List of top Biology Questions asked in CBSE CLASS XII

Immunity in our body is of two types: (i) Innate immunity and (ii) acquired immunity. Innate immunity is a non-specific defence mechanism, whereas acquired immunity is pathogen-specific; it is called specific immunity too. Acquired immunity is characterised by memory. Antibodies are specific to antigens and there are different types of antibodies produced in our body: they are IgA, IgE, IgG, IgC, and IgM. It shows primary response when it encounters the pathogen for the first time and secondary response during the subsequent encounters with the same antigen/pathogen.

(c) The organ transplants are often rejected if not taken from suitable compatible persons. [(i)] Mention the characteristic of our immune system that is responsible for the graft rejection. [(ii)] Name the type of immune response and the cell involved in it.

The symptoms of malaria do not appear immediately after the entry of sporozoites into the human body when bitten by female Anopheles mosquito. Explain why it happens.
Mention any four strategies adopted by flowering plants to prevent self-pollination.
Assertion (A): Restriction endonuclease recognises palindromic sequence in DNA and cuts them. Reason (R): Palindromic sequence has two unique recognition sites PstI and PvuI recognised by restriction endonuclease.
Assertion (A): The number of white winged moths decreased after industrialisation in England. Reason (R): Effects of industrialisation were more marked in rural areas of England.
For commercial and industrial production of citric acid, which one of the following microbes is used?
Select the correct statement from the following biotechnological procedures:
The decrease in the T-Lymphocytes count in human blood will finally result in

Assertion (A): The chronic use of alcohol by a person leads to cirrhosis. 
Reason (R): Alcohol addiction at times becomes the cause of mental and financial distress to the entire family of the addicted person.} 
 

A population of snakes lived in a desert with brown sand. Study the drawings given below showing the change in the population from ‘one’ to ‘two’ over time and answer the question that follows. Brown snakes and Grey snakes are represented by alleles A/a (Dominant/recessive). 

(a) If the frequency of the recessive trait is 9% in population-one, work out the frequency of homozygous dominant and heterozygous dominant snakes.
 

Mention any two applications of this technique.
Amniocentesis is a very useful and important technique, but due to some reason there is a statutory ban on amniocentesis. Justify this statement.
Why do plants discourage self-pollination? State any one reason.
Draw a T.S. of a mature anther of an angiosperm. Label its any three wall layers and mention their functions.
(a) Redraw the pie chart identifying the groups ’A’, ’B’, ’C’, and ’D’ in their respective positions. 
(b) Mention two examples of recently extinct animals.
pie chart identifying the groups ’A’, ’B’, ’C’, and ’D’ in their respective positions.
Explain the development of male gametophyte in an angiosperm.
Explain the mode of action of contraceptive pills taken by human females. Mention the schedule to be followed for an effective outcome.
Expression of different genes for different traits may show dominance, incomplete dominance, or co-dominance. Write about expression of such genes with examples for one of the above modes.
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With reference to flower color, two independent crosses were made, one between true breeding garden pea plants and another between true breeding Antirrhinum plants. Write the phenotypes of their F$_1$ progeny. Justify your answer giving reasons.
The picture given below shows:
Roots of a typical control tobacco crop plant (infected)

(a) Roots of a typical control tobacco crop plant (infected).
(b) Transgenic tobacco plant showing healthy roots even after deliberate infection by nematode.
Explain how this transformation was achieved in the tobacco plant.
Draw a schematic diagram of an antibody molecule and label any 4 parts. Mention their chemical nature. Name the cells which produce them.
Study the figures given below that depict the comparative age distribution of human populations in Sweden and Rwanda (International Data Base 2003) and answer the questions that follow:
\begin{figure[H] \centering \includegraphics[width=0.8\textwidth]{Screenshot 2025-01-17 115648.png} \caption{Population pyramids of Sweden and Rwanda showing age and gender distribution.} \label{fig:population_pyramids} \end{figure} (a) What can be inferred from the very broad base of Rwanda’s age pyramid? Support your answer with the data provided in the figure.}
Match the items in Column I with those in Column II and select the correctly matched option from those given below: 
Cross Phenotypic Ratio
(a) Differentiate between humoral immune response and cell-mediated immune response.
(b) Draw a schematic diagram of an antibody molecule and label any four parts.
(a) (i) Compare the pattern of inheritance of flower colour in garden pea plant (violet/white) with snapdragon plant (red/white) on the basis of the following:
1. F1 phenotypic expression.
2. Expected phenotypic and genotypic expression of F2 generation.
3. Conclusion you reached at the end of the comparison.
(ii) List any two characteristics of pattern of inheritance of human blood group ABO.
OR
(b) (i) Draw a schematic, self-explanatory labelled diagram of lac operon in a ‘switched on’ condition.
(ii) Why is regulation of lac operon referred to as negative regulation?