Q20. Explain how recombination frequency helps in gene mapping.

Answer:
Recombination frequency measures how often crossing over occurs between two genes. Genes located closer together have lower recombination frequency, while distant genes recombine more frequently. By analysing recombination percentages, geneticists can determine the relative positions of genes on chromosomes. This information is used to construct genetic maps, aiding in understanding genome organisation and identifying genes associated with traits or disorders.

Section E: Sex Determination, Mutation and Genetic Disorders

Q21. Describe sex determination in humans.

Answer:
Humans follow the XX–XY sex determination system. Females have two X chromosomes, while males have one X and one Y chromosome. The ovum always carries an X chromosome, whereas sperms carry either X or Y. Fertilisation by an X-bearing sperm produces a female, while a Y-bearing sperm produces a male. Thus, the male determines the sex of the offspring. This system ensures approximately equal probability of male and female births.

Q22. Explain different sex determination mechanisms found in nature.

Answer:
Various sex determination mechanisms exist across organisms. Humans follow the XX–XY system, while grasshoppers show XX–XO system. Birds exhibit the ZW–ZZ system, where females are heterogametic. Honeybees follow haplodiploidy, where males develop from unfertilised eggs. These mechanisms demonstrate evolutionary diversity in determining sex and help understand chromosomal behaviour.

Q23. Define mutation and explain its role in evolution.

Answer:
Mutation is a sudden heritable change in genetic material, either at gene or chromosome level. Mutations introduce new alleles into populations, creating genetic variation. Although many mutations are harmful or neutral, some provide advantages that are selected during evolution. Thus, mutations serve as the raw material for evolution, enabling species to adapt to changing environments.

Q24. Explain Mendelian genetic disorders with examples.

Answer:
Mendelian disorders are caused by mutations in single genes and follow Mendelian inheritance patterns. Haemophilia and colour blindness are X-linked recessive disorders, while sickle cell anaemia is an autosomal recessive disorder. These disorders demonstrate how defective alleles are transmitted across generations. Understanding them helps in diagnosis, genetic counselling, and management of inherited diseases.

Q25. Describe chromosomal disorders caused by nondisjunction.

Answer:
Chromosomal disorders result from abnormal chromosome number due to nondisjunction during meiosis. Down’s syndrome is caused by trisomy 21, Turner’s syndrome by monosomy X, and Klinefelter’s syndrome by XXY condition. These disorders affect physical and mental development. Studying them helps understand chromosome behaviour and the importance of accurate cell division.