Tamil Nadu Board 2026 Class 12 Zoology Set B Question Paper with Solutions

Concept:
Allergy is a hypersensitive immune response to harmless substances (allergens), mediated mainly by Immunoglobulin E (IgE).

Step 1: {\color{redRole of IgE.

IgE antibodies bind to allergens and trigger the release of histamine and other chemicals from mast cells and basophils.

Step 2: {\color{redResult of IgE action.

This leads to symptoms such as sneezing, itching, swelling, and inflammation.

Step 3: {\color{redEvaluating the options.


IgA \(\rightarrow\) Mucosal immunity (incorrect)
IgE \(\rightarrow\) Allergy and hypersensitivity (correct)
IgM \(\rightarrow\) First antibody in primary response (incorrect)
IgG \(\rightarrow\) Long-term immunity (incorrect)


Step 4: {\color{redConclusion.

Thus, allergy involves IgE. Quick Tip: Remember: Allergy → IgE → Histamine release.

Concept:
Hershey and Chase (1952) used bacteriophages to determine whether DNA or protein is the genetic material.

Step 1: {\color{redExperimental setup.


DNA was labeled with radioactive phosphorus (\(^{32}\)P)
Protein was labeled with radioactive sulphur (\(^{35}\)S)


Step 2: {\color{redInfection of bacteria.

Bacteriophages were allowed to infect bacterial cells.

Step 3: {\color{redObservation.

Only radioactive phosphorus (DNA) entered the bacterial cells, while protein remained outside.

Step 4: {\color{redInference.

Since DNA entered the cell and directed viral replication, it must be the genetic material.

Step 5: {\color{redEvaluating options.


DNA contains radioactive sulphur \(\rightarrow\) Incorrect (protein contains sulphur)
Protein gets into bacterial cells \(\rightarrow\) Incorrect
Viruses undergo transformation \(\rightarrow\) Incorrect
DNA is the genetic material \(\rightarrow\) Correct


Step 6: {\color{redConclusion.

Thus, the experiment proved that DNA is the genetic material. Quick Tip: Remember: \(^{32}\)P → DNA enters cell → DNA = genetic material.

Concept:
Pedigree symbols are used in genetics to represent family relationships and inheritance patterns.

Step 1: {\color{redUnderstanding the diagram.

The figure shows two offspring arising from a single point and connected by a horizontal line at the top.

Step 2: {\color{redInterpretation.

This indicates that the twins originate from the same zygote and are therefore genetically identical.

Step 3: {\color{redIdentification.

Such a symbol represents monozygotic (identical) twins.

Step 4: {\color{redEvaluating the options.


Male \(\rightarrow\) Represented by a square (incorrect)
Monozygotic twins \(\rightarrow\) Identical twins with connecting line (correct)
Mating \(\rightarrow\) Horizontal line between two individuals (incorrect)
Dizygotic twins \(\rightarrow\) No connecting line (incorrect)


Step 5: {\color{redConclusion.

Thus, the given pedigree symbol represents monozygotic twins. Quick Tip: Remember: Twins + connecting line = Monozygotic (identical).

Concept:
Convergent evolution occurs when unrelated organisms independently evolve similar structures or functions due to similar environmental pressures.

Step 1: {\color{redUnderstanding the structures.

Wings of birds and butterflies serve the same function (flight) but have different structural origins.

Step 2: {\color{redNature of organs.

These are analogous organs — same function but different origin.

Step 3: {\color{redInterpretation.

Such similarities arise due to convergent evolution.

Step 4: {\color{redEvaluating the options.


Divergent evolution \(\rightarrow\) Same origin, different function (incorrect)
Adaptive radiation \(\rightarrow\) Evolution from common ancestor (incorrect)
Variation \(\rightarrow\) General differences (incorrect)
Convergent evolution \(\rightarrow\) Same function, different origin (correct)


Step 5: {\color{redConclusion.

Thus, wings of birds and butterflies are an example of convergent evolution. Quick Tip: Remember: Analogous organs → Convergent evolution.

Concept:
Hormonal contraceptive pills (mainly estrogen and progesterone combinations) act on the female reproductive system to prevent pregnancy.

Step 1: {\color{redActions of contraceptive pills.


Inhibit ovulation by suppressing FSH and LH
Thicken cervical mucus, preventing sperm entry
Alter endometrium, making it unsuitable for implantation


Step 2: {\color{redEvaluate each option.


Changes in cervical mucus \(\rightarrow\) Correct action
Inhibition of spermatogenesis \(\rightarrow\) Occurs in males, not by female contraceptive pills (incorrect)
Alteration in uterine endometrium \(\rightarrow\) Correct action
Inhibition of ovulation \(\rightarrow\) Primary action (correct)


Step 3: {\color{redConclusion.

Thus, inhibition of spermatogenesis is not an action of hormonal contraceptive pills. Quick Tip: Remember: Pills act on ovulation, uterus, and cervical mucus — not on sperm production.

Concept:
Asexual reproduction produces offspring from a single parent without the involvement of gametes, resulting in genetically identical individuals called clones.

Step 1: {\color{redEvaluate Assertion.

Many organisms (such as bacteria, fungi, and some plants) reproduce asexually and produce clones. Hence, Assertion is true.

Step 2: {\color{redEvaluate Reason.

Asexual reproduction involves mitotic division, producing offspring that are genetically identical to the parent. Hence, Reason is true.

Step 3: {\color{redLink between Assertion and Reason.

The reason correctly explains why asexual reproduction results in clones.

Step 4: {\color{redConclusion.

Both Assertion and Reason are true, and the Reason correctly explains the Assertion. Quick Tip: Remember: Asexual reproduction → No variation → Clones.

Concept:
Orchidectomy refers to the surgical removal of one or both testes, commonly performed for medical conditions such as testicular cancer or as part of castration.

Step 1: {\color{redEvaluate Statement I.

Castration or removal of testes is indeed called orchidectomy. Hence, Statement I is correct.

Step 2: {\color{redEvaluate Statement II.

Orchidectomy primarily affects hormone production (testosterone) and fertility. It does not directly affect urination, as the urinary system is separate from the testes.

Step 3: {\color{redConclusion.

Statement I is correct, but Statement II is incorrect. Quick Tip: Remember: Testes → Reproduction & hormones, not urination.

Concept:
Commensalism is a type of interaction where one organism benefits while the other is neither harmed nor benefited.

Step 1: {\color{redUnderstanding the relationship.

The sucker fish (remora) attaches itself to the shark using a sucker-like organ.

Step 2: {\color{redBenefit to sucker fish.


Gets free transport
Feeds on food scraps left by the shark


Step 3: {\color{redEffect on shark.

The shark is neither harmed nor significantly benefited.

Step 4: {\color{redEvaluating the options.


Predation \(\rightarrow\) One kills another (incorrect)
Competition \(\rightarrow\) Both compete (incorrect)
Parasitism \(\rightarrow\) One harmed (incorrect)
Commensalism \(\rightarrow\) One benefits, other unaffected (correct)


Step 5: {\color{redConclusion.

Thus, the relationship is commensalism. Quick Tip: Remember: Remora + Shark → One benefits, other unaffected = Commensalism.

Concept:
An ecological niche refers to the functional role and position of a species within its ecosystem.

Step 1: {\color{redOrigin of the term.

The term “niche” in ecology was first introduced by Charles Elton in 1927.

Step 2: {\color{redMeaning of niche.

It includes how an organism interacts with biotic and abiotic factors, including its habitat, food, and role.

Step 3: {\color{redEvaluating the options.


John Ray \(\rightarrow\) Early taxonomist (incorrect)
Charles Darwin \(\rightarrow\) Evolution theory (incorrect)
Jordon \(\rightarrow\) Not associated (incorrect)
Charles Elton \(\rightarrow\) Introduced niche concept (correct)


Step 4: {\color{redConclusion.

Thus, the term “niche” was first used by Charles Elton. Quick Tip: Remember: Ecological niche → Charles Elton (1927).

Concept:
Parthenogenesis is a form of reproduction in which an organism develops from an unfertilized egg.

Step 1: {\color{redTypes of parthenogenesis.


Arrhenotoky \(\rightarrow\) Only males are produced (e.g., honeybees)
Thelytoky \(\rightarrow\) Only females are produced
Amphitoky (Deuterotoky) \(\rightarrow\) Both males and females are produced


Step 2: {\color{redEvaluate options.


Arrhenotoky \(\rightarrow\) Produces males (correct)
Thelytoky \(\rightarrow\) Produces females (incorrect)
Amphitoky \(\rightarrow\) Produces both sexes (incorrect)
Both (a) and (c) \(\rightarrow\) Incorrect


Step 3: {\color{redConclusion.

Thus, arrhenotoky is the type of parthenogenesis that produces only males. Quick Tip: Remember: Arrhenotoky → Males, Thelytoky → Females.

Concept:
Certain groups of organisms are more sensitive to environmental changes and habitat destruction.

Step 1: {\color{redUnderstanding amphibians.

Amphibians have a dual life (aquatic and terrestrial) and permeable skin, making them highly sensitive to environmental changes.

Step 2: {\color{redImpact of habitat destruction.

They depend on both land and water habitats, so destruction of either severely affects their survival.

Step 3: {\color{redEvaluate options.


Amphibians \(\rightarrow\) Highly sensitive and most affected (correct)
Mammals \(\rightarrow\) More adaptable (incorrect)
Echinoderms \(\rightarrow\) Marine, less directly affected (incorrect)
Birds \(\rightarrow\) Can migrate/adapt (incorrect)


Step 4: {\color{redConclusion.

Thus, amphibians are at higher risk of extinction due to habitat destruction. Quick Tip: Remember: Amphibians = Environmental indicators → Highly sensitive.

Concept:
The human genome consists of a very large number of base pairs and genes distributed across chromosomes.

Step 1: {\color{redEvaluate each pair.


Y-chromosome \(\rightarrow\) About 200–300 genes (correct)
Chromosome-1 \(\rightarrow\) Largest chromosome with \(\sim\)2968 genes (correct)
Dystrophin \(\rightarrow\) One of the largest genes (\(\sim\)2.4 million base pairs) (correct)
Human genome \(\rightarrow\) Actually contains about \(3 \times 10^9\) base pairs, not 300 (incorrect)


Step 2: {\color{redConclusion.

Thus, the wrongly matched pair is “Human genome - 300 bases”. Quick Tip: Remember: Human genome ≈ 3 billion base pairs (not 300!).

Concept:
Ethanol is produced by fermentation of sugars using microorganisms like yeast.

Step 1: {\color{redUnderstanding substrate requirement.

Fermentation requires a sugar-rich substrate.

Step 2: {\color{redRole of molasses.

Molasses, a by-product of sugar industry, is rich in sucrose and is widely used for ethanol production.

Step 3: {\color{redEvaluate options.


Molasses \(\rightarrow\) Rich in sugars, commonly used (correct)
Soya meal \(\rightarrow\) Protein-rich, not suitable (incorrect)
Corn meal \(\rightarrow\) Used in some countries, but not most common (incorrect)
Groundgram \(\rightarrow\) Not used (incorrect)


Step 4: {\color{redConclusion.

Thus, molasses is the most common substrate used for ethanol production. Quick Tip: Remember: Ethanol fermentation → Sugar source → Molasses.

Concept:
Disinfection of wastewater involves killing harmful microorganisms before releasing water into the environment.

Step 1: {\color{redUnderstanding ideal disinfectant.

An ideal disinfectant should be effective, economical, easy to apply, and capable of destroying pathogens.

Step 2: {\color{redEvaluate the options.


Boiling \(\rightarrow\) Effective but not practical for large-scale wastewater treatment (incorrect)
U-V Rays \(\rightarrow\) Used but less effective in turbid water (incorrect)
Ozonisation \(\rightarrow\) Effective but expensive (incorrect)
Chlorination \(\rightarrow\) Widely used, economical, and effective (correct)


Step 3: {\color{redConclusion.

Thus, chlorination is considered the ideal disinfectant for wastewater treatment. Quick Tip: Remember: Wastewater treatment → Chlorination (most common and economical).

Concept:
Vaccination and public awareness programs have been highly effective in controlling certain infectious diseases.

Step 1: {\color{redDiseases controlled by vaccines.


Smallpox \(\rightarrow\) Eradicated globally through vaccination
Polio \(\rightarrow\) Controlled through mass immunization programs
Diphtheria and Tetanus \(\rightarrow\) Controlled through routine vaccination


Step 2: {\color{redEvaluate other options.


Amoebiasis \(\rightarrow\) Not controlled by vaccines (incorrect)
AIDS \(\rightarrow\) No effective vaccine available (incorrect)
Common cold \(\rightarrow\) No vaccine due to many viral strains (incorrect)


Step 3: {\color{redConclusion.

Thus, the correct group is smallpox, polio, diphtheria, and tetanus. Quick Tip: Remember: Vaccine success → Smallpox, Polio, DPT diseases.