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WBBSE Madhyamik Life Science (English Adaptation) study materials
WBBSE Madhyamik Life Science (English Adaptation) study resources

WBBSE Madhyamik Life Science (English Adaptation) practice questions

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Question 1Control and Coordination in Living Organisms

In the human nervous system, the gap between two adjacent neurons across which a nerve impulse is transmitted chemically is called the:

  1. Neuromuscular junction
  2. Myelin sheath
  3. Node of Ranvier
  4. Synapse Correct answer.

Explanation

A synapse is the functional junction between the axon terminal of one neuron and the dendrite or cell body of the next neuron. The impulse crosses this gap by diffusion of a neurotransmitter such as acetylcholine. The node of Ranvier and myelin sheath are parts of a single axon, not a junction between two neurons.

Question 2Control and Coordination in Living Organisms

A person suffers damage to the medulla oblongata after a road accident. Which function is most directly at risk?

  1. Storage of long-term memory of events
  2. Maintenance of balance and posture during walking
  3. Interpretation of visual images
  4. Regulation of heartbeat and breathing rhythm Correct answer.

Explanation

The medulla oblongata contains the vital centres that control involuntary activities such as heartbeat, breathing and blood pressure. Visual interpretation occurs in the occipital lobe, balance is coordinated by the cerebellum, and long-term memory storage involves the cerebrum and limbic structures.

Question 3Control and Coordination in Living Organisms

Which of the following is a plant hormone that promotes cell elongation and is produced mainly at the growing shoot apex?

  1. Abscisic acid
  2. Ethylene
  3. Auxin Correct answer.
  4. Cytokinin

Explanation

Auxin, chiefly indole acetic acid, is synthesised at the shoot apex and moves downward, promoting cell elongation on the shaded side and causing bending towards light. Abscisic acid is a growth inhibitor, ethylene ripens fruit, and cytokinin mainly promotes cell division.

Question 4Control and Coordination in Living Organisms

A student touches a hot pan and pulls her hand away before she consciously feels the heat. The pathway responsible is best described as:

  1. A reflex arc in which the spinal cord acts as the integrating centre Correct answer.
  2. A hormonal loop involving the adrenal medulla only
  3. A cerebellar reflex that bypasses sensory neurons
  4. A voluntary response initiated in the cerebral cortex

Explanation

In a reflex action the sensory neuron carries the impulse to the spinal cord, where an interneuron relays it directly to a motor neuron, producing a quick protective response without waiting for the brain. The brain receives the information at the same time but conscious perception is slower. Every reflex arc still begins with a sensory receptor and a sensory neuron.

Question 5Control and Coordination in Living Organisms

The hormone secreted by the thyroid gland that lowers blood calcium level is:

  1. Parathormone
  2. Insulin
  3. Calcitonin Correct answer.
  4. Thyroxine

Explanation

Calcitonin from the thyroid gland lowers blood calcium by promoting deposition of calcium in bone and reducing its release. Parathormone from the parathyroid glands has the opposite effect and raises blood calcium. Thyroxine controls metabolic rate and insulin controls blood glucose.

Question 6Control and Coordination in Living Organisms

The autonomic nervous system is best characterised as the part of the nervous system that:

  1. Controls skeletal muscles under conscious will
  2. Controls involuntary activities of smooth muscle, cardiac muscle and glands Correct answer.
  3. Contains only sensory neurons and no motor neurons
  4. Carries impulses from the eye to the occipital lobe

Explanation

The autonomic nervous system regulates involuntary effectors such as smooth muscle of the gut, cardiac muscle and glands, and is divided into sympathetic and parasympathetic divisions. Skeletal muscle under conscious control is served by the somatic nervous system, and sensory pathways belong to the afferent division.

Question 7Control and Coordination in Living Organisms

Geotropism in a germinating bean root is demonstrated when the root grows downward into the soil. This response occurs because:

  1. Auxin accumulates on the upper side and stimulates elongation of root cells there
  2. Ethylene accumulates on the lower side and stimulates root cell division
  3. Auxin accumulates on the lower side and inhibits elongation of root cells there Correct answer.
  4. Light destroys auxin on the lower side of the root

Explanation

Gravity causes auxin to accumulate on the lower side of a horizontally placed root. Root cells are far more sensitive to auxin than shoot cells, so the higher concentration inhibits elongation on the lower side, and the upper side grows faster, bending the root downward. In shoots the same uneven distribution promotes elongation on the lower side and causes upward bending.

Question 8Control and Coordination in Living Organisms

Which sequence correctly traces the path of a nerve impulse in a reflex arc?

  1. Receptor → sensory neuron → spinal cord → motor neuron → effector Correct answer.
  2. Effector → sensory neuron → spinal cord → motor neuron → receptor
  3. Receptor → motor neuron → spinal cord → sensory neuron → effector
  4. Receptor → interneuron → sensory neuron → brain → effector

Explanation

A reflex arc begins at a receptor that detects the stimulus, and the impulse travels along the sensory neuron to the spinal cord. There an interneuron passes it to a motor neuron, which carries it to the effector muscle or gland. The direction is always receptor to effector, never the reverse.

Question 9Control and Coordination in Living Organisms

A patient shows excessive thirst, frequent urination and high blood glucose. A deficiency of which hormone is most likely responsible?

  1. Thyroxine
  2. Insulin Correct answer.
  3. Glucagon
  4. Adrenaline

Explanation

These are classic signs of diabetes mellitus, in which the beta cells of the pancreatic islets secrete too little insulin. Without insulin, cells cannot take up glucose efficiently, blood glucose rises and excess glucose is excreted in urine, drawing water with it. Adrenaline and glucagon raise blood glucose, and thyroxine affects metabolic rate.

Question 10Control and Coordination in Living Organisms

In a nerve fibre, the impulse travels much faster along a myelinated axon than along an unmyelinated one because:

  1. Myelinated axons release more neurotransmitter at each synapse
  2. The impulse jumps from one node of Ranvier to the next, a process called saltatory conduction Correct answer.
  3. Myelin conducts the electrical impulse like a copper wire
  4. Myelin prevents depolarisation, so no ion movement is needed

Explanation

Myelin is an insulating lipid sheath that blocks ion flow across the membrane except at the nodes of Ranvier. Depolarisation therefore occurs only at successive nodes, so the impulse appears to jump forward, which greatly increases conduction velocity. This jumping mechanism is called saltatory conduction.

Question 11Control and Coordination in Living Organisms

A potted plant kept near a window grows with its stem bending towards the light. Which explanation is correct?

  1. Auxin is evenly distributed but the illuminated side grows faster
  2. Auxin moves to the shaded side, where it promotes greater cell elongation Correct answer.
  3. Light destroys auxin on the shaded side, so that side grows faster
  4. Auxin moves to the illuminated side, where it promotes greater cell elongation

Explanation

Unilateral light causes lateral transport of auxin from the illuminated side to the shaded side of the shoot. The higher auxin concentration on the shaded side stimulates greater elongation of cells there, so the stem bends towards the light. This is positive phototropism.

Question 12Control and Coordination in Living Organisms

Which statement about the cerebellum is correct?

  1. It is the main centre for regulating body temperature and hunger
  2. It secretes hormones that control the pituitary gland
  3. It coordinates muscular activity and maintains balance and posture Correct answer.
  4. It receives all sensory impulses and produces conscious sensation

Explanation

The cerebellum lies behind the brain stem and coordinates voluntary muscular movements, maintaining balance, posture and smooth execution of movement. Body temperature and hunger are regulated by the hypothalamus, and conscious sensation is a function of the cerebral cortex.

Question 13Control and Coordination in Living Organisms

The pituitary gland is often called the master gland because:

  1. Its hormones regulate the activity of several other endocrine glands Correct answer.
  2. It is the largest endocrine gland in the human body
  3. It produces insulin and glucagon for glucose regulation
  4. It directly controls all reflex actions of the spinal cord

Explanation

The anterior pituitary secretes trophic hormones such as TSH, ACTH and gonadotropins that stimulate the thyroid, adrenal cortex and gonads. Because its secretions govern other glands, it is termed the master gland. It is small in size and does not control spinal reflexes or secrete insulin.

Question 14Control and Coordination in Living Organisms

A person in danger shows increased heart rate, dilated pupils and raised blood glucose. These changes are mainly due to:

  1. Increased secretion of insulin from the pancreas
  2. Increased secretion of calcitonin from the thyroid
  3. Increased parasympathetic activity from the vagus nerve
  4. Increased secretion of adrenaline from the adrenal medulla Correct answer.

Explanation

Adrenaline released from the adrenal medulla during emergencies prepares the body for fight or flight. It speeds the heart, dilates the pupils, raises blood glucose and increases breathing rate. Parasympathetic activity slows the heart, and insulin lowers blood glucose.

Question 15Control and Coordination in Living Organisms

The hormone that promotes cell division and delays ageing of leaves is:

  1. Abscisic acid
  2. Cytokinin Correct answer.
  3. Auxin
  4. Gibberellin

Explanation

Cytokinins stimulate cell division in meristems and delay senescence by preventing the breakdown of chlorophyll and proteins in leaves. Auxin and gibberellin mainly promote elongation, while abscisic acid promotes dormancy and leaf fall.

Question 16Control and Coordination in Living Organisms

A student explains that the nervous system and the endocrine system both coordinate body activities. Which difference between them is correctly stated?

  1. Nervous responses are rapid and short-lived, while hormonal responses are slower and longer-lasting Correct answer.
  2. Nervous responses travel through blood, while hormones travel along nerve fibres
  3. The nervous system uses chemical messengers, while the endocrine system uses electrical impulses
  4. Hormones act only on the cells that secrete them, while nerve impulses act on distant organs

Explanation

Nerve impulses travel along neurons and produce quick responses that end soon after the stimulus stops. Hormones are secreted into the blood, reach target cells more slowly and usually exert effects over a longer period. Both systems are coordinating mechanisms but differ in speed and duration.

Question 17Control and Coordination in Living Organisms

The three main parts of a neuron are:

  1. Nucleus, myelin sheath and synapse
  2. Dendrite, receptor and effector
  3. Axon, node of Ranvier and neurotransmitter
  4. Cell body, dendrites and axon Correct answer.

Explanation

A neuron consists of a cell body containing the nucleus, short branched dendrites that receive impulses, and a long axon that carries impulses away. The myelin sheath and nodes of Ranvier are coverings and features of the axon, not separate main parts, and a synapse is a junction between neurons.

Question 18Control and Coordination in Living Organisms

A gardener wants to induce early flowering and uniform ripening in a fruit crop. Which pair of plant hormones would be most relevant?

  1. Ethylene for flowering and cytokinin for ripening
  2. Gibberellin for flowering and ethylene for ripening Correct answer.
  3. Auxin for flowering and abscisic acid for ripening
  4. Abscisic acid for flowering and auxin for ripening

Explanation

Gibberellins can promote bolting and early flowering in several plants, and ethylene is the hormone that triggers ripening of climacteric fruits. Abscisic acid inhibits growth and cytokinin delays senescence, so neither matches these commercial aims.

Question 19Control and Coordination in Living Organisms

Which of the following correctly pairs a hormone with its source gland?

  1. Insulin - islets of Langerhans of the pancreas Correct answer.
  2. Glucagon - anterior pituitary
  3. Thyroxine - adrenal cortex
  4. Adrenaline - thyroid gland

Explanation

Insulin is secreted by the beta cells of the islets of Langerhans in the pancreas. Thyroxine comes from the thyroid, adrenaline from the adrenal medulla, and glucagon from the alpha cells of the pancreatic islets.

Question 20Control and Coordination in Living Organisms

In a laboratory demonstration, the tip of a coleoptile is removed and the stump is exposed to unilateral light. What is the expected result?

  1. The stump bends away from the light, because auxin accumulates on the illuminated side
  2. The stump does not bend, because the source of auxin has been removed Correct answer.
  3. The stump grows straight but faster, because auxin is no longer inhibited
  4. The stump bends towards the light, because auxin is produced in the stump

Explanation

Auxin is produced at the tip of the coleoptile, so removing the tip eliminates the hormone supply. Without auxin there is no unequal elongation, and the stump shows little or no bending towards light. This classic experiment established that the tip is the site of auxin production and light perception.

Question 21Control and Coordination in Living Organisms

A patient's blood report shows low blood calcium and muscle twitching. Which hormonal disturbance best explains this?

  1. Deficiency of parathormone from the parathyroid glands Correct answer.
  2. Excess of calcitonin from the thyroid gland
  3. Deficiency of thyroxine from the thyroid gland
  4. Excess of insulin from the pancreatic islets

Explanation

Parathormone raises blood calcium by promoting release of calcium from bone and its reabsorption in the kidney. Its deficiency lowers blood calcium, which increases excitability of nerves and muscles and causes twitching or tetany. Excess calcitonin would lower calcium but is not the usual cause of this clinical picture, and thyroxine and insulin do not regulate calcium.

Question 22Control and Coordination in Living Organisms

A drug blocks the release of acetylcholine at the neuromuscular junction. Which immediate consequence is expected?

  1. Sensory impulses fail to reach the spinal cord from the receptor
  2. Nerve impulses reach the muscle but the muscle fails to contract Correct answer.
  3. The myelin sheath of the motor neuron is destroyed
  4. The hormone insulin is no longer secreted by the pancreas

Explanation

Acetylcholine is the neurotransmitter that carries the impulse across the neuromuscular junction from the motor neuron to the muscle fibre. If its release is blocked, the impulse cannot cross the gap, so the muscle membrane is not depolarised and contraction fails. Sensory pathways and hormone secretion are unaffected by this junction-specific block.

Question 23Control and Coordination in Living Organisms

Which observation would best support the conclusion that a particular response in a plant is a tropism rather than a nastic movement?

  1. The direction of the response depends on the direction of the external stimulus Correct answer.
  2. The response occurs rapidly and is independent of the direction of the stimulus
  3. The response is caused by a hormone moving through the phloem
  4. The response occurs only in dead cells of the plant body

Explanation

A tropism is a directional growth response in which the direction of bending is determined by the direction of the stimulus, as in phototropism and geotropism. Nastic movements such as the closing of a touch-me-not leaf are non-directional with respect to the stimulus. Hormone transport and cell viability are not the defining criteria.

Question 24Control and Coordination in Living Organisms

A student claims that a spinal reflex such as the knee jerk is entirely independent of the brain. Which statement best evaluates this claim?

  1. The reflex is integrated in the brain and only relayed through the spinal cord
  2. The claim is fully correct, since the brain plays no role in any reflex action
  3. The reflex is integrated in the spinal cord, but the brain is still informed and can modify the response Correct answer.
  4. The claim is fully correct, since the spinal cord replaces the brain in all nervous functions

Explanation

The reflex arc is completed within the spinal cord, so the response occurs without waiting for the brain. However, sensory information is also carried upward to the brain, which can influence or inhibit the reflex through descending pathways. Thus the reflex is spinal in its integration but not wholly isolated from the brain.

Question 25Continuity of Life: Cell Division, Reproduction and Heredity

During which phase of mitosis do the sister chromatids separate and move to opposite poles of the cell?

  1. Telophase
  2. Metaphase
  3. Prophase
  4. Anaphase Correct answer.

Explanation

In anaphase, the centromeres split and the sister chromatids are pulled apart by spindle fibres towards opposite poles. This ensures that each daughter nucleus receives one copy of each chromosome. Prophase involves chromosome condensation, metaphase alignment, and telophase nuclear reformation.

Question 26Continuity of Life: Cell Division, Reproduction and Heredity

Which of the following is the main purpose of meiosis in sexually reproducing organisms?

  1. To halve the chromosome number and generate genetic variation Correct answer.
  2. To increase the number of chromosomes in gametes
  3. To produce genetically identical daughter cells
  4. To repair damaged tissues by replacing old cells

Explanation

Meiosis reduces the chromosome number by half, producing haploid gametes, and introduces genetic variation through crossing over and independent assortment. This is essential for sexual reproduction so that fertilisation restores the diploid number. Mitosis, by contrast, produces identical cells for growth and repair.

Question 27Continuity of Life: Cell Division, Reproduction and Heredity

In a cross between a pure-breeding tall pea plant (TT) and a pure-breeding dwarf pea plant (tt), what will be the phenotype of the F1 generation?

  1. Half tall and half dwarf
  2. All tall Correct answer.
  3. All dwarf
  4. Three tall to one dwarf

Explanation

The F1 offspring will all be heterozygous (Tt) because each parent contributes one allele. Since the tall allele (T) is dominant over the dwarf allele (t), every F1 plant will show the tall phenotype. The 3:1 ratio appears only in the F2 generation after self-pollination of the F1.

Question 28Continuity of Life: Cell Division, Reproduction and Heredity

Which of the following structures is directly involved in the formation of the spindle fibres during cell division?

  1. Centriole Correct answer.
  2. Golgi body
  3. Ribosome
  4. Lysosome

Explanation

Centrioles migrate to opposite poles during prophase and organise the spindle fibres that attach to chromosomes. Ribosomes synthesise proteins, Golgi bodies modify and package proteins, and lysosomes contain digestive enzymes. None of these other organelles form the spindle apparatus.

Question 29Continuity of Life: Cell Division, Reproduction and Heredity

A human somatic cell contains 46 chromosomes. How many chromosomes will be present in each daughter cell after mitosis?

  1. 46 Correct answer.
  2. 23
  3. 92
  4. 12

Explanation

Mitosis produces two daughter cells that are genetically identical to the parent cell, so the chromosome number is maintained. If the parent cell has 46 chromosomes, each daughter cell will also have 46 chromosomes. Reduction to 23 occurs only in meiosis.

Question 30Continuity of Life: Cell Division, Reproduction and Heredity

Which of the following is a primary function of the placenta in human reproduction?

  1. Production of sperm cells
  2. Secretion of digestive enzymes
  3. Storage of amniotic fluid
  4. Exchange of nutrients and gases between mother and foetus Correct answer.

Explanation

The placenta is the organ of exchange between the maternal and foetal blood, allowing nutrients and oxygen to pass to the foetus and waste products to be removed. It also secretes hormones such as progesterone, but its main role is exchange. It does not produce sperm or store amniotic fluid.

Question 31Continuity of Life: Cell Division, Reproduction and Heredity

Which of the following is the correct sequence of stages in the human menstrual cycle?

  1. Menstruation → Luteal phase → Ovulation → Follicular phase
  2. Follicular phase → Ovulation → Luteal phase → Menstruation Correct answer.
  3. Ovulation → Follicular phase → Luteal phase → Menstruation
  4. Luteal phase → Ovulation → Follicular phase → Menstruation

Explanation

The menstrual cycle begins with the follicular phase, during which an egg matures. Ovulation then occurs, followed by the luteal phase, and if fertilisation does not occur, menstruation follows. This order reflects the hormonal changes controlled by FSH, LH, oestrogen, and progesterone.

Question 32Continuity of Life: Cell Division, Reproduction and Heredity

Which of the following is a difference between mitosis and meiosis?

  1. Mitosis produces four daughter cells while meiosis produces two
  2. Mitosis produces diploid cells while meiosis produces haploid cells Correct answer.
  3. Mitosis involves crossing over while meiosis does not
  4. Mitosis occurs in gametes while meiosis occurs in somatic cells

Explanation

Mitosis maintains the chromosome number, producing diploid daughter cells, whereas meiosis halves the chromosome number, producing haploid cells. Mitosis produces two cells and meiosis produces four. Crossing over occurs in meiosis, not mitosis.

Question 33Continuity of Life: Cell Division, Reproduction and Heredity

In a dihybrid cross between two heterozygous pea plants (RrYy × RrYy), what is the expected phenotypic ratio in the offspring?

  1. 9:3:3:1 Correct answer.
  2. 1:2:1
  3. 3:1
  4. 1:1:1:1

Explanation

When two heterozygotes for two independently assorting traits are crossed, the offspring show a 9:3:3:1 phenotypic ratio. This ratio reflects the independent assortment of alleles during gamete formation. A 3:1 ratio is typical of a monohybrid cross, and 1:1:1:1 is from a test cross.

Question 34Continuity of Life: Cell Division, Reproduction and Heredity

A woman with blood group AB marries a man with blood group O. What blood groups are possible in their children?

  1. A and B only Correct answer.
  2. Only AB
  3. A, B, AB, and O
  4. A, B, and AB

Explanation

The woman with AB blood has genotype I^A I^B and the man with O blood has genotype ii. Their children can inherit I^A from the mother and i from the father (blood group A) or I^B from the mother and i from the father (blood group B). Thus only A and B are possible; AB and O are not.

Question 35Continuity of Life: Cell Division, Reproduction and Heredity

In humans, red-green colour blindness is an X-linked recessive trait. A colour-blind man marries a woman with normal vision who is a carrier. What is the probability that a son will be colour-blind?

  1. 50% Correct answer.
  2. 0%
  3. 100%
  4. 25%

Explanation

The man is X^c Y and the carrier woman is X^C X^c. A son receives the Y chromosome from his father and one of the mother's X chromosomes. There is a 50% chance he receives X^c and is colour-blind, and a 50% chance he receives X^C and has normal vision. Daughters, however, have a 50% chance of being carriers and 50% chance of being colour-blind.

Question 36Continuity of Life: Cell Division, Reproduction and Heredity

Which of the following best explains why the offspring of sexual reproduction show more genetic variation than those of asexual reproduction?

  1. Asexual reproduction always produces sterile offspring
  2. Sexual reproduction produces more offspring than asexual reproduction
  3. Sexual reproduction involves meiosis and fertilisation, which reshuffle alleles Correct answer.
  4. Sexual reproduction involves mitosis, which introduces new mutations

Explanation

Meiosis generates genetic variation through crossing over and independent assortment, and fertilisation combines alleles from two different parents. These processes create new combinations of alleles in the offspring. Asexual reproduction, relying on mitosis, produces genetically identical clones.

Question 37Continuity of Life: Cell Division, Reproduction and Heredity

A cell in the G1 phase of the cell cycle has 20 chromosomes. After completing the S phase, how many chromatids will be present in the cell?

  1. 10
  2. 20
  3. 40 Correct answer.
  4. 80

Explanation

During the S phase, each chromosome replicates to form two sister chromatids. If there were 20 chromosomes in G1, after S phase there will still be 20 chromosomes but each consists of two chromatids, giving 40 chromatids in total. The chromosome number remains 20 until anaphase.

Question 38Continuity of Life: Cell Division, Reproduction and Heredity

Which of the following is the most likely consequence of a failure of cytokinesis after mitosis?

  1. Four daughter cells with half the normal chromosome number
  2. Apoptosis of the cell immediately
  3. Two daughter cells with half the normal chromosome number
  4. One cell with double the normal chromosome number Correct answer.

Explanation

If mitosis occurs but cytokinesis fails, the cell does not divide into two. The nuclear division produces two nuclei within one cell, resulting in a single cell with double the normal chromosome number (polyploidy). This is common in some plant cells and can lead to polyploid organisms.

Question 39Continuity of Life: Cell Division, Reproduction and Heredity

Which of the following is a function of the hormone testosterone in human males?

  1. Stimulation of milk production
  2. Development of male secondary sexual characteristics Correct answer.
  3. Induction of ovulation
  4. Regulation of the menstrual cycle

Explanation

Testosterone is produced by the interstitial cells of the testes and is responsible for the development of male secondary sexual characteristics such as deepening of the voice, growth of facial hair, and muscle development. It also stimulates sperm production. Milk production and ovulation are regulated by female hormones.

Question 40Continuity of Life: Cell Division, Reproduction and Heredity

In a cross between a heterozygous tall pea plant (Tt) and a dwarf pea plant (tt), what proportion of the offspring will be dwarf?

  1. 0%
  2. 50% Correct answer.
  3. 100%
  4. 25%

Explanation

The cross Tt × tt produces offspring with genotypes Tt and tt in a 1:1 ratio. The tt genotype produces dwarf plants, so 50% of the offspring will be dwarf. This is a test cross, which reveals the genotype of the tall parent.

Question 41Continuity of Life: Cell Division, Reproduction and Heredity

Which of the following statements about the law of independent assortment is correct?

  1. All genes on the same chromosome assort independently
  2. Independent assortment occurs only in mitosis
  3. Alleles of different genes assort independently during gamete formation Correct answer.
  4. Alleles of the same gene separate during gamete formation

Explanation

The law of independent assortment states that alleles of different genes assort independently of one another during gamete formation. This is true for genes located on different chromosomes or far apart on the same chromosome. The separation of alleles of the same gene is described by the law of segregation.

Question 42Continuity of Life: Cell Division, Reproduction and Heredity

A human female produces ova with either an X chromosome. A human male produces sperm with either an X or a Y chromosome. What is the probability that a fertilised egg will develop into a male?

  1. 0%
  2. 100%
  3. 50% Correct answer.
  4. 25%

Explanation

The sex of the offspring is determined by the sperm. Half of the sperm carry an X chromosome and half carry a Y chromosome. Fertilisation with an X-bearing sperm gives a female (XX), and with a Y-bearing sperm gives a male (XY), so the probability of a male is 50%.

Question 43Continuity of Life: Cell Division, Reproduction and Heredity

Which of the following is the correct order of the stages of meiosis I?

  1. Prophase I → Metaphase I → Anaphase I → Telophase I Correct answer.
  2. Metaphase I → Prophase I → Anaphase I → Telophase I
  3. Prophase I → Anaphase I → Metaphase I → Telophase I
  4. Anaphase I → Prophase I → Metaphase I → Telophase I

Explanation

Meiosis I proceeds through prophase I, metaphase I, anaphase I, and telophase I. During prophase I, homologous chromosomes pair and crossing over occurs. In metaphase I, homologous pairs align at the equator, and in anaphase I, they separate.

Question 44Continuity of Life: Cell Division, Reproduction and Heredity

A plant with genotype AaBb is self-pollinated. Assuming independent assortment, what fraction of the offspring will be homozygous recessive for both traits (aabb)?

  1. 1/8
  2. 1/2
  3. 1/16 Correct answer.
  4. 1/4

Explanation

For each gene, the probability of obtaining the homozygous recessive genotype from a cross of two heterozygotes is 1/4. Since the genes assort independently, the combined probability is 1/4 × 1/4 = 1/16. This is consistent with the 9:3:3:1 ratio where the double recessive is the smallest class.

Question 45Continuity of Life: Cell Division, Reproduction and Heredity

Which of the following events occurs during prophase I of meiosis but not during prophase of mitosis?

  1. Disappearance of the nucleolus
  2. Formation of spindle fibres
  3. Condensation of chromosomes
  4. Crossing over between homologous chromosomes Correct answer.

Explanation

Crossing over, the exchange of genetic material between non-sister chromatids of homologous chromosomes, occurs only in prophase I of meiosis. Chromosome condensation, spindle formation, and nucleolar disappearance occur in both mitosis and meiosis. Crossing over is a major source of genetic variation.

Question 46Continuity of Life: Cell Division, Reproduction and Heredity

A woman is a carrier of the allele for haemophilia, an X-linked recessive disorder. Her husband does not have haemophilia. What is the probability that their daughter will be a carrier?

  1. 25%
  2. 100%
  3. 0%
  4. 50% Correct answer.

Explanation

The carrier woman has genotype X^H X^h, and the normal man has genotype X^H Y. A daughter receives one X from each parent. She will be a carrier if she inherits X^h from her mother and X^H from her father, which occurs with probability 1/2. The other 1/2 probability is for a non-carrier daughter (X^H X^H).

Question 47Continuity of Life: Cell Division, Reproduction and Heredity

In a certain plant, flower colour is controlled by a gene with incomplete dominance. When a red-flowered plant (RR) is crossed with a white-flowered plant (rr), the F1 plants have pink flowers. What phenotypic ratio is expected in the F2 generation when the F1 plants are self-pollinated?

  1. All pink
  2. 1 red : 2 pink : 1 white Correct answer.
  3. 3 red : 1 white
  4. 1 red : 1 white

Explanation

With incomplete dominance, the heterozygote (Rr) shows an intermediate phenotype, pink. When two pink F1 plants (Rr × Rr) are crossed, the genotypes in the F2 are 1 RR : 2 Rr : 1 rr, which correspond to 1 red : 2 pink : 1 white. This is because each genotype has a distinct phenotype.

Question 48Continuity of Life: Cell Division, Reproduction and Heredity

A couple has a child with blood group O. The mother has blood group A and the father has blood group B. What are the possible genotypes of the parents?

  1. Mother I^A i, father I^B i Correct answer.
  2. Mother I^A I^A, father I^B I^B
  3. Mother I^A i, father I^B I^B
  4. Mother I^A I^B, father ii

Explanation

For a child to have blood group O (genotype ii), each parent must contribute an i allele. Therefore, the mother with blood group A must be heterozygous I^A i, and the father with blood group B must be heterozygous I^B i. If either parent were homozygous, they could not pass on an i allele.

Question 49Continuity of Life: Cell Division, Reproduction and Heredity

In a species of bird, the male is ZZ and the female is ZW. A sex-linked recessive trait is carried on the Z chromosome. If a normal male (heterozygous) is crossed with a female showing the recessive trait, what proportion of the male offspring will show the trait?

  1. 25%
  2. 100%
  3. 0%
  4. 50% Correct answer.

Explanation

Let the recessive allele be z and the dominant allele be Z. The heterozygous male is Zz, and the affected female is Zw (since she has only one Z chromosome and it carries z). Male offspring receive one Z from each parent. They will be affected (zz) if they inherit z from both parents, which happens with probability 1/2. The other 1/2 will be Zz and normal.

Question 50Continuity of Life: Cell Division, Reproduction and Heredity

Which structure formed during meiosis is responsible for the exchange of segments between non-sister chromatids of a homologous pair?

  1. Cell plate
  2. Chiasma Correct answer.
  3. Spindle fibre
  4. Centromere

Explanation

During prophase I, homologous chromosomes pair as bivalents and non-sister chromatids exchange segments at points called chiasmata, a process known as crossing over. This recombination produces new allele combinations on the chromatids.