In Mendel's dihybrid cross, the 9 : 3 : 3 : 1 phenotypic ratio obtained in F₂ is a direct consequence of:

independent assortment of the two gene pairs during gamete formation

the law of dominance acting on one gene pair

crossing over between the two genes

incomplete dominance at both loci

The peppered moth Biston betularia became predominantly dark-coloured in industrial England within a few decades. This is best cited as evidence for:

natural selection acting on pre-existing variation

mutation being directed by the environment

the inheritance of characters acquired during life

genetic drift in a small population

In the Hershey–Chase experiment, bacteriophages grown in radioactive medium were used to infect E. coli, which was then agitated in a blender and centrifuged. The result that established DNA as the genetic material was that:

radioactive phosphorus, but not radioactive sulphur, was found inside the bacterial cells

radioactive sulphur, but not radioactive phosphorus, was found inside the bacterial cells

both isotopes were found inside the bacterial cells in equal amounts

neither isotope could be detected inside the bacterial cells

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CBSE Class 12 Biology Board Paper 2 — 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 2 — 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
The layer of the blastocyst that attaches to the endometrium and later forms the embryonic part of the placenta is the:
A.trophoblast✓ Correct
B.inner cell mass
C.zona pellucida
D.corona radiata
Answer: A. trophoblast
Explanation: The blastocyst has an outer trophoblast and an inner cell mass; the trophoblast adheres to the uterine lining and grows into the chorionic villi of the placenta, while the inner cell mass forms the embryo proper. The zona pellucida is shed before implantation can even begin.
Q2Easy
The genetic material of the Tobacco Mosaic Virus and of the Qβ bacteriophage is:
A.double-stranded DNA
B.single-stranded DNA
C.RNA✓ Correct
D.a DNA–RNA hybrid
Answer: C. RNA
Explanation: Both TMV and the Qβ bacteriophage carry RNA as their hereditary material, which is why they were used to show that the gene need not always be DNA. An RNA genome mutates faster because it lacks the proof-reading and repair that a double-stranded DNA genome enjoys.
Q3Easy
Among the four major causes of biodiversity loss often grouped together as the evil quartet, the one responsible for the largest number of extinctions is:
A.co-extinction
B.habitat loss and fragmentation✓ Correct
C.over-exploitation
D.invasion by alien species
Answer: B. habitat loss and fragmentation
Explanation: Habitat loss and fragmentation head the list, the clearing of tropical rainforest for cultivation being the standard example, and fragmentation additionally dooms mammals that need large territories and birds that migrate. Over-exploitation and alien invasions are genuine threats but account for far fewer losses overall.
Q4Medium
A flower that never opens and is therefore compelled to be self-pollinated in the bud is described as:
A.chasmogamous
B.dichogamous
C.herkogamous
D.cleistogamous✓ Correct
Answer: D. cleistogamous
Explanation: Cleistogamous flowers stay shut, so the anthers and stigma lie side by side and autogamy is assured even when no pollinator arrives, though the plant gives up recombination with other individuals. Chasmogamous flowers, as in Commelina, open and expose their sex organs and can therefore be cross-pollinated.
Q5Medium
In Mendel's dihybrid cross, the 9 : 3 : 3 : 1 phenotypic ratio obtained in F₂ is a direct consequence of:
A.independent assortment of the two gene pairs during gamete formation✓ Correct
B.the law of dominance acting on one gene pair
C.crossing over between the two genes
D.incomplete dominance at both loci
Answer: A. independent assortment of the two gene pairs during gamete formation
Explanation: Independent assortment lets each heterozygous pair segregate without reference to the other, so the F₁ makes four kinds of gamete in equal numbers and their random union gives the 9 : 3 : 3 : 1 array. Dominance decides which phenotype is seen within each class but cannot by itself create four classes, and incomplete dominance at both loci would give nine distinct phenotypes instead of four.
Q6Medium
The peppered moth Biston betularia became predominantly dark-coloured in industrial England within a few decades. This is best cited as evidence for:
A.mutation being directed by the environment
B.the inheritance of characters acquired during life
C.natural selection acting on pre-existing variation✓ Correct
D.genetic drift in a small population
Answer: C. natural selection acting on pre-existing variation
Explanation: Melanic moths already existed at low frequency; soot-blackened trunks simply made the light form conspicuous to predatory birds, so the dark form survived better and its frequency climbed. The environment selected among variants that were already present, which is quite different from the Lamarckian reading in which the environment instructs the moth to change.
Q7Hard
A woman with normal vision whose father was colour-blind marries a man with normal vision. The proportion of their sons expected to be colour-blind is:
A.all of them
B.none of them
C.one-half✓ Correct
D.one-quarter
Answer: C. one-half
Explanation: Writing the colour-blindness allele as Xᶜ, the woman must be a carrier Xᶜ X because she received one Xᶜ from her affected father, while her husband is X Y. Sons take their single X from their mother, so half of them receive Xᶜ and are colour-blind; one-quarter is the fraction of all the children taken together, sons and daughters, not the fraction of sons.
Q8Hard
In the Hershey–Chase experiment, bacteriophages grown in radioactive medium were used to infect E. coli, which was then agitated in a blender and centrifuged. The result that established DNA as the genetic material was that:
A.radioactive phosphorus, but not radioactive sulphur, was found inside the bacterial cells✓ Correct
B.radioactive sulphur, but not radioactive phosphorus, was found inside the bacterial cells
C.both isotopes were found inside the bacterial cells in equal amounts
D.neither isotope could be detected inside the bacterial cells
Answer: A. radioactive phosphorus, but not radioactive sulphur, was found inside the bacterial cells
Explanation: DNA contains phosphorus but no sulphur, while protein contains sulphur but virtually no phosphorus, so ³²P labels the phage DNA and ³⁵S labels the protein coat. Radioactivity from ³²P was recovered in the bacterial pellet while ³⁵S stayed in the supernatant with the emptied coats, proving that only the DNA entered the cell and directed the making of new phages.

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Questions students ask about this paper

Is this an official CBSE sample paper?

No — and that is deliberate. This is an original paper written in the CBSE board pattern by our subject team, not a copy of CBSE's official Sample Question Paper. It means you get fresh questions you have not already seen on a dozen other sites, with instant scoring and worked solutions rather than a PDF.

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.

How long should this paper take?

30 minutes for 20 questions, roughly 90 seconds each. The timer runs whether or not you are watching it, which is the point: board marks are lost to pacing at least as often as to gaps in knowledge.

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