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Biology 30 Diploma study materials
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Biology 30 Diploma practice questions

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Question 1Biology 30

Which molecule carries the genetic instructions from the nucleus to the ribosome during protein synthesis?

  1. Deoxyribonucleic acid
  2. Ribosomal RNA
  3. Transfer RNA
  4. Messenger RNA Correct answer.

Explanation

Messenger RNA is transcribed from DNA in the nucleus and carries the codon sequence to the ribosome, where it is translated into a polypeptide. DNA itself remains in the nucleus, and the other RNA types perform different roles in translation.

Question 2Biology 30

A diploid cell with 24 chromosomes undergoes meiosis. How many chromosomes does each resulting gamete contain?

  1. 6
  2. 12 Correct answer.
  3. 24
  4. 48

Explanation

Meiosis halves the chromosome number, so a diploid cell with 24 chromosomes produces haploid gametes containing 12 chromosomes each. Two successive divisions follow one round of replication, reducing the chromosome number by half.

Question 3Biology 30

Which hormone is released by the posterior pituitary and acts to increase water reabsorption by the kidney tubules?

  1. Adrenocorticotropic hormone
  2. Antidiuretic hormone Correct answer.
  3. Oxytocin
  4. Aldosterone

Explanation

Antidiuretic hormone, also called vasopressin, is released from the posterior pituitary and increases the permeability of kidney collecting ducts to water. This concentrates urine and conserves body water.

Question 4Biology 30

In a cross between two heterozygous tall pea plants (Tt x Tt), what is the expected phenotypic ratio of the offspring?

  1. 3 tall : 1 short Correct answer.
  2. 1 tall : 1 short
  3. All tall
  4. 1 tall : 3 short

Explanation

A monohybrid cross of two heterozygotes produces genotypes in a 1:2:1 ratio, but because the tall allele is dominant, three of four offspring show the tall phenotype. The phenotypic ratio is therefore 3 tall to 1 short.

Question 5Biology 30

Which structure of the human respiratory system is the primary site of gas exchange with the blood?

  1. Trachea
  2. Bronchus
  3. Larynx
  4. Alveolus Correct answer.

Explanation

Alveoli are tiny air sacs with thin walls and a dense capillary network, giving a large surface area for diffusion of oxygen into the blood and carbon dioxide out. The other listed structures conduct air but do not exchange gases significantly.

Question 6Biology 30

Which part of the human nervous system controls involuntary functions such as heart rate, breathing, and digestion?

  1. Somatic nervous system
  2. Autonomic nervous system Correct answer.
  3. Sensory nervous system
  4. Central nervous system

Explanation

The autonomic nervous system regulates involuntary effectors including cardiac muscle, smooth muscle, and glands. It is divided into sympathetic and parasympathetic divisions that often act in opposition.

Question 7Biology 30

Which process converts atmospheric nitrogen gas into ammonia that can be used by plants?

  1. Nitrification
  2. Denitrification
  3. Ammonification
  4. Nitrogen fixation Correct answer.

Explanation

Nitrogen fixation is carried out by bacteria such as Rhizobium in root nodules and by some free-living soil bacteria, converting inert nitrogen gas into ammonia. This is the only step that makes atmospheric nitrogen available to living things.

Question 8Biology 30

Which property of water allows it to absorb a large amount of heat with only a small change in temperature?

  1. High specific heat capacity Correct answer.
  2. Low density when frozen
  3. Universal solvent ability
  4. Cohesion between molecules

Explanation

Water's high specific heat capacity means a large amount of energy must be added or removed to change its temperature. This stabilizes body temperature and moderates the temperature of aquatic environments.

Question 9Biology 30

During glycolysis, one molecule of glucose is converted into two molecules of pyruvate. What is the net production of ATP and NADH per glucose?

  1. 4 ATP and 2 NADH
  2. 2 ATP and 2 NADH Correct answer.
  3. 2 ATP and 4 NADH
  4. 1 ATP and 1 NADH

Explanation

Glycolysis uses two ATP to prime the pathway but produces four ATP by substrate-level phosphorylation, giving a net gain of two ATP. Two molecules of NADH are also produced per glucose.

Question 10Biology 30

A mutation changes a single DNA base, but the resulting polypeptide has the same amino acid sequence as the original. What type of mutation is this?

  1. Missense mutation
  2. Nonsense mutation
  3. Silent mutation Correct answer.
  4. Frameshift mutation

Explanation

A silent mutation alters a codon but not the amino acid it specifies, because the genetic code is degenerate and several codons encode the same amino acid. The protein sequence is therefore unchanged.

Question 11Biology 30

In a population at Hardy-Weinberg equilibrium, the frequency of the recessive allele is 0.3. What proportion of the population is expected to be heterozygous?

  1. 0.49
  2. 0.42 Correct answer.
  3. 0.21
  4. 0.09

Explanation

With q = 0.3, p = 0.7. The heterozygote frequency is 2pq = 2(0.7)(0.3) = 0.42, so 42 percent of the population is expected to be heterozygous.

Question 12Biology 30

Which statement best describes the role of the corpus luteum in the menstrual cycle?

  1. It secretes progesterone to maintain the uterine lining Correct answer.
  2. It releases the oocyte during ovulation
  3. It secretes testosterone to trigger menstruation
  4. It produces follicle-stimulating hormone

Explanation

After ovulation, the ruptured follicle becomes the corpus luteum, which secretes progesterone and some estrogen. Progesterone maintains the thickened endometrium and inhibits further ovulation until it degenerates if fertilization does not occur.

Question 13Biology 30

Which event occurs during the rising phase of an action potential in a neuron?

  1. Chloride channels open and chloride enters the cell
  2. Sodium channels open and sodium enters the cell Correct answer.
  3. The sodium-potassium pump reverses direction
  4. Potassium channels open and potassium leaves the cell

Explanation

Depolarization is caused by voltage-gated sodium channels opening, allowing sodium ions to rush into the axon. This raises the membrane potential toward threshold and reverses the polarity.

Question 14Biology 30

Two populations of the same species are separated by a river for many generations and gradually become reproductively isolated. Which term best describes this process?

  1. Genetic drift
  2. Artificial selection
  3. Allopatric speciation Correct answer.
  4. Sympatric speciation

Explanation

Allopatric speciation occurs when a geographic barrier physically separates populations, preventing gene flow. Over time, mutations and natural selection in each isolated group can lead to reproductive isolation.

Question 15Biology 30

In a food web, only about 10 percent of the energy at one trophic level is transferred to the next. Which explanation best accounts for this loss?

  1. Most energy is lost as heat through cellular respiration Correct answer.
  2. Energy is stored permanently in decomposers
  3. Energy is converted entirely into biomass
  4. Energy is destroyed completely at each level

Explanation

Organisms use much of the energy they consume for metabolism and release it as heat during cellular respiration. The remainder is lost in wastes, so only a small fraction is stored as new biomass available to the next trophic level.

Question 16Biology 30

Which of the following best describes the function of the loop of Henle in the mammalian kidney?

  1. It reabsorbs glucose and amino acids
  2. It filters blood plasma to form filtrate
  3. It establishes a concentration gradient in the medulla for water reabsorption Correct answer.
  4. It secretes hydrogen ions to regulate blood pH

Explanation

The loop of Henle creates a countercurrent multiplier that produces a high solute concentration in the medulla. This gradient allows water to be reabsorbed osmotically from the collecting duct under the influence of antidiuretic hormone.

Question 17Biology 30

A woman with blood type AB and a man with blood type O have a child. Which blood types are possible for the child?

  1. Only type O
  2. Type AB or type O
  3. Type A or type B Correct answer.
  4. Only type AB

Explanation

The mother has genotype IA IB and the father has genotype ii. The child inherits either IA or IB from the mother and i from the father, giving genotype IAi or IBi, which are blood types A and B respectively.

Question 18Biology 30

Which mechanism of evolution is most likely to have a large effect on small populations because it involves random fluctuations in allele frequencies?

  1. Natural selection
  2. Genetic drift Correct answer.
  3. Gene flow
  4. Mutation

Explanation

Genetic drift is the random change in allele frequencies due to chance events in sampling which individuals reproduce. Its effects are much more pronounced in small populations, where chance can eliminate or fix alleles rapidly.

Question 19Biology 30

A man who is heterozygous for a recessive autosomal disorder marries a woman who is homozygous recessive for the same disorder. What is the probability that a child will express the disorder?

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

Explanation

The father produces gametes carrying the dominant allele or the recessive allele with equal probability, while the mother can only pass on the recessive allele. Half the offspring will therefore be homozygous recessive and express the disorder.

Question 20Biology 30

Which statement best explains why the enzyme catalase increases the rate of hydrogen peroxide breakdown?

  1. It provides energy directly to the reaction
  2. It increases the activation energy of the reaction
  3. It increases the concentration of hydrogen peroxide
  4. It lowers the activation energy of the reaction Correct answer.

Explanation

Enzymes speed reactions by lowering the activation energy needed for the reaction to proceed, allowing it to occur at a higher rate under physiological conditions. The enzyme is not consumed and does not change the equilibrium of the reaction.

Question 21Biology 30

Which of the following best describes the role of reverse transcriptase in a retrovirus such as HIV?

  1. It degrades host DNA to prevent immune responses
  2. It transcribes DNA into RNA for viral protein synthesis
  3. It synthesizes a DNA copy from the viral RNA genome Correct answer.
  4. It translates viral RNA directly into proteins

Explanation

Reverse transcriptase reads the viral RNA and synthesizes a complementary DNA strand, reversing the usual direction of transcription. The resulting DNA can integrate into the host genome and direct production of new viral particles.

Question 22Biology 30

In a population of birds, individuals with medium-sized beaks are favored because they can eat both large and small seeds, while birds with very large or very small beaks are less successful. Which type of selection is occurring?

  1. Disruptive selection
  2. Directional selection
  3. Stabilizing selection Correct answer.
  4. Sexual selection

Explanation

Stabilizing selection favors intermediate phenotypes and reduces extreme variation in a trait. Because medium-beaked birds have the highest fitness, the population mean is maintained and the range narrows.

Question 23Biology 30

A woman carries a mutation in a mitochondrial gene. Which statement best describes the likely inheritance pattern in her children?

  1. All sons will be affected but daughters will not
  2. Neither sons nor daughters will inherit the mutation
  3. All children will inherit the mutation from their mother Correct answer.
  4. Only daughters will inherit the mutation

Explanation

Mitochondrial DNA is inherited maternally because the zygote receives its mitochondria almost entirely from the egg. A mother carrying a mitochondrial mutation can pass it to all of her children, although the severity may vary due to heteroplasmy.

Question 24Biology 30

During an immune response, a helper T cell recognizes an antigen presented on a macrophage. What is the most direct consequence of this recognition?

  1. The helper T cell secretes antibodies against the antigen
  2. The macrophage directly destroys the antigen-presenting cell
  3. The helper T cell activates B cells and cytotoxic T cells Correct answer.
  4. The helper T cell differentiates into a plasma cell

Explanation

Helper T cells release cytokines that stimulate B cells to proliferate and differentiate into antibody-secreting plasma cells and activate cytotoxic T cells. They coordinate the adaptive response rather than performing these effector functions themselves.

Question 25Biology 30

A researcher finds that a bacterial population exposed to an antibiotic develops resistance within a few generations. Which explanation is most consistent with evolutionary theory?

  1. Individual bacteria learn to resist the antibiotic and pass the learned trait to offspring
  2. Pre-existing resistant bacteria survive and reproduce, increasing in frequency Correct answer.
  3. The antibiotic is diluted by the bacteria over time
  4. The antibiotic causes bacteria to mutate in order to survive

Explanation

Antibiotics act as a selective pressure on existing genetic variation. Bacteria that already carry resistance alleles survive and reproduce, so the frequency of resistance in the population rises over generations.

Question 26Biology 30

Which molecule stores the genetic instructions that are translated into a polypeptide during protein synthesis?

  1. Transfer RNA
  2. Messenger RNA
  3. Deoxyribonucleic acid Correct answer.
  4. Ribosomal RNA

Explanation

DNA holds the hereditary instructions in its base sequence, and these instructions are transcribed into messenger RNA before translation. DNA itself is not read directly by ribosomes during polypeptide assembly. The RNA types assist in delivering or decoding the message rather than storing it permanently.

Question 27Biology 30

A cell is placed in a hypertonic solution. What is the most likely immediate effect on an animal cell?

  1. Water enters the cell and the cell swells
  2. Water leaves the cell and the cell shrinks Correct answer.
  3. The cell maintains its volume because the membrane is impermeable
  4. Solutes move out of the cell until concentrations equalize

Explanation

In a hypertonic solution the surrounding fluid has a higher solute concentration than the cytoplasm, so water moves out of the cell by osmosis. Animal cells lack a rigid wall and therefore lose volume and shrink. Solute movement is limited because the membrane is selectively permeable to ions.

Question 28Biology 30

Which structure in a eukaryotic cell is the primary site of aerobic ATP production through the electron transport chain?

  1. Smooth endoplasmic reticulum
  2. Golgi apparatus
  3. Lysosome
  4. Mitochondrion Correct answer.

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, which generate most of the ATP from aerobic respiration. The other organelles listed carry out packaging, digestion, or lipid synthesis rather than oxidative phosphorylation.

Question 29Biology 30

A test cross is performed between an organism showing a dominant trait and a homozygous recessive individual. Half of the offspring show the recessive trait. What is the genotype of the unknown parent?

  1. Homozygous dominant
  2. Homozygous recessive
  3. Cannot be determined from the data
  4. Heterozygous Correct answer.

Explanation

A heterozygous parent crossed with a homozygous recessive parent yields roughly equal numbers of dominant and recessive offspring. A homozygous dominant parent would produce only dominant offspring in this cross. The observed one-to-one ratio therefore identifies the unknown parent as heterozygous.

Question 30Biology 30

Which type of selection favors individuals at both extremes of a phenotypic range and can produce two distinct phenotypes?

  1. Directional selection
  2. Artificial selection
  3. Disruptive selection Correct answer.
  4. Stabilizing selection

Explanation

Disruptive selection acts against intermediate phenotypes and favors both extremes, which can split a population into two forms. Stabilizing selection favors intermediates, and directional selection favors one extreme. Artificial selection is human-driven breeding rather than a natural pattern of selection.

Question 31Biology 30

Which hormone is released by the anterior pituitary and stimulates the thyroid gland to produce its hormones?

  1. Antidiuretic hormone
  2. Thyroid-stimulating hormone Correct answer.
  3. Adrenocorticotropic hormone
  4. Insulin

Explanation

Thyroid-stimulating hormone from the anterior pituitary signals the thyroid gland to release thyroxine and triiodothyronine. Insulin comes from pancreatic beta cells, and antidiuretic hormone is released from the posterior pituitary. Adrenocorticotropic hormone targets the adrenal cortex instead.

Question 32Biology 30

Which feature of an enzyme explains why it is highly specific for one substrate?

  1. It has an active site whose shape complements the substrate Correct answer.
  2. It lowers the activation energy of any reaction in the cell
  3. It raises the temperature of the surrounding solution
  4. It is consumed during the reaction it catalyzes

Explanation

Enzyme specificity arises from the active site, whose three-dimensional shape and chemical properties allow only complementary substrates to bind. This binding positions reacting groups and lowers activation energy for that particular reaction. Enzymes are not consumed and do not act by heating the environment.

Question 33Biology 30

Which blood vessel carries oxygenated blood away from the heart to the body's tissues?

  1. Pulmonary artery
  2. Vena cava
  3. Aorta Correct answer.
  4. Coronary sinus

Explanation

The aorta is the large systemic artery that receives oxygenated blood from the left ventricle and distributes it to the body. The pulmonary artery carries deoxygenated blood to the lungs, and the vena cava returns deoxygenated blood to the heart. The coronary sinus drains blood from the heart muscle itself.

Question 34Biology 30

In a dihybrid cross between two heterozygotes for independently assorting genes, what fraction of the offspring is expected to show both recessive traits?

  1. 3/16
  2. 1/16 Correct answer.
  3. 9/16
  4. 1/4

Explanation

For each independently assorting gene, a heterozygous cross gives a 1/4 chance of the recessive phenotype. Multiplying the independent probabilities gives 1/4 times 1/4, or 1/16, for both recessive traits together.

Question 35Biology 30

A segment of DNA has the template strand sequence 3'-TAC GGA-5'. Assuming standard genetic code pairing, what mRNA sequence is transcribed from this template?

  1. 5'-ATG CCT-3'
  2. 5'-TAC GGA-3'
  3. 5'-UAC GGA-3'
  4. 5'-AUG CCU-3' Correct answer.

Explanation

Transcription builds mRNA complementary to the template strand, replacing thymine with uracil. Complementary bases are A with U, T with A, G with C, and C with G, giving AUG CCU. The direction is antiparallel, so the mRNA is written 5' to 3'.

Question 36Biology 30

During which stage of meiosis does crossing over between homologous chromosomes occur?

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

Explanation

Crossing over occurs during prophase I, when homologous chromosomes pair as tetrads and exchange segments at chiasmata. This recombination creates new combinations of alleles on the resulting chromosomes. Later stages separate chromosomes but do not exchange segments between homologues.

Question 37Biology 30

A population of 500 individuals has 180 individuals with the recessive phenotype. Assuming Hardy-Weinberg equilibrium, what is the approximate frequency of the dominant allele?

  1. 0.36
  2. 0.64
  3. 0.60 Correct answer.
  4. 0.40

Explanation

The recessive phenotype frequency is 180/500, or 0.36, which equals q squared. Taking the square root gives q equal to 0.6, so the dominant allele frequency p equals 1 minus 0.6, or 0.4. Care must be taken not to confuse the allele frequency with the genotype frequency.

Question 38Biology 30

Which statement best describes the role of NAD+ in cellular respiration?

  1. It hydrolyzes glucose into two molecules of pyruvate
  2. It accepts electrons and hydrogen to become NADH, carrying them to the electron transport chain Correct answer.
  3. It directly phosphorylates ADP to form ATP in the cytoplasm
  4. It donates electrons to oxygen at the end of the electron transport chain

Explanation

NAD+ is reduced to NADH when it accepts electrons and hydrogen during glycolysis and the Krebs cycle. NADH then delivers these electrons to the electron transport chain, where they drive proton pumping and ATP synthesis. NAD+ itself is not the terminal electron acceptor or the enzyme that splits glucose.

Question 39Biology 30

A person's blood pressure is measured as 130/85 mm Hg. What does the second number represent?

  1. Pressure in the arteries when the ventricles relax Correct answer.
  2. Pressure in the arteries when the ventricles contract
  3. Pressure exerted by the atria during filling
  4. Pressure inside the veins returning blood to the heart

Explanation

The lower number is the diastolic pressure, recorded while the ventricles are relaxed and the heart refills. The higher number is systolic pressure, measured during ventricular contraction. This distinction reflects the pressure changes within the arterial system across the cardiac cycle.

Question 40Biology 30

Which part of a neuron receives signals from other neurons and conducts them toward the cell body?

  1. Axon
  2. Axon terminal
  3. Dendrite Correct answer.
  4. Myelin sheath

Explanation

Dendrites are branched extensions that receive chemical signals and carry electrical changes toward the cell body. The axon conducts impulses away from the cell body toward other cells. The myelin sheath insulates the axon to speed conduction rather than receiving input.

Question 41Biology 30

Two true-breeding plants, one with red flowers and one with white flowers, are crossed. All the offspring have pink flowers. Which inheritance pattern does this illustrate?

  1. Codominance
  2. Complete dominance
  3. Sex-linked inheritance
  4. Incomplete dominance Correct answer.

Explanation

In incomplete dominance the heterozygote shows an intermediate phenotype, here a blend of red and white pigment. Codominance would instead show both parental traits expressed distinctly at the same time. The uniform pink offspring indicate a single gene with an intermediate heterozygous phenotype.

Question 42Biology 30

Which statement best describes the function of the placenta during pregnancy?

  1. It exchanges nutrients, gases, and wastes between maternal and fetal blood Correct answer.
  2. It secretes digestive enzymes to break down maternal food
  3. It produces sperm and eggs for the developing embryo
  4. It stores the embryo until implantation occurs

Explanation

The placenta is the interface where materials are exchanged between maternal and fetal circulations across a thin barrier. Oxygen and nutrients pass to the fetus while carbon dioxide and wastes pass to the mother. The two blood supplies are kept separate, so cells do not mix directly.

Question 43Biology 30

A bacterial population is exposed to a new antibiotic, and a few cells survive because they already carry a resistance gene. Over several generations the population becomes mostly resistant. Which mechanism best explains this outcome?

  1. Pre-existing resistant cells survive and reproduce, increasing resistance in the population Correct answer.
  2. The antibiotic is diluted by the bacteria until it no longer works
  3. Bacteria learn to resist the antibiotic and pass the learned trait to offspring
  4. The antibiotic causes mutations that create resistance in individual cells

Explanation

Natural selection acts on existing variation: cells that already carry resistance survive and reproduce, so resistance alleles become more common. The antibiotic does not create the mutation; it only changes which variants reproduce. This is a shift in allele frequencies across generations, not learning by individual cells.

Question 44Biology 30

In a food chain, grasses are eaten by grasshoppers, which are eaten by frogs, which are eaten by snakes. If the grasses contain 20 000 kJ of energy, approximately how much energy is available to the snakes?

  1. 20 kJ
  2. 200 kJ Correct answer.
  3. 2 kJ
  4. 2000 kJ

Explanation

Energy transfer between trophic levels is roughly 10 percent per step. From grass to grasshoppers is about 2000 kJ, to frogs about 200 kJ, and to snakes about 20 kJ. The snake occupies the fourth trophic level, so three transfers of 10 percent each apply.

Question 45Biology 30

Which statement best explains why the cell cycle includes checkpoints?

  1. They prevent all cells from ever undergoing apoptosis
  2. They allow cells to divide without replicating their DNA
  3. They ensure DNA is replicated and chromosomes are correctly attached before the cell proceeds Correct answer.
  4. They increase the rate of mutation to generate genetic variation

Explanation

Checkpoints monitor conditions such as DNA integrity and chromosome attachment to the spindle, pausing the cycle until problems are corrected. This control prevents the transmission of damaged or incomplete genetic material to daughter cells. Loss of checkpoint control can contribute to uncontrolled division.

Question 46Biology 30

A genetic counsellor studies a pedigree in which an affected father and unaffected mother have an affected son and an unaffected daughter. The trait appears in every generation and affects about half the offspring of an affected parent. Which inheritance pattern is most consistent with these observations?

  1. X-linked recessive
  2. Autosomal dominant Correct answer.
  3. Mitochondrial inheritance
  4. Autosomal recessive

Explanation

Appearance in every generation and transmission from an affected parent to roughly half the offspring fit autosomal dominant inheritance. An affected father with an unaffected mother can pass the dominant allele to sons and daughters alike. Autosomal recessive would typically skip generations unless the mother were a carrier, and mitochondrial inheritance passes only through mothers.

Question 47Biology 30

A hormone is secreted by the anterior pituitary and its target organ releases a second hormone. Rising levels of the second hormone reduce secretion of the first. Which control mechanism is described?

  1. Antagonistic muscle action
  2. Neurotransmitter reuptake
  3. Negative feedback Correct answer.
  4. Positive feedback

Explanation

In negative feedback, the product of a pathway suppresses the signal that stimulated it, keeping hormone levels within a stable range. Here the target hormone inhibits the pituitary hormone, damping the response. Positive feedback would instead amplify the initial signal.

Question 48Biology 30

In a population of beetles, a mutation produces a new allele that confers resistance to a pesticide. Before the pesticide is applied, the allele is present at a very low frequency. After several years of spraying, its frequency rises sharply. Which conclusion is best supported?

  1. The resistance allele was selected for because resistant beetles survived and reproduced Correct answer.
  2. The pesticide caused the beetles to need the allele and so they developed it
  3. The pesticide directly caused the resistance mutation in individual beetles
  4. The allele frequency changed because the population size increased

Explanation

The mutation existed before exposure, and spraying changed the selective environment so that resistant beetles had higher survival and reproductive success. This increases the allele frequency over generations. Selection acts on existing variation rather than creating the beneficial allele on demand.

Question 49Biology 30

A cell with a mutation that prevents the formation of the spindle apparatus enters mitosis. Which outcome is most likely?

  1. The cell will undergo meiosis instead of mitosis
  2. DNA replication will not occur, so the cell will arrest in G1 phase
  3. The cell will complete mitosis normally because the spindle is not required
  4. Chromosomes will fail to separate properly, producing daughter cells with abnormal chromosome numbers Correct answer.

Explanation

The spindle apparatus attaches to chromosomes and pulls sister chromatids to opposite poles during anaphase. Without a functional spindle, chromosomes cannot be distributed evenly, leading to unequal chromosome numbers in daughter cells. DNA replication occurs earlier in S phase and would not be directly prevented by this mutation.

Question 50Biology 30

An individual has one allele for sickle-cell trait and one normal allele. Which statement best explains why this heterozygous condition provides some protection against malaria?

  1. The presence of abnormal hemoglobin reduces the ability of the malaria parasite to reproduce in red blood cells Correct answer.
  2. Heterozygotes produce no red blood cells at all
  3. The normal allele is completely inactivated in heterozygotes
  4. The sickle-cell allele prevents mosquitoes from biting the individual

Explanation

In heterozygotes, the altered hemoglobin affects the red blood cell environment in ways that reduce parasite survival and reproduction. This gives a selective advantage in regions where malaria is common, maintaining the allele at higher frequency. The normal allele remains functional, and red blood cells are still produced.