CBSE Class 12

Pre Boards

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Sample Papers

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 Multiple Choice QuestionsShort Answer Type

21. What is inbreeding depression and how is it caused in organisms? Write any two advantages of inbreeding.

Inbreeding depression is the reduction in the fertility and productivity of an organism due to continuous inbreeding. Inbreeding is the mating of closely-related individuals within the same breed for 4-6 generations. Continued inbreeding, especially close inbreeding usually reduces the fertility and even productivity of the organism. Two advantages of inbreeding are:

(i) Superior quality of breed of organisms is obtained.

(ii) Pure lines can be developed.



The diagram above is that of a typical biogas plant. Explain the seQ uence of events occurring in a biogas plant. Identify a, b and c.

The sequences of events occurring in a biogas plant are:

(i) Hydrolysis: Through hydrolysis the complex organic molecules are broken down into simple sugars, amino acids and fatty acids.

(ii) Acidogenesis: In this step further breakdown of the remaining components by acidogenic (fermentative) bacteria takes place.

(iii) Acetogenesis: In this step simple molecule obtained by acidogenesis phase are further digested by acetogens, to produce largely acetic acid as well as carbon dioxide and hydrogen.

(iv) Methanogenesis: In this step methanogens utilize the intermediate products of the previous stages and convert them into methane, carbon dioxide and water.

The parts of biogas plant:

The part labelled (a) is sludge

The part labelled (b) is a gas holder, and

The part labelled (c) is inlet to feed slurry



(a) Identify (A) and (B) illustrations in the following:



(b) Write the term given to (A) and (C) and why?

(c) Expand PCR. Mention its importance in biotechnology.

(a)  A represents - Sticky end.

      B represents - Foreign DNA insert.

(b) The term used for A is Sticky ends, it is called so because they form hydrogen bonds with their complementary cut counterparts. Their stickiness of the ends facilitates the action of the enzyme DNA ligase.

C are called the palindromic nucleotide sequence. These are named so because the sequence of base pairs reads same on the two strands when orientation of reading is kept the same

(c) PCR stands for Polymerase Chain Reaction. PCR is a technique used to synthesize multiple copies of the gene (or DNA) of interest in vitro using two sets of primers (small chemically synthesized oligonucleotides that are complementary to the regions of DNA) and the enzyme DNA polymerase. It is extensively used in the process of gene manipulation.



(a) Sickle celled anaemia in humans is a result of point mutation. Explain.

(b) Write the genotypes of both the parents who have produced a sickle celled anaemic offspring.

(a) Sickle cell anaemia in humans is a result of point mutation because a single base change in the gene for beta globin chain results in the change of amino acid residue glutamate to valine leads to the replacement of GAG by GUG codon at sixth position.

(b) Genotypes of parents who have produced a sickle celled anaemic offspring can be either 

HbA/ Hbs and HbA/ Hbs 

or,  Hbs/ Hbs and HbA/ Hbs


 Multiple Choice QuestionsLong Answer Type


(a) Draw a labelled diagram of the human female reproductive system. 

(b) Enumerate the events in the ovary of a human female during: 

(i) Follicular phase

(ii) Luteal phase of menstrual cycle



(i) Follicular phase: Follicular phase is also called the proliferative phase. During this phase the FSH released by the pituitary stimulates the growth of the primary ovarian follicle and also causes maturation of the primary oocyte in this follicle. The follicular cells of the Graafian follicle secrete oestrogen. Due to an increase level of estrogen the uterine endometrium becomes thick, more vascular and more glandular. This phase lasts for about 10 to 14 days, or until ovulation occurs. The luteinizing hormone (LH) released by the pituitary induces the mature Graafian follicle to burst and release the egg or ovum into the fallopian tube. This is called ovulation.

(ii) Luteal phase: The luteal phase is also called secretory phase or the post -ovulatory phase. It follows the ovulation. During this phase the corpus luteum continues to release the hormone, progesterone. It lasts for about 12 - 14 days and extends from the 16th - 28th day of the menstrual cycle.


 Multiple Choice QuestionsShort Answer Type

26. How are dominance, co-dominance and incomplete dominance patterns of inheritance different from each other?

Dominance: is the condition when in a pair of dissimilar factors or alleles, one dominates the other that is one allele expresses itself in the presence or absence of the other allele. For example, the allele for tall trait T is dominant over the trait for dwarfness t.

Co-dominance: Co-dominance is the phenomenon in which both the alleles of a contrasting character are expressed in heterozygous condition. Both the alleles of a gene are equally dominant. ABO blood group system is an example of co-dominance.

Incomplete dominance: Incomplete dominance is a phenomenon in which one allele shows incomplete dominance over the other member of the allelic pair for a character. For example, a monohybrid cross between the plants having red flowers and white flowers in Antirrhinum species will result in all pink flower plants in F1generation.



How can crop varieties be made disease resistant to overcome food crisis in India? Explain.

Name one disease resistant variety in India of:

(a) Wheat to leaf and stripe rust

(b) Brassica to white rust

A wide range of fungal, bacterial and viral pathogens affect the yield of cultivated crops. Disease resistance can be provided by conventional breeding, mutational breeding or genetic engineering.

(i) Conventional breeding: It includes the basic steps of screening, germplasm, hybridisation, selection, testing and release.

(ii) Mutational breeding: In this method, genetic variations are created, which then result in the creation of traits not found in the parental type.

 Genetic engineering:

(1) Certain wild varieties have disease-resistant characteristics, but they are low yielding.

(2) Disease-resistant genes from such varieties are introduced in high-yielding varieties through recombinant DNA technology.

One disease resistant variety in India of:

(a) Wheat to leaf and stripe rust: Himgiri

(b) Brassica to white rust: Pusa swarnim



Write the source and the effect on the human body of the following drugs:

(i) Morphine

(ii) Cocaine

(iii) Marijuana

The source and effect of the following drugs are:

(i) Morphine:

 Source- latex of poppy plants (Papaver somniferum)

Effect- It is a depressant; slows down body functions

(ii) Cocaine:

Source: Coca plant Erythroxylum coca, found in South America Effect: Stimulates the CNS, producing a sense of euphoria and increased energy; excessive dosages cause hallucination.

(iii) Marijuana:

Source: Inflorescences of the plant Cannabis sativa

Effect: Affects the cardiovascular system.


Name the type of interaction seen in each of the following examples:

(i) Ascaris worms living in the intestine of human

(ii) Wasp pollinating fig inflorescence

(iii) Clown fish living among the tentacles of sea-anemone

(iv) Mycorrhizae living on the roots of higher plants

(v) Orchid growing on a branch of a mango tree

(vi) Disappearance of smaller barnacles when Balanus dominated in the Coast of Scotland.

(i) Ascaris worms living in the intestine of human -Parasitism

(ii) Wasp pollinating fig inflorescence - Mutualism

(iii) Clown fish living among the tentacles of sea-anemone - Commensalism

(iv) Mycorrhizae living on the roots of higher plants- Mutualism

(v) Orchid growing on a branch of a mango tree - Commensalism

(vi) Disappearance of smaller barnacles when Balanus dominated in the Coast of Scotland - Competition



The base sequence in one of the strands of DNA is TAGCATGAT.

(i) Give the base sequence of its complementary strand.

(ii) How are these base pairs held together in a DNA molecule?

(iii) Explain the base complementarity rules. Name the scientist who framed this rule.

(i) The base sequence of the complementary strand will be - ATCGTACTA.

(ii) The base pairs in the DNA molecules are held together by hydrogen bonds. There are two hydrogen bonds between adenine and thymine and three hydrogen bonds between guanine and cytosine.

(iii) Base Complementarity Rule: A purine always pairs with a pyrimidine in a DNA molecule i.e. 

A will pair with T and G will pair with C.

The ratio of A and T or C and G will always be 1.

Erwin Chargaff framed the base complementarity rule.