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OAE Biology practice questions

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Question 1Nature of Science

A researcher predicts that a certain enzyme will show higher activity at 40 °C than at 25 °C. She runs both treatments and finds the opposite result. What is the most scientifically appropriate next step?

  1. Report only the 40 °C trial that matched the prediction
  2. Discard the data because they contradict the prediction
  3. Change the prediction after the fact so it matches the data
  4. Revise or reject the hypothesis in light of the new evidence Correct answer.

Explanation

Scientific hypotheses are provisional claims that must be evaluated against evidence, so a failed prediction prompts revision rather than concealment. Discarding valid data or rewriting the prediction after seeing results undermines the self-correcting nature of science.

Question 2Nature of Science

In a controlled experiment on plant growth, which variable is the independent variable?

  1. The final height of each plant measured in centimeters
  2. The temperature of the growth chamber, held constant
  3. The average height of the control group at the end
  4. The amount of light each plant receives, set by the researcher Correct answer.

Explanation

The independent variable is the factor the researcher deliberately manipulates, such as the light level assigned to each group. Height is the measured dependent variable, and held-constant factors are controlled variables.

Question 3Nature of Science

Which statement best describes the role of a control group in a well-designed biological experiment?

  1. It guarantees that the experiment will produce statistically significant results
  2. It replaces the need for repeated trials or replication
  3. It allows the researcher to test several independent variables at once
  4. It provides a baseline for comparison so the effect of the treatment can be isolated Correct answer.

Explanation

A control group experiences all conditions except the treatment, giving a reference point against which the treated group is compared. This isolation of the treatment effect is what allows causal claims to be made.

Question 4Nature of Science

A student measures the mass of the same sample five times and obtains very similar values, but the true mass is known to be considerably different. Which description fits these measurements?

  1. High both precision and accuracy
  2. High precision but low accuracy Correct answer.
  3. High accuracy but low precision
  4. Low both precision and accuracy

Explanation

Precision refers to how closely repeated measurements agree with one another, while accuracy refers to closeness to the true value. Consistent but offset readings are precise yet inaccurate, often reflecting systematic error.

Question 5Nature of Science

Which of the following is the best example of a scientific theory as the term is used in biology?

  1. A tentative guess a scientist makes before starting an experiment
  2. A single observation that has not yet been repeated
  3. The theory of evolution by natural selection, supported by extensive converging evidence Correct answer.
  4. An opinion held by a majority of researchers at one laboratory

Explanation

In science, a theory is a well-substantiated explanatory framework supported by many lines of evidence, not a mere guess. Evolution by natural selection exemplifies such a framework in biology.

Question 6Nature of Science

A scientist wants to determine whether a new fertilizer increases crop yield. Which procedure best reflects sound experimental design?

  1. Apply two different fertilizers to the same plot and record the total yield
  2. Apply fertilizer to all plots and compare yields with last year's harvest
  3. Apply fertilizer to one set of plots and no fertilizer to comparable plots, keeping other conditions the same Correct answer.
  4. Apply fertilizer to the plots nearest the greenhouse and compare with distant plots

Explanation

A sound test compares treated and untreated groups that are otherwise equivalent, so any yield difference can be attributed to the fertilizer. Holding other conditions constant prevents confounding variables from explaining the result.

Question 7Nature of Science

Which practice most directly reduces the influence of bias when assigning subjects to treatment groups?

  1. Letting each subject pick which group to join
  2. Random assignment of subjects to groups Correct answer.
  3. Assigning all volunteers to the treatment group
  4. Choosing subjects the researcher expects to respond well

Explanation

Random assignment distributes unknown confounding traits roughly evenly across groups, reducing systematic bias. Researcher selection or self-selection can concentrate particular characteristics in one group.

Question 8Nature of Science

In scientific communication, what is the primary purpose of peer review before publication?

  1. To have independent experts evaluate the methods and evidence for quality and validity Correct answer.
  2. To decide which author will receive credit for the discovery
  3. To guarantee that the findings will be replicated by other laboratories
  4. To translate the manuscript into multiple languages for wider readership

Explanation

Peer review subjects a manuscript to scrutiny by independent experts who assess the soundness of methods, analysis, and interpretation. It improves quality control but does not certify that findings are true or replicable.

Question 9Nature of Science

A study reports that students who eat breakfast earn higher test scores, and the authors conclude that eating breakfast causes higher scores. Which concern most weakens this causal claim?

  1. Test scores were recorded as numerical values rather than categories
  2. Students who eat breakfast may differ systematically in other ways that affect scores Correct answer.
  3. The study included students from more than one classroom
  4. The researchers used a standardized test rather than a teacher-made quiz

Explanation

Observational associations can reflect confounding variables, such as socioeconomic status or study habits, that influence both breakfast habits and scores. Without controlling such factors, a causal conclusion is not justified.

Question 10Nature of Science

A researcher graphs data with a line of best fit and reports an R² value of 0.92. What does this value most directly indicate?

  1. The measurements contain 8% random error
  2. There is a 92% probability that the hypothesis is true
  3. The independent variable causes 92% of the changes in the dependent variable
  4. About 92% of the variation in the dependent variable is accounted for by the model Correct answer.

Explanation

R², the coefficient of determination, expresses the proportion of variance in the response variable explained by the fitted model. A high R² describes fit, not causation or the probability that a hypothesis is correct.

Question 11Nature of Science

Which change would most improve the reliability of an experiment's results?

  1. Removing outliers that contradict the hypothesis
  2. Reporting results to three decimal places
  3. Increasing the number of independent trials and subjects Correct answer.
  4. Using a single large trial instead of several smaller ones

Explanation

Reliability improves when results are consistent across repeated trials, and larger sample sizes reduce the influence of random variation. Additional decimal places or selective removal of data do not increase confidence in repeatability.

Question 12Nature of Science

A scientist uses a model to predict how a pollutant spreads through a lake. Which statement best reflects the appropriate use of such a model?

  1. Its predictions are valid because mathematical models are objective
  2. It should be accepted only if it predicts every observed detail exactly
  3. It eliminates the need for empirical sampling of the lake
  4. Its predictions should be validated against field measurements before being relied upon Correct answer.

Explanation

Models are simplifications of reality, so their outputs must be checked against empirical data to establish their validity. Agreement with observation, not the mere use of mathematics, justifies confidence in predictions.

Question 13Nature of Science

Two groups of researchers investigate the same question and obtain conflicting results. Which response is most consistent with scientific norms?

  1. Average the two results and report the mean as the answer
  2. Assume the group with more funding produced the correct result
  3. Ignore both studies until a consensus emerges without further work
  4. Investigate differences in methods and sample conditions to explain the discrepancy Correct answer.

Explanation

Conflicting findings are resolved by examining methodological differences, replication, and accumulated evidence. Authority or simple averaging does not resolve scientific disagreement.

Question 14Nature of Science

Which statement correctly distinguishes a scientific hypothesis from a scientific law?

  1. A law explains why a phenomenon occurs, while a hypothesis only describes it
  2. A hypothesis applies only to biology, while a law applies only to physics
  3. A hypothesis is a testable proposed explanation, while a law describes a consistent observed relationship Correct answer.
  4. A hypothesis is always true, while a law is always provisional

Explanation

A hypothesis offers a testable explanation for observations, whereas a scientific law summarizes a consistent relationship that has held under many observations. Neither is inherently permanent or confined to one discipline.

Question 15Nature of Science

A student records a length as 12.50 cm using a ruler marked in millimeters. What does the final zero suggest about the measurement?

  1. It shows the ruler was calibrated incorrectly
  2. It proves the measurement is exactly 12.5 cm with no uncertainty
  3. It is a placeholder with no meaning and should be dropped
  4. It indicates the estimated digit and conveys the precision of the instrument Correct answer.

Explanation

Significant figures communicate the precision of a measurement, and the final recorded digit is typically an estimate. Retaining that zero indicates the level of precision the instrument supports.

Question 16Nature of Science

A scientist collects water samples from a stream at the same time each day for a year to assess seasonal nutrient changes. Which term best characterizes this approach?

  1. Systematic longitudinal sampling Correct answer.
  2. Randomized controlled trial
  3. Retrospective case study
  4. Double-blind experiment

Explanation

Repeated sampling over an extended period at consistent intervals is a systematic longitudinal design suited to detecting seasonal trends. It is observational rather than a controlled intervention.

Question 17Nature of Science

Which of the following best illustrates the principle of falsifiability in biology?

  1. A claim that all outcomes are consistent with a proposed mechanism
  2. A claim that a species has an innate drive that cannot be observed
  3. A claim that an unseen life force guides all living processes
  4. A claim that a specific drug reduces blood pressure in a measurable, testable way Correct answer.

Explanation

A falsifiable claim makes specific predictions that observations could show to be wrong, as with a measurable drug effect. Claims compatible with every possible outcome cannot be tested.

Question 18Nature of Science

A researcher reports a p-value of 0.03 for a difference between treatment and control groups. What is the most accurate interpretation?

  1. There is a 3% chance the treatment has no effect
  2. The treatment effect is 3% larger than the control effect
  3. The result is practically important because p is below 0.05
  4. If no real difference existed, results this extreme would occur about 3% of the time by chance Correct answer.

Explanation

A p-value estimates the probability of observing data at least as extreme as those obtained, assuming the null hypothesis is true. It is not the probability that the null is true, nor a measure of effect size or practical importance.

Question 19Nature of Science

A team publishes a method that other labs cannot reproduce. Which factor is most likely to undermine confidence in the original findings?

  1. The original team used statistical software to analyze data
  2. The original findings were described in a press release
  3. Incomplete reporting of procedures and conditions needed for replication Correct answer.
  4. The original study was published in a peer-reviewed journal

Explanation

Reproducibility depends on transparent reporting of methods, materials, and conditions so others can repeat the work. Failure to replicate may stem from missing procedural details or from results that do not hold up.

Question 20Nature of Science

A scientist claims that a new supplement boosts memory based on a study of 15 volunteers who all received the supplement. Which criticism is most fundamental?

  1. The absence of a control group makes it impossible to attribute any improvement to the supplement Correct answer.
  2. Memory cannot be measured scientifically
  3. Fifteen volunteers is too few to compute any statistics
  4. Supplements are never effective, so the claim is false

Explanation

Without a comparison group, improvements could arise from practice effects, placebo responses, or other factors. A control group is essential to isolate the supplement's effect from alternative explanations.

Question 21Nature of Science

A researcher wants to test whether a gene affects wing length in fruit flies. Which approach best controls for genetic background differences?

  1. Compare flies from two different species collected in the wild
  2. Compare genetically similar strains that differ only in the gene of interest Correct answer.
  3. Measure wing length in one strain and compare with published values
  4. Use flies of many genetic backgrounds and average the results

Explanation

Using strains that are genetically similar except for the gene of interest isolates that gene's effect from other genetic variation. Comparing different species or averaging across diverse backgrounds introduces confounding factors.

Question 22Nature of Science

A meta-analysis combines results from 30 studies on a treatment and finds a small pooled effect with wide confidence intervals. What is the most defensible conclusion?

  1. The individual studies must have been fraudulent
  2. The evidence suggests a small effect but uncertainty remains substantial Correct answer.
  3. The treatment is proven effective because 30 studies were combined
  4. The treatment is proven ineffective because the effect is small

Explanation

A pooled estimate with wide confidence intervals indicates imprecision, so the effect is plausible but not firmly established. Combining studies increases power but does not eliminate uncertainty or prove efficacy.

Question 23Nature of Science

A student argues that because a correlation between pesticide use and amphibian decline is strong, the pesticide must be the cause. Which response best applies the principles of causal inference?

  1. The correlation must be spurious because causation cannot be studied in nature
  2. A strong correlation coefficient is sufficient proof of a causal relationship
  3. Causation can be assumed whenever both variables are measured precisely
  4. Correlation is consistent with causation but alternative explanations and temporal order must be examined Correct answer.

Explanation

Correlation is necessary but not sufficient for causation; researchers must consider confounders, temporal precedence, and mechanisms. Well-designed observational and experimental studies can support causal inferences in nature.

Question 24Nature of Science

A journal receives a manuscript in which the authors selectively report only trials that supported their hypothesis. Which ethical principle is most directly violated?

  1. Confidentiality of peer reviewers
  2. Honest and complete reporting of results Correct answer.
  3. Informed consent of human subjects
  4. Proper attribution of authorship

Explanation

Selective reporting of favorable results, sometimes called reporting bias, violates the norm of honest and complete disclosure of findings. It distorts the evidence base regardless of other ethical requirements.

Question 25Nature of Science

A model predicts population growth under ideal conditions, but observed populations plateau as resources become limited. How should the scientist best respond?

  1. Conclude that population ecology cannot be modeled
  2. Revise the model to incorporate resource limitation and test the revised predictions Correct answer.
  3. Report the original prediction as confirmed despite the plateau
  4. Discard all data showing the plateau as measurement error

Explanation

When predictions diverge from observation, models are refined by incorporating the missing factors, such as density-dependent resource limits, and then retested. This iterative revision is central to scientific progress.

Question 26Nature of Science

A biology student measures the mass of the same tissue sample five times and obtains values of 4.98 g, 5.01 g, 4.99 g, 5.00 g, and 5.02 g, while the true mass is 5.40 g. Which description best characterizes these measurements?

  1. They are both precise and accurate
  2. They are precise but not accurate Correct answer.
  3. They are accurate but not precise
  4. They are neither precise nor accurate

Explanation

Precision refers to how closely repeated measurements agree with one another, while accuracy refers to how close they are to the true value. Here the five values cluster tightly together, showing high precision, but all fall well below the true mass of 5.40 g, showing low accuracy. This pattern often indicates a systematic error such as an uncalibrated balance.

Question 27Nature of Science

A researcher wants to know whether a bacterial strain can use citrate as its sole carbon source. Which experimental design best allows a valid conclusion about that specific question?

  1. Measure the pH of the medium before and after adding the bacteria
  2. Inoculate the strain into a medium containing only citrate as the carbon source and compare growth with an identical medium that also contains glucose Correct answer.
  3. Stain the bacteria and examine their cell shape under a microscope
  4. Inoculate the strain into a rich medium and observe whether colonies appear after incubation

Explanation

Testing a single nutritional capability requires a defined medium in which citrate is the only available carbon source, with a comparison treatment providing a known usable carbon source such as glucose. Growth on citrate alone supports the conclusion, while growth on glucose confirms the organism was viable and the conditions suitable. Including both conditions controls for inoculation error and general growth failure.

Question 28Nature of Science

A researcher clones a gene into a plasmid, transforms bacteria, and selects on an antibiotic, but obtains no colonies on the selection plate while the positive-control plasmid yields many colonies. Which interpretation best reflects scientific reasoning about this result?

  1. The absence of colonies proves that the inserted gene is lethal to the host bacterium
  2. The negative result points to a specific failure in the cloning or transformation step that should be investigated before concluding the gene is toxic Correct answer.
  3. The experiment should be repeated with a different antibiotic until colonies appear
  4. The result should be discarded because negative results carry no scientific value

Explanation

Because the positive-control plasmid produced colonies, the competent cells, antibiotic, and plating conditions were functional, so the failure localizes to the cloning or transformation of the experimental construct. A well-reasoned response treats this as a diagnostic signal and tests alternative explanations such as ligation failure, insert orientation, or a faulty plasmid preparation. Concluding lethality immediately would be premature without ruling out these technical causes.

Question 29Molecular and Cellular Biology

Which molecule serves as the primary energy currency of the cell by donating a terminal phosphate group to energy-requiring reactions?

  1. ATP Correct answer.
  2. NADPH
  3. Pyruvate
  4. Glucose

Explanation

ATP is the cell's universal energy currency; hydrolysis of its terminal phosphoanhydride bond releases free energy that drives endergonic reactions. NADPH is an electron carrier, while glucose and pyruvate are metabolic fuels rather than direct phosphate donors.

Question 30Molecular and Cellular Biology

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

  1. Solutes move out to equalize concentration
  2. Water leaves and the cell shrinks Correct answer.
  3. The cell maintains its volume unchanged
  4. Water enters and the cell swells

Explanation

In a hypertonic solution the surrounding solute concentration exceeds that inside the cell, so water exits by osmosis. The resulting loss of water causes the animal cell to shrink. Solute movement does not occur passively to equalize concentration in this situation.

Question 31Molecular and Cellular Biology

Which organelle is the principal site of aerobic ATP production through oxidative phosphorylation?

  1. Golgi apparatus
  2. Ribosome
  3. Lysosome
  4. Mitochondrion Correct answer.

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, which together carry out oxidative phosphorylation. Ribosomes synthesize proteins, the Golgi modifies and sorts them, and lysosomes degrade material.

Question 32Molecular and Cellular Biology

During DNA replication, which enzyme adds nucleotides to the growing strand by matching them to the template?

  1. DNA ligase
  2. DNA polymerase Correct answer.
  3. Primase
  4. Helicase

Explanation

DNA polymerase catalyzes the addition of nucleotides complementary to the template strand, extending the new chain in the 5' to 3' direction. Ligase joins fragments, helicase unwinds the duplex, and primase lays down RNA primers.

Question 33Molecular and Cellular Biology

Which process converts glucose into two molecules of pyruvate while generating a net gain of two ATP?

  1. The citric acid cycle
  2. Beta oxidation
  3. Glycolysis Correct answer.
  4. The Calvin cycle

Explanation

Glycolysis splits one glucose into two pyruvate molecules in the cytoplasm, with a net yield of two ATP and two NADH. The citric acid cycle oxidizes acetyl-CoA, the Calvin cycle fixes carbon in photosynthesis, and beta oxidation degrades fatty acids.

Question 34Molecular and Cellular Biology

Which structural feature distinguishes a eukaryotic cell from a prokaryotic cell?

  1. Presence of ribosomes
  2. Presence of a plasma membrane
  3. Presence of a membrane-bound nucleus Correct answer.
  4. Presence of DNA

Explanation

Eukaryotic cells enclose their genetic material within a membrane-bound nucleus, whereas prokaryotes keep DNA in a nucleoid region. Both cell types possess a plasma membrane, ribosomes, and DNA.

Question 35Molecular and Cellular Biology

Which type of bond links amino acids together in a polypeptide chain?

  1. Glycosidic bond
  2. Ionic bond
  3. Peptide bond Correct answer.
  4. Hydrogen bond

Explanation

A peptide bond forms between the carboxyl group of one amino acid and the amino group of the next, releasing water. Hydrogen and ionic bonds contribute to higher-order protein structure, while glycosidic bonds link sugars.

Question 36Molecular and Cellular Biology

Which property of the phospholipid bilayer allows small nonpolar molecules to cross the membrane most readily?

  1. Its net negative charge
  2. Its rigid cholesterol layer
  3. Its high protein content
  4. Its hydrophobic interior Correct answer.

Explanation

The fatty acid tails create a hydrophobic core, so small nonpolar molecules such as oxygen dissolve through it easily. Charged and large polar molecules require transport proteins instead.

Question 37Molecular and Cellular Biology

A mutation changes a single DNA base but the encoded amino acid sequence remains the same. What type of mutation is this?

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

Explanation

Because the genetic code is degenerate, several codons specify the same amino acid, so a base substitution can leave the protein unchanged. Such a change is called a silent mutation. Missense and nonsense mutations alter the amino acid sequence or create a stop codon.

Question 38Molecular and Cellular Biology

In a cross between two heterozygous individuals (Aa x Aa) for a trait showing complete dominance, what phenotypic ratio is expected among the offspring?

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

Explanation

A monohybrid cross of two heterozygotes yields a 3:1 phenotypic ratio because the dominant phenotype appears in both AA and Aa genotypes. The underlying genotypic ratio is 1:2:1, but the phenotypes collapse to three dominant to one recessive.

Question 39Molecular and Cellular Biology

Which stage of mitosis is characterized by chromosomes aligning along the cell's equator?

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

Explanation

During metaphase, spindle fibers attach to kinetochores and position chromosomes at the metaphase plate along the cell equator. Prophase involves condensation, anaphase involves chromatid separation, and telophase involves nuclear reformation.

Question 40Molecular and Cellular Biology

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

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

Explanation

Crossing over takes place in prophase I, when homologous chromosomes pair as tetrads and exchange segments at chiasmata. This recombination increases genetic variation among the resulting gametes.

Question 41Molecular and Cellular Biology

Which statement best describes the role of tRNA in translation?

  1. It delivers amino acids to the ribosome according to the mRNA codon Correct answer.
  2. It catalyzes the formation of peptide bonds
  3. It carries the genetic message from DNA to the ribosome
  4. It forms the structural core of the ribosome

Explanation

Each tRNA carries a specific amino acid and has an anticodon that base-pairs with a complementary mRNA codon, ensuring correct placement of residues. mRNA carries the message, rRNA forms the ribosome, and the ribosome's catalytic activity joins the amino acids.

Question 42Molecular and Cellular Biology

Which enzyme transcribes DNA into mRNA in eukaryotic cells?

  1. DNA polymerase III
  2. Reverse transcriptase
  3. Restriction endonuclease
  4. RNA polymerase II Correct answer.

Explanation

RNA polymerase II synthesizes messenger RNA from a DNA template in eukaryotes. DNA polymerase replicates DNA, reverse transcriptase copies RNA into DNA, and restriction endonucleases cleave DNA at specific sequences.

Question 43Molecular and Cellular Biology

A cell that normally produces an enzyme loses that ability after a mutation in a regulatory region upstream of the gene. Which explanation is most consistent with this observation?

  1. The ribosome began translating the wrong reading frame
  2. The mRNA gained a longer poly-A tail
  3. The coding sequence acquired a nonsense mutation
  4. A promoter or enhancer element no longer binds its transcription factor Correct answer.

Explanation

Regulatory regions such as promoters and enhancers control transcription by binding transcription factors; a mutation there can abolish expression even with an intact coding sequence. The scenario specifies a regulatory-region change rather than a coding or translational defect.

Question 44Molecular and Cellular Biology

In the light-dependent reactions of photosynthesis, what is the primary source of electrons that replace those lost by photosystem II?

  1. ATP
  2. NADPH
  3. Water Correct answer.
  4. Carbon dioxide

Explanation

Photosystem II obtains replacement electrons by splitting water, releasing oxygen as a byproduct. NADPH is produced later and donates electrons in the Calvin cycle, while carbon dioxide is fixed rather than used as an electron source.

Question 45Molecular and Cellular Biology

Which level of protein structure is defined by the overall three-dimensional folding of a single polypeptide, including interactions between distant side chains?

  1. Quaternary structure
  2. Secondary structure
  3. Primary structure
  4. Tertiary structure Correct answer.

Explanation

Tertiary structure describes the complete three-dimensional shape of one polypeptide, stabilized by hydrophobic interactions, hydrogen bonds, ionic bonds, and disulfide bridges. Secondary structure refers to local patterns such as helices and sheets.

Question 46Molecular and Cellular Biology

An enzyme-catalyzed reaction rate increases with substrate concentration but then plateaus. What does the plateau indicate?

  1. All enzyme active sites are saturated with substrate Correct answer.
  2. The substrate has been completely consumed
  3. The reaction has become endergonic
  4. The enzyme has been irreversibly denatured

Explanation

At high substrate concentrations every active site is occupied, so the reaction reaches its maximum velocity and adding more substrate cannot increase the rate further. This saturation behavior is described by Michaelis-Menten kinetics.

Question 47Molecular and Cellular Biology

Which molecule is the final electron acceptor in the mitochondrial electron transport chain under aerobic conditions?

  1. Oxygen Correct answer.
  2. Water
  3. FAD
  4. NAD+

Explanation

Oxygen accepts electrons at complex IV and combines with protons to form water, allowing the chain to keep operating. NAD+ and FAD are electron carriers that deliver electrons earlier in the chain.

Question 48Molecular and Cellular Biology

A cell is treated with a drug that blocks the sodium-potassium pump in the plasma membrane. What is the most direct consequence for ion gradients?

  1. Intracellular sodium rises and intracellular potassium falls Correct answer.
  2. Intracellular sodium falls and intracellular potassium rises
  3. Both sodium and potassium become evenly distributed without energy cost
  4. Calcium influx increases while sodium is unaffected

Explanation

The sodium-potassium pump normally exports sodium and imports potassium against their gradients using ATP. Blocking it allows these ions to leak down their gradients, so intracellular sodium rises and intracellular potassium falls over time.

Question 49Molecular and Cellular Biology

In a dihybrid cross between two individuals heterozygous for both genes (AaBb x AaBb), assuming independent assortment, what fraction of offspring is expected to be homozygous recessive for both traits?

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

Explanation

Each gene independently yields a 1/4 probability of the homozygous recessive genotype, so the combined probability is 1/4 x 1/4 = 1/16. Independent assortment allows multiplication of the separate gene probabilities.

Question 50Molecular and Cellular Biology

A frameshift mutation occurs near the beginning of a coding sequence. Which outcome is most likely?

  1. A single amino acid substitution with little effect
  2. Enhanced protein function due to a new stop codon
  3. No change in the protein because the code is degenerate
  4. A completely altered downstream amino acid sequence Correct answer.

Explanation

Inserting or deleting a number of bases not divisible by three shifts the reading frame, so every codon downstream is read differently. This typically produces a truncated or nonfunctional protein rather than a single substitution.