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Class 9 Biology Chapter 5 Molecular Basis of Inheritance: Previous Year Questions & Answers (2020–2025)
Class 9 Biology Chapter 5 focuses on the Molecular Basis of Inheritance—one of the most critical chapters for board exams and competitive entrance tests. Understanding DNA structure, genes, chromosomes, and how traits pass from parents to children is fundamental to modern biology. This page brings you curated previous year questions (2020–2025) from CBSE boards, along with expert-verified answers, to help you master inheritance concepts with confidence. Whether you're preparing for periodic tests or final exams, these real board-level questions build clarity and exam readiness.
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What Is the Molecular Basis of Inheritance?
The molecular basis of inheritance explains how genetic information is stored, replicated, and transmitted from parent cells to daughter cells. At the core lies DNA (deoxyribonucleic acid), a double-stranded molecule composed of nucleotides. Each nucleotide contains a sugar (deoxyribose), a phosphate group, and a nitrogenous base. The NCERT Class 9 Biology curriculum emphasizes that DNA is the genetic material responsible for heredity, and its structure was discovered by Watson and Crick. Understanding DNA's role is essential for answering board exam questions about inheritance patterns.
DNA Structure and Nucleotides: NCERT Fundamentals
DNA is made of four types of nucleotides, each containing one of four bases: adenine (A), guanine (G), cytosine (C), and thymine (T). These bases pair specifically—A with T, and G with C—forming base pairs that hold the two DNA strands together. The sugar-phosphate backbone forms the outer structure, while base pairs form the interior. NCERT Class 9 Chapter 5 diagrams clearly show this double helix structure. Previous year CBSE questions frequently ask students to identify nucleotide components or explain complementary base pairing, making this section crucial for exam success.
Genes, Chromosomes, and Heredity Explained
A gene is a segment of DNA that codes for a specific protein or trait. Chromosomes are thread-like structures composed of DNA and proteins, found in the cell nucleus. Humans have 23 pairs of chromosomes (46 total), with genes located at fixed positions called loci. During reproduction, chromosomes are inherited from both parents, passing genes to offspring. NCERT emphasizes that the combination of alleles (different forms of a gene) determines an organism's traits. Board questions often test understanding of gene segregation, chromosome inheritance, and how traits skip generations.
Cell Division: Mitosis, Meiosis, and Genetic Transmission
Mitosis produces two identical daughter cells, maintaining the chromosome number and ensuring genetic continuity in body cells. Meiosis, however, reduces chromosome number by half, creating four genetically unique gametes (sex cells). This reduction is critical for sexual reproduction—when sperm and egg unite, the chromosome number is restored in offspring. NCERT Class 9 explains that meiosis introduces genetic variation through crossing over and random assortment. Previous year CBSE exams regularly ask about the stages of meiosis, why genetic variation matters, and how chromosome numbers change during cell division.
Mendel's Laws of Inheritance and Their Molecular Basis
Gregor Mendel's laws—segregation, independent assortment, and dominance—describe how traits are inherited. At the molecular level, these laws operate through genes and alleles on chromosomes. When gametes form during meiosis, alleles separate (law of segregation), and different gene pairs assort independently (law of independent assortment). NCERT Class 9 connects Mendel's observations to DNA structure and explains that molecular mechanisms underlie visible inheritance patterns. Board questions test your ability to solve Punnett squares, predict offspring traits, and explain inheritance in genetic terms.
Previous Year CBSE Questions: Common Patterns (2020–2025)
CBSE board exams repeatedly ask short-answer and long-answer questions on this chapter. Common question types include: (1) Define genes and alleles; (2) Draw and label DNA structure; (3) Explain how traits are inherited through generations; (4) Differentiate mitosis from meiosis; (5) Solve inheritance problems using Punnett squares; (6) Explain genetic variation and its significance. Questions often carry 2, 3, or 5 marks and require clear, concise explanations backed by NCERT diagrams. Practicing these past questions builds pattern recognition and confidence for final board exams.
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Across India, CBSE families trust CBSETUTOR.ai as their go-to 24x7 AI tutor for mastering Biology Chapter 5 and all CBSE Classes 6–12 subjects. Our AI-powered platform provides instant, personalized explanations of molecular inheritance, step-by-step solutions to previous year questions, interactive chapter quizzes, and Hindi-medium support for non-English learners. With detailed NCERT alignment and exam-focused content, CBSETUTOR.ai helps thousands of students clarify doubts, boost marks, and ace board exams without coaching center fees. Study anytime, anywhere—your personal CBSE tutor is always ready.
Answer Strategies for 2-Mark and 5-Mark Questions
For 2-mark questions: Define the concept in one sentence, then provide one key supporting detail or example. For instance, 'Gene: A segment of DNA coding for a specific trait. Example: The gene for eye color is located on chromosome 19.' For 5-mark questions: Structure your answer with a definition, explanation of the mechanism, diagram reference (if applicable), and a real-world or NCERT-based example. Always link molecular concepts (DNA, genes, alleles) to observable traits (phenotypes). Practice answers should mirror NCERT language and include proper scientific terminology to maximize marks.
Genetics Vocabulary: Must-Know Terms for Board Exams
Master these NCERT-defined terms: Genome (all genes in an organism), Genotype (genetic makeup), Phenotype (observable traits), Dominant allele (expressed trait), Recessive allele (hidden unless homozygous), Heterozygous (two different alleles), Homozygous (two identical alleles), Locus (gene location on chromosome), and Mutation (change in DNA sequence). CBSE exams test your precision with terminology. Define each term clearly and distinguish between similar concepts (e.g., gene vs. allele, mitosis vs. meiosis). Flashcards and practice questions help lock these definitions into memory.
How to Use NCERT Diagrams for Higher Exam Marks
NCERT Chapter 5 includes essential diagrams: DNA double helix structure, nucleotide composition, chromosome structure, mitotic and meiotic stages, and Punnett squares for monohybrid and dihybrid crosses. In exams, labeling diagrams correctly earns substantial marks. Practice drawing and labeling these diagrams from memory, then cross-check with NCERT. When answering questions, reference specific diagram details (e.g., 'As shown in NCERT Fig. 5.1, base pairing follows Chargaff's rules'). Integration of diagrams with written explanations demonstrates deep understanding and boosts your final score.
Frequently asked questions
What is the difference between a gene and an allele?+
A gene is a DNA segment coding for a trait; an allele is a variant form of that gene. For example, the gene for flower color has alleles for red or white. One gene, multiple possible alleles.
Why is meiosis important for genetic variation?+
Meiosis creates four genetically unique gametes through crossing over and random assortment of chromosomes. This variation is essential for evolution, adaptation, and genetic diversity in populations.
How can I access CBSETUTOR.ai's free-trial resources for this chapter?+
Visit CBSETUTOR.ai's platform to explore free-trial access to Chapter 5 video explanations, sample questions, and instant AI doubt-solving. No credit card required to start learning Biology today.
Does CBSETUTOR.ai provide support in Hindi medium for Chapter 5?+
Yes. CBSETUTOR.ai supports Hindi-medium CBSE students with full chapter content, answers, and explanations in Hindi. Choose your preferred language during sign-up for seamless learning.
What are the most common previous year CBSE questions on DNA structure?+
Common questions ask to label DNA nucleotides, explain base pairing rules, differentiate DNA from RNA, and describe semi-conservative replication. Mastering these ensures success on 2- and 3-mark questions.
How do I solve a Punnett square for a dihybrid cross?+
Write parental genotypes, determine gamete types for each parent, create a 4×4 grid (16 cells), and fill in offspring genotypes. Count phenotypic ratios (typically 9:3:3:1). Practice with NCERT examples.
Is Chapter 5 Molecular Basis of Inheritance important for competitive exams like NEET?+
Absolutely. Inheritance, DNA replication, protein synthesis, and genetic variation are core NEET topics. Strong Class 9 foundation from NCERT Chapter 5 supports NEET preparation significantly.
What is the typical mark distribution for inheritance questions in CBSE Class 9 exams?+
Usually 8–12 marks out of 80 in the biology paper. Questions span multiple choice, short-answer (2 marks), and long-answer (5 marks). Consistent practice with previous years ensures comprehensive coverage.
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