Which of the following is an example of active immunity?

antibodies produced by a person's own body after a vaccination

antibodies passed from the mother to the foetus across the placenta

preformed antitoxin injected after a snake bite

antibodies received by an infant through colostrum

The technique of introducing foreign DNA into a plant cell by bombarding it with microscopic gold or tungsten particles coated with the DNA is called:

biolistics, or the gene gun method

Haemophilia and Duchenne muscular dystrophy appear far more often in men than in women because:

a man has only one X chromosome, so a single recessive allele on it is expressed

the alleles concerned are dominant in men and recessive in women

the Y chromosome carries the same alleles in a dominant form

the alleles are carried on mitochondrial DNA

Which pair of organism and industrial product is correctly matched?

Aspergillus niger — citric acid

Clostridium butylicum — citric acid

Monascus purpureus — butyric acid

Morgan crossed Drosophila carrying two mutant characters and found that in the progeny the two mutant characters stayed together far more often than a 9 : 3 : 3 : 1 expectation allows. He explained this by proposing that:

the two genes lay close together on the same chromosome, so recombination between them was rare

the two genes lay on different chromosomes and assorted independently

one of the two genes was dominant over the other

both characters were controlled by a single gene with multiple alleles

Which sequence arranges human ancestors correctly from the earliest to the most recent?

Australopithecus → Homo habilis → Homo erectus → Homo sapiens

Homo habilis → Australopithecus → Homo erectus → Homo sapiens

Australopithecus → Homo erectus → Homo habilis → Homo sapiens

Homo erectus → Homo habilis → Australopithecus → Homo sapiens

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CBSE Class 12 Biology Board Paper 4 — MCQs with Answers

Original paper · CBSE patternWritten by our subject team, not a reprint of an official sample paper.

Questions: 20
Time: 30 min
Marking: +1, no negative marking
Mix: 4 Easy · 8 Medium · 8 Hard

Board Paper 4 — CBSE Class 12 Biology is a full 20-question objective paper for Class 12 Biology, written in the CBSE board pattern and marked the way the board marks: +1 for a correct answer and 0 for a wrong one — no negative marking, exactly like the real board paper. It follows Section A of a real CBSE paper: Q1–Q16 are single-correct MCQs, Q17–Q18 are case-based questions built on a short source, and Q19–Q20 are assertion–reason items using the board's own four option strings.

The difficulty sits deliberately a notch above the real board — 4 Easy · 8 Medium · 8 Hard — because a paper you can clear comfortably tells you nothing on exam day. You get 30 minutes — about 90 seconds a question, the pace the real paper demands. 8 of the questions are solved in full below, with the correct option and a worked explanation for each; the remaining 12 are timed and scored when you sign in free.

Sit it under exam conditions rather than open-book. The score you get on a timed full-length paper is the only honest signal of whether your Biology revision is holding together across chapters — which is precisely what the board tests and what chapter-wise practice cannot tell you.

8 solved questions from this paper

Answer and worked explanation shown for each. The remaining 12 are timed and scored when you sign in free.

Q1Easy
Which of the following is an example of active immunity?
A.antibodies passed from the mother to the foetus across the placenta
B.antibodies produced by a person's own body after a vaccination✓ Correct
C.preformed antitoxin injected after a snake bite
D.antibodies received by an infant through colostrum
Answer: B. antibodies produced by a person's own body after a vaccination
Explanation: Active immunity develops when a person's own lymphocytes meet an antigen, as after a vaccine or a natural infection; it takes time to build but lasts long because memory cells persist. Placental transfer, colostrum and injected antitoxin all supply ready-made antibodies, which is passive immunity and fades within weeks.
Q2Easy
The technique of introducing foreign DNA into a plant cell by bombarding it with microscopic gold or tungsten particles coated with the DNA is called:
A.microinjection
B.electroporation
C.the disarmed vector method
D.biolistics, or the gene gun method✓ Correct
Answer: D. biolistics, or the gene gun method
Explanation: Biolistics fires high-velocity DNA-coated metal particles at the cells and is the standard way of getting past a rigid plant cell wall. Microinjection uses a fine needle and suits animal cells, while electroporation opens transient pores in a membrane with a brief electric pulse.
Q3Easy
In an ecosystem, the organisms that break down the complex organic matter of dead remains into simple inorganic substances are called:
A.producers
B.primary consumers
C.decomposers✓ Correct
D.secondary consumers
Answer: C. decomposers
Explanation: Bacteria and fungi secrete digestive enzymes onto detritus and absorb the products, and the release of inorganic nutrients from the humus is the step called mineralisation. Without them the nutrients locked up in dead bodies would never return to the soil for producers to use again.
Q4Medium
Haemophilia and Duchenne muscular dystrophy appear far more often in men than in women because:
A.a man has only one X chromosome, so a single recessive allele on it is expressed✓ Correct
B.the alleles concerned are dominant in men and recessive in women
C.the Y chromosome carries the same alleles in a dominant form
D.the alleles are carried on mitochondrial DNA
Answer: A. a man has only one X chromosome, so a single recessive allele on it is expressed
Explanation: These are X-linked recessive conditions, and a male is hemizygous, so one defective allele on his single X is enough to produce the disease, whereas a female must inherit the allele on both her X chromosomes. The Y chromosome carries no normal copy of these genes that could mask the defective one.
Q5Medium
The hormone whose sharp mid-cycle surge triggers ovulation in the human menstrual cycle is:
A.follicle stimulating hormone
B.luteinising hormone✓ Correct
C.progesterone
D.human chorionic gonadotropin
Answer: B. luteinising hormone
Explanation: Rising oestrogen from the maturing follicle provokes a sharp LH surge around the fourteenth day, and it is this peak that ruptures the Graafian follicle and later converts what remains of it into the corpus luteum. FSH also rises, but its role is follicular growth, and hCG appears only after implantation has occurred.
Q6Medium
Which pair of organism and industrial product is correctly matched?
A.Clostridium butylicum — citric acid
B.Monascus purpureus — butyric acid
C.Lactobacillus — statins
D.Aspergillus niger — citric acid✓ Correct
Answer: D. Aspergillus niger — citric acid
Explanation: Citric acid is produced commercially by the fungus Aspergillus niger, butyric acid by Clostridium butylicum and lactic acid by Lactobacillus. Monascus purpureus yields statins, the blood-cholesterol-lowering drugs that act by competitively inhibiting the enzyme responsible for cholesterol synthesis.
Q7Hard
Morgan crossed Drosophila carrying two mutant characters and found that in the progeny the two mutant characters stayed together far more often than a 9 : 3 : 3 : 1 expectation allows. He explained this by proposing that:
A.the two genes lay on different chromosomes and assorted independently
B.the two genes lay close together on the same chromosome, so recombination between them was rare✓ Correct
C.one of the two genes was dominant over the other
D.both characters were controlled by a single gene with multiple alleles
Answer: B. the two genes lay close together on the same chromosome, so recombination between them was rare
Explanation: Physical linkage keeps alleles that lie on the same chromosome travelling together, and the closer the two genes are the fewer the crossovers that separate them, so parental combinations dominate the progeny. Morgan and Sturtevant then used recombination frequency itself as a measure of map distance, white and yellow showing only 1.3% recombination against 37.2% for white and miniature wing.
Q8Hard
Which sequence arranges human ancestors correctly from the earliest to the most recent?
A.Homo habilis → Australopithecus → Homo erectus → Homo sapiens
B.Australopithecus → Homo erectus → Homo habilis → Homo sapiens
C.Homo erectus → Homo habilis → Australopithecus → Homo sapiens
D.Australopithecus → Homo habilis → Homo erectus → Homo sapiens✓ Correct
Answer: D. Australopithecus → Homo habilis → Homo erectus → Homo sapiens
Explanation: Australopithecines lived roughly 3 to 4 million years ago, Homo habilis about 2 million years ago with a brain of 650 to 800 cc, Homo erectus about 1.5 million years ago with roughly 900 cc, and modern Homo sapiens arose within the last 0.2 million years. Brain size and tool use increase steadily along this order, so any sequence placing habilis before Australopithecus reverses the trend.

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Is this an official CBSE sample paper?

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How is this Class 12 Biology paper marked?

+1 for a correct answer and 0 for a wrong one — no negative marking, exactly like the real board paper. So attempt every question — there is no penalty for a wrong answer on a board paper, and leaving a question blank can only cost you.

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