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Class 12 Biology
CBQ Practice

Competency Based Questions · 3 chapters · 6 CBQ sets

Question types:Case StudySource BasedAssertion–Reason
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Ch 2

Sexual Reproduction in Flowering Plants

2 sets
CBQ 1Case StudyDouble Fertilisation in Angiosperms4 marks

Read the passage

In angiosperms, a remarkable process called double fertilisation occurs. A pollen grain germinates on the stigma and produces a pollen tube. The pollen tube carries two male gametes. One male gamete (n) fuses with the egg cell (n) to form the zygote (2n) — this is syngamy. The other male gamete (n) fuses with the secondary nucleus (2n) in the central cell to form the Primary Endosperm Nucleus (PEN, 3n) — this is triple fusion. The zygote develops into the embryo and the PEN develops into the endosperm, which provides nutrition to the developing embryo. This entire process, unique to angiosperms, is called double fertilisation — discovered by Nawaschin in 1898.
1

The product of fusion of one male gamete with the secondary nucleus is:

1M
(A)Zygote (2n)
(B)Endosperm (3n)
(C)Pollen grain (n)
(D)Antipodal cells (n)
2

Double fertilisation was first observed by:

1M
(A)Gregor Mendel
(B)Nawaschin
(C)Darwin
(D)Strasburger
3

What is the role of the endosperm in angiosperms?

1M
(A)It forms the seed coat
(B)It provides nutrition to the developing embryo
(C)It forms the fruit wall
(D)It carries out pollination
4

Distinguish between syngamy and triple fusion in double fertilisation.

1M
CBQ 2Assertion–Reason1 mark
A
Assertion

Wind-pollinated flowers are usually small, dull-coloured, and produce large quantities of pollen.

R
Reason

Wind-pollinated flowers do not need to attract insects or other animals, so they lack attractive features like bright colours and nectar, but produce abundant lightweight pollen to increase the chance of reaching the stigma.

(A) Both A and R are true and R is the correct explanation of A
(B) Both A and R are true but R is not the correct explanation of A
(C) A is true but R is false
(D) A is false but R is true
Ch 5

Principles of Inheritance and Variation

2 sets
CBQ 1Case StudyMendel's Dihybrid Cross4 marks

Read the passage

Gregor Mendel crossed pea plants with two pairs of contrasting characters: seed colour (Yellow, Y dominant; green, y recessive) and seed shape (Round, R dominant; wrinkled, r recessive). The parental plants were YYRR (yellow round) × yyrr (green wrinkled). The F₁ generation was YyRr (yellow round). When Mendel selfed the F₁ plants, the F₂ generation showed four phenotypes: yellow round, yellow wrinkled, green round, and green wrinkled in a 9:3:3:1 ratio. This experiment led Mendel to formulate his Law of Independent Assortment, which states that the factors for different characters assort independently of each other during gamete formation when not on the same chromosome.
1

The phenotypic ratio observed in the F₂ generation of a dihybrid cross is:

1M
(A)3:1
(B)1:2:1
(C)9:3:3:1
(D)1:1:1:1
2

How many different types of gametes does the F₁ plant (YyRr) produce?

1M
(A)2
(B)4
(C)8
(D)16
3

Mendel's Law of Independent Assortment applies when the two genes are:

1M
(A)Located on the same chromosome close together
(B)Both recessive
(C)Located on different (non-homologous) chromosomes
(D)Codominant alleles
4

In the F₂ of a dihybrid cross (YyRr × YyRr), how many plants (out of 16) will have the genotype YyRr? What phenotype will they show?

1M
CBQ 2Assertion–Reason1 mark
A
Assertion

In humans, sex is determined by the type of sex chromosome contributed by the father (sperm), not the mother (egg).

R
Reason

All eggs produced by human females carry the X chromosome, while sperm can carry either X or Y. If an X-bearing sperm fertilises the egg, a girl (XX) is born; if a Y-bearing sperm fertilises the egg, a boy (XY) is born.

(A) Both A and R are true and R is the correct explanation of A
(B) Both A and R are true but R is not the correct explanation of A
(C) A is true but R is false
(D) A is false but R is true
Ch 13

Organisms and Populations

2 sets
CBQ 1Case StudyPopulation Growth Models4 marks

Read the passage

Ecologists study how populations grow over time. Two main growth models are used. In the exponential growth model (J-curve), resources are assumed unlimited: dN/dt = rN, where N = population size, t = time, r = intrinsic rate of natural increase. This gives a J-shaped growth curve. In the logistic growth model (S-curve or sigmoid curve), resources are limited and growth slows as population approaches the carrying capacity (K): dN/dt = rN(K−N)/K. The term (K−N)/K is the 'unutilised growth potential'. When N = K, growth stops. Organisms like bacteria show exponential growth in unlimited lab conditions, while most natural populations follow the logistic model. The concept of carrying capacity is important in wildlife conservation and fisheries management.
1

In the logistic growth equation dN/dt = rN(K−N)/K, the population growth rate is maximum when:

1M
(A)N = K
(B)N = K/2
(C)N = 0
(D)N > K
2

The 'J-shaped' growth curve in population ecology represents:

1M
(A)Logistic growth with limited resources
(B)Exponential growth with unlimited resources
(C)Population decline due to predation
(D)Seasonal fluctuation in population
3

Carrying capacity (K) in ecology refers to:

1M
(A)The maximum growth rate of a population
(B)The maximum population size a given environment can sustainably support
(C)The minimum viable population for survival
(D)The number of offspring per individual per year
4

Compare exponential and logistic growth in terms of the shape of the curve and the key assumption of each model.

1M
CBQ 2Assertion–Reason1 mark
A
Assertion

Interspecific competition generally has a more severe effect on both competing species than intraspecific competition.

R
Reason

Members of the same species have identical resource requirements and compete more intensely with each other than members of different species.

(A) Both A and R are true and R is the correct explanation of A
(B) Both A and R are true but R is not the correct explanation of A
(C) A is true but R is false
(D) A is false but R is true

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