In short: CBSE Class 12 Biology (70 marks theory) includes 2-4 assertion-reason questions in Section A, each worth 1 mark. The paper has 33 questions across 5 sections, with 50% competency-based content. A-R questions test whether you understand the relationship between biological facts — not just whether you remember them. This post gives you 30 practice A-R pairs with detailed explanations.

The Assertion-Reason Format in Biology

Biology A-R questions follow the same four-option format as Chemistry and Physics:

Option	What It Means
(a)	Both A and R are true, and R is the correct explanation of A
(b)	Both A and R are true, but R is NOT the correct explanation of A
(c)	A is true but R is false
(d)	A is false but R is true

Biology A-R questions are particularly tricky because the subject has many facts that are true but causally unrelated. A statement about DNA structure and a statement about Mendel's experiments might both be true, but one does not explain the other.

The Biology chapter-wise marks weightage shows that Genetics (Unit VI) and Reproduction (Unit V) carry the most marks — and A-R questions frequently come from these units.

Reproduction in Organisms (Chapters 1-4)

Q1. Assertion: In angiosperms, double fertilisation results in the formation of a diploid zygote and a triploid endosperm. Reason: One male gamete fuses with the egg cell (syngamy) and the other fuses with the two polar nuclei (triple fusion).

Answer: (a) — Double fertilisation involves two fusion events: syngamy (n + n = 2n zygote) and triple fusion (n + 2n = 3n endosperm). The reason correctly explains both products.

Q2. Assertion: The megaspore mother cell undergoes meiosis to form four megaspores. Reason: In most angiosperms, only one megaspore is functional and develops into the embryo sac.

Answer: (b) — Both statements are true. However, the fact that only one megaspore survives does not explain why the mother cell undergoes meiosis. The reason describes what happens after meiosis, not why meiosis occurs.

Q3. Assertion: Human males are heterogametic. Reason: Males produce two types of gametes — one with X chromosome and one with Y chromosome.

Answer: (a) — The production of two different types of gametes (X-bearing and Y-bearing sperm) is exactly what defines heterogamety.

Q4. Assertion: The placenta acts as a temporary endocrine gland during pregnancy. Reason: The placenta secretes hormones like hCG, hPL, estrogen, and progesterone.

Answer: (a) — The secretion of these hormones is what makes the placenta function as an endocrine gland. The reason directly explains the assertion.

Q5. Assertion: Test-tube babies are produced by in vivo fertilisation. Reason: In IVF, the zygote is transferred into the fallopian tube at the 8-blastomere stage.

Answer: (d) — A is false (test-tube babies are produced by in vitro fertilisation, not in vivo). R is true (the ZIFT technique transfers zygotes/early embryos into the fallopian tube).

Genetics and Inheritance (Chapters 5-6)

Q6. Assertion: When a cross between a tall plant (TT) and a dwarf plant (tt) is made, all F1 plants are tall. Reason: The tall character is dominant over the dwarf character.

Answer: (a) — Dominance of the tall allele (T) masks the recessive dwarf allele (t) in heterozygous (Tt) F1 plants.

Q7. Assertion: Haemophilia is more common in males than in females. Reason: Haemophilia is an X-linked recessive disorder, and males have only one X chromosome.

Answer: (a) — With only one X chromosome, a single recessive allele is sufficient to express haemophilia in males. Females need two copies (homozygous recessive).

Q8. Assertion: A dihybrid cross produces a 9:3:3:1 phenotypic ratio in F2. Reason: The two genes assort independently during gamete formation (Law of Independent Assortment).

Answer: (a) — Independent assortment creates four types of gametes in equal proportions, leading to the 9:3:3:1 ratio.

Q9. Assertion: Codominance is different from incomplete dominance. Reason: In codominance, both alleles express themselves fully in the heterozygote.

Answer: (a) — In codominance (e.g., AB blood group), both A and B antigens are expressed. In incomplete dominance, a blended phenotype appears. The reason explains the distinction.

Q10. Assertion: A cross between a homozygous red-flowered plant and a white-flowered plant gives pink flowers in F1 (in Mirabilis jalapa). Reason: This is an example of codominance.

Answer: (c) — A is true (pink flowers in F1). R is false (this is incomplete dominance, not codominance — the phenotype is blended, not both parental traits expressed separately).

Q11. Assertion: Down syndrome is caused by trisomy of chromosome 21. Reason: Non-disjunction during cell division can lead to an extra copy of chromosome 21.

Answer: (a) — Non-disjunction is the mechanism that produces the extra chromosome, directly causing Down syndrome.

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Molecular Basis of Inheritance (Chapter 6)

Q12. Assertion: DNA replication is semi-conservative. Reason: Each daughter DNA molecule has one parental strand and one newly synthesised strand.

Answer: (a) — The definition of semi-conservative replication in R directly explains A. This was experimentally proved by Meselson and Stahl.

Q13. Assertion: RNA polymerase does not require a primer to start transcription. Reason: RNA polymerase has the ability to initiate de novo synthesis of RNA.

Answer: (a) — Unlike DNA polymerase, RNA polymerase can start new RNA chains without a primer, which is what makes transcription different from replication initiation.

Q14. Assertion: The genetic code is degenerate. Reason: Some amino acids are coded by more than one codon.

Answer: (a) — Degeneracy means multiple codons can code for the same amino acid (e.g., leucine has 6 codons). The reason is the definition of degeneracy.

Q15. Assertion: mRNA is polycistronic in prokaryotes. Reason: Prokaryotic mRNA codes for multiple proteins from a single transcript.

Answer: (a) — In prokaryotes, an operon produces a single mRNA that codes for multiple proteins (polycistronic). The reason explains the assertion.

Q16. Assertion: Introns are removed from pre-mRNA during RNA processing (splicing). Reason: Introns code for important proteins in eukaryotes.

Answer: (c) — A is true (introns are spliced out). R is false (introns are non-coding sequences — they do NOT code for proteins).

Evolution (Chapter 7)

Q17. Assertion: Natural selection acts on the phenotype, not the genotype. Reason: The environment directly interacts with the observable traits of an organism.

Answer: (a) — Survival and reproduction depend on how an organism functions in its environment (phenotype), not on the DNA sequence directly.

Q18. Assertion: Industrial melanism in peppered moths is an example of natural selection. Reason: During the Industrial Revolution, dark-coloured moths had a survival advantage on soot-covered trees.

Answer: (a) — The changed environment (soot-covered trees) favoured dark moths, demonstrating directional selection.

Q19. Assertion: Hardy-Weinberg equilibrium states that allele frequencies remain constant in a population. Reason: This equilibrium requires random mating, no mutation, no migration, large population size, and no natural selection.

Answer: (a) — The five conditions listed in R are necessary for maintaining constant allele frequencies as stated in A. For more on evolution and genetics questions, see the Biology diagram and labelling guide.

Q20. Assertion: Homologous organs provide evidence for divergent evolution. Reason: Homologous organs have similar structure but different functions due to adaptation to different environments.

Answer: (a) — Organs with the same embryological origin that have diversified in function demonstrate divergent evolution from a common ancestor.

Q21. Assertion: Analogous organs are evidence for convergent evolution. Reason: Analogous organs have different origins but similar functions due to similar environmental pressures.

Answer: (a) — Wings of insects and birds have different origins but serve the same function (flight), showing convergent evolution.

Ecology (Chapters 13-16)

Q22. Assertion: Energy flow in an ecosystem is unidirectional. Reason: Energy is transferred from one trophic level to the next, with only about 10% being passed on.

Answer: (a) — Energy moves in one direction (producers → consumers) with progressive loss as heat at each level. The 10% rule explains why it cannot flow back.

Q23. Assertion: Decomposers play a crucial role in nutrient cycling. Reason: Decomposers break down dead organic matter and release nutrients back into the soil.

Answer: (a) — The breakdown process described in R is exactly how decomposers facilitate nutrient recycling.

Q24. Assertion: Biodiversity is highest in tropical regions. Reason: Tropical regions have been climatically stable over long periods, allowing speciation.

Answer: (a) — Long-term climatic stability, high solar energy, and large area provide more time and resources for species diversification.

Q25. Assertion: Habitat fragmentation threatens biodiversity. Reason: Fragmentation increases the edge habitat relative to the core habitat.

Answer: (b) — Both are true. Fragmentation does threaten biodiversity, and it does increase edge relative to core. However, the main reason fragmentation threatens biodiversity is that it reduces population sizes, isolates populations, and restricts gene flow — the edge effect is a contributing factor, not the primary explanation.

Q26. Assertion: The pyramid of energy is always upright. Reason: Energy decreases at each successive trophic level.

Answer: (a) — Since energy is lost as heat at each level, each higher level has less energy than the one below — making the pyramid always upright.

Biotechnology (Chapters 11-12)

Q27. Assertion: Restriction enzymes are called molecular scissors. Reason: They cut DNA at specific palindromic recognition sequences.

Answer: (a) — The specific cutting ability described in R is why they are metaphorically called scissors.

Q28. Assertion: PCR (Polymerase Chain Reaction) is used to amplify specific DNA segments. Reason: PCR uses Taq polymerase, which is thermostable and functions at high temperatures.

Answer: (b) — Both are true. PCR does amplify DNA, and Taq polymerase is thermostable. However, the thermostability of Taq explains why PCR can use repeated heating cycles — not why PCR amplifies DNA in general (amplification is due to repeated denaturation-annealing-extension cycles).

Q29. Assertion: Bt cotton is resistant to bollworm attack. Reason: Bt cotton produces Cry protein, which is toxic to certain insect larvae.

Answer: (a) — The Cry protein (from Bacillus thuringiensis) kills bollworm larvae, making the cotton plant resistant.

Q30. Assertion: Gene therapy involves replacing defective genes with functional ones. Reason: The first successful gene therapy was for ADA (adenosine deaminase) deficiency.

Answer: (b) — Both statements are true. Gene therapy does involve gene replacement, and the first case was indeed ADA deficiency. However, the historical fact about ADA therapy does not explain what gene therapy is. The reason is an example, not an explanation.

How to Approach Biology Assertion-Reason Questions

The Decision Tree

Follow this exact sequence for every A-R question:

Step 1: Is Assertion true or false?

If false → check if R is true → Answer is (d)
If true → go to Step 2

Step 2: Is Reason true or false?

If false → Answer is (c)
If true → go to Step 3

Step 3: Does R explain A?

If yes → Answer is (a)
If no → Answer is (b)

Biology-Specific Tips

Biology has many "both true but unrelated" (option b) situations because the subject is fact-heavy. Two statements from the same chapter can both be correct without one causing the other.

Example of (b) pattern: "Assertion: Frogs are amphibians. Reason: Frogs have moist skin." Both are true, but frogs are classified as amphibians because they live in both water and land — not because of moist skin.

For diagram-based questions that often accompany Biology, see the diagram drawing and labelling guide.

Common Mistakes in Biology A-R Questions

Treating examples as explanations. "The first gene therapy was for ADA deficiency" is a fact, not an explanation of what gene therapy is. Students choose (a) when (b) is correct.
Not verifying the Reason independently. A statement like "introns code for proteins" sounds reasonable if you do not think carefully. Always ask: "Is this biochemically correct?"
Confusing similar terms. Codominance vs incomplete dominance, homologous vs analogous, polycistronic vs monocistronic — A-R questions specifically target these confusions.
Assuming (a) is always correct. In actual CBSE papers, (b), (c), and (d) each appear regularly. Do not default to (a).
Rushing. A-R questions in Biology require careful reading of both statements. Spend 45-60 seconds per question — do not try to finish in 20 seconds.

Check the board exam time management strategy for allocating time across the full Biology paper.

FAQs

How many assertion-reason questions are in CBSE Class 12 Biology 2027?

Typically 2-4 out of 16 MCQs in Section A are assertion-reason format, carrying 1 mark each. The exact number varies by year but the format is guaranteed in every paper.

Which Biology chapters are most common for A-R?

Genetics (Mendel's Laws, sex-linked inheritance), Molecular Biology (DNA replication, transcription), Evolution (natural selection, Hardy-Weinberg), and Ecology (energy flow, biodiversity) are the top sources.

Are the options always the same (a), (b), (c), (d)?

Yes. CBSE uses the standardised four-option format across all subjects. Memorise the options so you can focus on evaluating the statements rather than re-reading the choices.

How do I distinguish between option (a) and (b)?

Ask yourself: "If someone asked me WHY the assertion is true, would I answer with the reason?" If yes → (a). If you would give a different explanation → (b).

Can A-R questions come from practical/lab-based topics?

Rarely. A-R questions are almost always theory-based. However, concepts demonstrated in practicals (like gel electrophoresis, DNA fingerprinting) can appear in theory A-R format.

Should I study A-R from coaching material or CBSE sources?

Start with CBSE sample papers and DIKSHA platform questions — these match the actual difficulty. Then supplement with the CBSE sample paper practice sets for more practice.

CBSE Class 12 Biology Assertion-Reason Questions Chapter-Wise Practice 2027