Genetics Quiz: Experimental Controls And Replicates

What this quiz covers

This quiz focuses on Experimental Controls And Replicates, giving you a quick way to practice the rules, question types, and explanations that matter most for Genetics.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Repeating the entire QTL mapping experiment with a larger F2 population derived from the same parental lines.
2. Bioinformatically scanning the QTL interval for candidate genes known to be involved in plant growth or hormone pathways.
3. Re-measuring the height and re-genotyping the original F2 population to eliminate any potential data collection errors.
4. Creating a near-isogenic line (NIL) where only the identified chromosome 3 QTL region from the 'tall' parent is introgressed into the 'short' parent's genetic background. (correct answer)

Explanation: When you encounter QTL mapping questions, remember that identifying a statistical association is just the first step—proving causation requires controlled experimentation that isolates the genetic factor of interest. The most definitive way to validate causal responsibility is creating a near-isogenic line (NIL) where only the QTL region from the tall parent is introgressed into the short parent's background. This approach isolates the effect of that specific chromosomal segment while keeping all other genetic factors constant. If height differences persist in the NIL compared to the recipient parent, you've demonstrated that genes within that region directly cause the trait variation. Answer D represents the gold standard for QTL validation. Option A (larger F2 population) would increase statistical power and mapping resolution, but wouldn't prove causation—it's still just correlation analysis. Option B (bioinformatic scanning) is useful for generating hypotheses about candidate genes, but computational predictions don't establish causal relationships without experimental validation. Option C (re-measuring and re-genotyping) addresses technical accuracy but doesn't advance beyond the original association study—you'd still only have correlational evidence. The key distinction here is between association and causation. Statistical mapping identifies genomic regions correlated with traits, but environmental factors, population structure, or linked genes could create false associations. Only controlled genetic experiments like NIL construction can definitively prove that specific DNA sequences cause phenotypic variation. Remember: QTL mapping finds associations, but functional validation requires isolating the genetic factor through techniques like NILs, complementation tests, or gene editing.

Question 2

1. A mutant strain of worms lacking KinaseX, grown on medium with the inhibitor.
2. Wild-type worms grown on medium containing only DMSO at the same concentration used for the inhibitor. (correct answer)
3. Wild-type worms grown on medium with a different, known kinase inhibitor to serve as a positive control.
4. Wild-type worms grown on medium with the inhibitor, but at a range of different concentrations.

Explanation: The correct answer is B. This is the vehicle control. DMSO is known to have biological effects of its own, including influencing lifespan in some model organisms. Without this control, any observed effect on lifespan could be due to the DMSO solvent rather than the inhibitor itself. (A) This is an excellent control to test the specificity of the inhibitor (a genetic control), but the vehicle control is more fundamental to the initial experiment's validity. (C) A positive control is useful but not as essential as the vehicle control. (D) A dose-response curve is a good idea for characterizing the effect, but it doesn't replace the need for a proper vehicle control at each dose.

Question 3

1. Pool all 9 measurements from the treated group and all 9 from the control group and perform a t-test with n=9 for each group.
2. For each mouse, average the Cq values of the 3 technical replicates, then perform a t-test comparing the 3 average values from the treated group to the 3 average values from the control group. (correct answer)
3. Perform six separate t-tests, each comparing the 3 technical replicates of a treated mouse to the 3 technical replicates of a control mouse.
4. For each mouse, select the technical replicate with the Cq value closest to the median and perform a t-test on these selected single values (n=3 per group).

Explanation: The correct answer is B. This approach correctly treats the biological replicates (the individual mice) as the independent units of statistical analysis (n=3 per group). The technical replicates are averaged to get a more precise estimate for each biological replicate, but they do not increase the biological sample size. (A) is an example of pseudoreplication, which artificially inflates the sample size and the statistical significance by treating non-independent technical replicates as independent samples. (C) is an incorrect application of statistical testing. (D) introduces bias and discards data that informs the measurement variability for each sample.

Question 4

1. Sequencing the gut contents of the mice to confirm that the antibiotic treatment was effective at depleting bacteria.
2. Repeating the experiment using a different antibiotic to ensure the effect is not a side effect of the specific drug used.
3. Testing another inbred mouse strain to see if the phenomenon is generalizable across different genetic backgrounds.
4. Performing a fecal microbiota transplant (FMT) from untreated mice into the antibiotic-treated mice to see if it reverses the anxiety-like behavior. (correct answer)

Explanation: When you encounter questions about establishing causality in biological research, think about the hierarchy of evidence. Correlation studies show associations, but to prove causation, you need experiments that can reverse or restore the observed effects. The strongest evidence for causality comes from demonstrating that restoring the presumed cause (gut microbiota) reverses the observed effect (anxiety-like behavior). Answer D does exactly this through fecal microbiota transplant (FMT). If depleting the microbiota causes anxiety, then restoring it should reduce anxiety back to baseline levels. This "rescue experiment" approach is the gold standard for proving causation in biological systems because it shows the relationship works in both directions. Let's examine why the other options fall short: Answer A (sequencing gut contents) only confirms the depletion worked but doesn't establish causation—it's still just correlative evidence. Answer B (different antibiotic) helps rule out drug-specific side effects, which is valuable, but doesn't move beyond correlation to causation. Answer C (testing different mouse strains) addresses generalizability across genetic backgrounds, but again doesn't establish the causal mechanism. While options B and C represent good scientific practices for strengthening correlative findings, they don't provide the direct causal evidence that option D offers through its ability to reverse the phenotype. Remember this pattern: when you see research questions about proving causation, look for the experiment that can restore or reverse the effect by manipulating the proposed cause. Rescue experiments provide the most compelling evidence for causal relationships in genetics and biology.

Question 5

1. To provide a sufficient supply of nucleotides for cell division to proceed normally after the pulse.
2. To synchronize the cell population at the beginning of S phase for more consistent measurements.
3. To dilute the intracellular pool of ³H-thymidine, effectively stopping the incorporation of the radioactive label into new DNA. (correct answer)
4. To act as a negative control for the scintillation counter, establishing a background radiation level.

Explanation: The correct answer is C. The purpose of the chase is to flood the cell with non-radioactive ("cold") substrate, which outcompetes any remaining radioactive ("hot") substrate for incorporation. This effectively ends the labeling period at a precise time, ensuring that the measured radioactivity reflects only the synthesis that occurred during the defined pulse window. (A) While cells need nucleotides, this is not the primary purpose of the chase. (B) Synchronization is a separate procedure performed before the pulse. (D) A proper negative control would be cells that were never exposed to ³H-thymidine.

Question 6

1. The study design is correlational and cannot rule out confounding variables between the urban and rural sites. (correct answer)
2. The sample size of 20 nests per site is too small to draw meaningful conclusions about bird populations.
3. The lack of technical replication in measuring nesting success compromises the data's reliability.
4. The distance between the sites is too great, introducing geographic variation that is not controlled for.

Explanation: The correct answer is A. This is an observational study, not a controlled experiment. The urban and rural sites differ in many ways besides light levels (e.g., food source availability, predator types, noise pollution, air quality). Any of these confounding factors could be the true cause of the difference in nesting success. The study shows a correlation, but it cannot establish causation. (B) While larger samples are always better, the confounding issue is a more fundamental flaw in the design. (C) Nesting success is a discrete biological outcome; it is the biological replicate, so 'technical replicate' is not the appropriate concept. (D) Geographic variation is one of the many potential confounding variables mentioned in choice A.

Question 7

1. Co-transfecting pGeneY-Luc with a plasmid expressing a known, potent activator of the GeneY promoter. (correct answer)
2. Transfecting a plasmid containing a strong, constitutive promoter (like CMV) driving the luciferase gene.
3. Co-transfecting pGeneY-Luc with an empty vector that does not express TF1.
4. Measuring the endogenous mRNA levels of GeneY in cells overexpressing TF1 using qPCR.

Explanation: The correct answer is A. This control demonstrates that the pGeneY-Luc reporter construct is responsive to activation and that the cell system is capable of producing a detectable signal. It specifically validates the promoter part of the reporter system. (B) is a positive control for luciferase expression and detection in general, but it doesn't test the responsiveness of the specific GeneY promoter. (C) is the essential negative control for the experiment, establishing the baseline luciferase activity. (D) is a different experimental method to test the same hypothesis, not a control for the reporter assay itself.

Question 8

1. Allowing the original breeding pair to produce three more litters and pooling the data from all 600+ progeny.
2. Setting up five new, independent breeding pairs with the same parental genotypes and analyzing the pooled progeny. (correct answer)
3. Having a different lab technician recount the 160 progeny from the first litter to check for counting errors.
4. Performing the same cross using mice that have been moved to a different room in the animal facility.

Explanation: The correct answer is B. True biological replication aims to account for biological variability. Using multiple independent breeding pairs controls for pair-specific effects, such as a particular male's poor fertility, a female's poor maternal care, or stochastic variations in meiosis and fertilization that could skew ratios from a single pair. (A) increases the sample size from a single replicate, which is helpful but does not account for variability between breeding pairs. (C) is a check for measurement error, not biological replication. (D) tests for an environmental effect but is not as fundamental as replicating the biological units (the pairs) themselves.

Question 9

1. A population of the same cell line that remains completely untreated.
2. A population of the same cell line transfected with the plasmid delivery vehicle only.
3. A population of the same cell line transfected with a similar plasmid encoding Cas9 and a non-targeting (scrambled) gRNA. (correct answer)
4. A population of a different cell line in which GENEX is known to be non-functional, transfected with the same plasmid.

Explanation: The correct answer is C. A non-targeting gRNA controls for the cellular stress and potential off-target effects of expressing the Cas9 nuclease and a gRNA, as well as the transfection process itself. This setup isolates the phenotype specifically caused by the targeting of GENEX. (A) is a baseline but does not control for the effects of transfection or Cas9/gRNA expression. (B) is a vehicle control that accounts for the transfection reagent's effect but not for the expression of the CRISPR components. (D) is not a proper control as it uses a different cell line, introducing confounding genetic variables.

Question 10

1. Performing the IP on a cell line in which the gene encoding TF-A has been knocked out.
2. Performing the IP without adding any antibody to the chromatin lysate.
3. Performing the IP using an antibody that recognizes a different transcription factor with well-known binding sites.
4. Performing the IP using a non-specific immunoglobulin G (IgG) from the same host species as the anti-TF-A antibody. (correct answer)

Explanation: When evaluating ChIP-seq experimental design, you need to understand that negative controls help distinguish true protein-DNA binding from non-specific interactions during the immunoprecipitation process. The goal is to identify what DNA gets pulled down due to random antibody binding rather than specific transcription factor interactions. Answer D is correct because using non-specific IgG from the same host species (e.g., rabbit IgG if your anti-TF-A antibody came from rabbit) controls for non-specific antibody binding to chromatin, beads, or plastic surfaces. This matched control has the same basic antibody structure and properties as your experimental antibody but lacks the specific binding domain for TF-A. Any DNA sequences that appear in both your experimental sample and the IgG control likely represent background noise rather than true TF-A binding sites. Answer A is problematic because knockout cells test whether TF-A is necessary for the signal, but don't control for non-specific antibody interactions—the anti-TF-A antibody could still bind non-specifically to other proteins or surfaces. Answer B (no antibody) doesn't control for antibody-related artifacts since you're removing the antibody entirely rather than testing its specificity. Answer C (different transcription factor antibody) introduces variables related to that other transcription factor's binding properties and doesn't properly control for non-specific interactions of your specific anti-TF-A antibody. Remember: effective negative controls should be as similar as possible to your experimental condition except for the one variable you're testing—in this case, specific antigen recognition.

Question 11

1. Re-sequencing the region around the SNP in the original 5,000 Finnish individuals using a more accurate sequencing technology.
2. Expanding the original study by recruiting and genotyping an additional 5,000 individuals from Finland and re-analyzing the combined data.
3. Genotyping the same SNP in a newly recruited cohort of 5,000 individuals with and without type 2 diabetes from Japan. (correct answer)
4. Performing a different set of statistical tests on the original genotype data from the 5,000 Finnish individuals to confirm the p-value.

Explanation: The correct answer is C. An independent replication requires testing the same hypothesis in a new, non-overlapping cohort of subjects, preferably from a different population to show that the association is not specific to the original group's genetic background or environment. (A) is a technical replication, checking the accuracy of the original genotyping. (B) is an extension of the original study to increase its statistical power, not an independent test of the finding. (D) is a test of statistical robustness, not a biological replication.

Question 12

1. Mutant flies raised on a standard food medium with neither DrugX nor ethanol.
2. Wild-type flies raised on the food medium containing DrugX dissolved in 5% ethanol.
3. Mutant flies raised on a food medium containing 5% ethanol but without DrugX. (correct answer)
4. Wild-type flies raised on a standard food medium with neither DrugX nor ethanol.

Explanation: The correct answer is C. A vehicle control is used to assess the effect of the solvent or delivery medium in which the experimental compound is dissolved. In this case, it is essential to determine if the 5% ethanol itself has any effect on the crumpled-wing phenotype, independent of DrugX. (A) is the negative control for the phenotype, but it doesn't account for the vehicle. (B) tests for drug toxicity in a wild-type background, which is a different question. (D) is a wild-type baseline control.

Question 13

1. The paired design, using normal and tumor tissue from the same patient, introduces bias.
2. The experiment lacks technical replicates because each sample was run on only one microarray chip.
3. The sample size of three per condition is too low to detect statistically significant expression changes.
4. There are no true biological replicates, as all samples originate from a single individual. (correct answer)

Explanation: The correct answer is D. The most significant flaw is the lack of biological replicates from different individuals. Any observed gene expression differences may be idiosyncratic to this one patient's specific genetic background, environmental exposures, or tumor subtype. The results are not generalizable to the broader population of liver cancer patients. (A) is incorrect; a paired design is actually a strength because it controls for inter-individual genetic variation. (B) and (C) are potential issues, but the lack of biological replicates is a more fundamental flaw that invalidates any general conclusions, regardless of statistical power or technical precision.

Question 14

1. It serves as a form of technical replication, ensuring the consistency of the experimental procedure.
2. It controls for potential off-target effects, as it is unlikely two different siRNAs would share the same unintended targets. (correct answer)
3. It demonstrates that silencing the gene is effective regardless of which region of the mRNA is targeted.
4. It provides an internal positive control for transfection efficiency and a negative control for sequence-independent effects.

Explanation: The correct answer is B. The main weakness of RNAi is the potential for an siRNA to silence unintended genes that have similar sequences (off-target effects), leading to a misleading phenotype. By using two different siRNAs that target the same gene but have different sequences, researchers can be more confident that the observed phenotype is due to the silencing of the intended target, as it's highly improbable that both siRNAs would produce the same phenotype via the same off-target mechanism. (A) This is not technical replication, which would involve repeating the experiment with the same siRNA. (C) While true, this is a consequence, not the primary reason. (D) Neither siRNA acts as a control in this sense; a separate non-targeting siRNA is used as a negative control.

Question 15

1. A population of the same cell line that remains completely untreated.
2. A population of the same cell line transfected with the plasmid delivery vehicle only.
3. A population of the same cell line transfected with a similar plasmid encoding Cas9 and a non-targeting (scrambled) gRNA. (correct answer)
4. A population of a different cell line in which GENEX is known to be non-functional, transfected with the same plasmid.

Explanation: The correct answer is C. A non-targeting gRNA controls for the cellular stress and potential off-target effects of expressing the Cas9 nuclease and a gRNA, as well as the transfection process itself. This setup isolates the phenotype specifically caused by the targeting of GENEX. (A) is a baseline but does not control for the effects of transfection or Cas9/gRNA expression. (B) is a vehicle control that accounts for the transfection reagent's effect but not for the expression of the CRISPR components. (D) is not a proper control as it uses a different cell line, introducing confounding genetic variables.

Question 16

1. Pool all 9 measurements from the treated group and all 9 from the control group and perform a t-test with n=9 for each group.
2. For each mouse, average the Cq values of the 3 technical replicates, then perform a t-test comparing the 3 average values from the treated group to the 3 average values from the control group. (correct answer)
3. Perform six separate t-tests, each comparing the 3 technical replicates of a treated mouse to the 3 technical replicates of a control mouse.
4. For each mouse, select the technical replicate with the Cq value closest to the median and perform a t-test on these selected single values (n=3 per group).

Explanation: The correct answer is B. This approach correctly treats the biological replicates (the individual mice) as the independent units of statistical analysis (n=3 per group). The technical replicates are averaged to get a more precise estimate for each biological replicate, but they do not increase the biological sample size. (A) is an example of pseudoreplication, which artificially inflates the sample size and the statistical significance by treating non-independent technical replicates as independent samples. (C) is an incorrect application of statistical testing. (D) introduces bias and discards data that informs the measurement variability for each sample.

Question 17

1. Performing a Bradford assay to precisely quantify and load an equal mass of protein from each cell lysate.
2. After transferring, staining the membrane with Ponceau S to visually confirm that protein bands are present in all lanes.
3. After probing for p-ERK, stripping the membrane and re-probing it with an antibody against a ubiquitously expressed housekeeping protein like β-actin. (correct answer)
4. After probing for p-ERK, stripping the membrane and re-probing it with an antibody that recognizes total ERK (both phosphorylated and unphosphorylated).

Explanation: The correct answer is C. Probing for a housekeeping protein is the gold standard for a loading control. It provides a quantifiable internal standard on the final blot, accounting for both initial quantification errors and any inconsistencies in gel loading or transfer efficiency. (A) is a crucial first step but is prone to pipetting error; the loading control on the blot is the final verification. (B) is a qualitative check but is not suitable for quantification. (D) is an important control to determine if the total amount of ERK protein is changing, but it does not control for the loading of all other proteins in the lysate; β-actin does.

Question 18

1. The paired design, using normal and tumor tissue from the same patient, introduces bias.
2. The experiment lacks technical replicates because each sample was run on only one microarray chip.
3. The sample size of three per condition is too low to detect statistically significant expression changes.
4. There are no true biological replicates, as all samples originate from a single individual. (correct answer)

Explanation: The correct answer is D. The most significant flaw is the lack of biological replicates from different individuals. Any observed gene expression differences may be idiosyncratic to this one patient's specific genetic background, environmental exposures, or tumor subtype. The results are not generalizable to the broader population of liver cancer patients. (A) is incorrect; a paired design is actually a strength because it controls for inter-individual genetic variation. (B) and (C) are potential issues, but the lack of biological replicates is a more fundamental flaw that invalidates any general conclusions, regardless of statistical power or technical precision.

Question 19

1. It serves as a form of technical replication, ensuring the consistency of the experimental procedure.
2. It controls for potential off-target effects, as it is unlikely two different siRNAs would share the same unintended targets. (correct answer)
3. It demonstrates that silencing the gene is effective regardless of which region of the mRNA is targeted.
4. It provides an internal positive control for transfection efficiency and a negative control for sequence-independent effects.

Explanation: The correct answer is B. The main weakness of RNAi is the potential for an siRNA to silence unintended genes that have similar sequences (off-target effects), leading to a misleading phenotype. By using two different siRNAs that target the same gene but have different sequences, researchers can be more confident that the observed phenotype is due to the silencing of the intended target, as it's highly improbable that both siRNAs would produce the same phenotype via the same off-target mechanism. (A) This is not technical replication, which would involve repeating the experiment with the same siRNA. (C) While true, this is a consequence, not the primary reason. (D) Neither siRNA acts as a control in this sense; a separate non-targeting siRNA is used as a negative control.

Question 20

1. Transforming yeast with the Bait plasmid and an empty Prey plasmid and plating on selective media. (correct answer)
2. Performing a screen with a Bait protein known to interact with a specific Prey protein.
3. Transforming yeast with an empty Bait plasmid and the Prey library plasmids and plating on selective media.
4. Confirming positive interactions by re-testing them with a different reporter gene system, such as LacZ.

Explanation: The correct answer is A. This control directly tests for autoactivation by the Bait protein. If yeast transformed with the Bait alone (along with an empty prey vector) can grow on the selective medium, it means the Bait is activating the reporter on its own, and any results from the full screen will be unreliable. (B) is a positive control to ensure the system works. (C) is a control for Prey proteins that can autoactivate, which is also important, but bait autoactivation is often tested first as it would invalidate the entire screen. (D) is a strategy for validating hits after the initial screen, not a control for a primary artifact.