Environmental Effects on Phenotype
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AP Biology › Environmental Effects on Phenotype
Genetically identical yeast cells are grown in either glucose-rich medium or lactose-rich medium. In lactose medium, cells produce high levels of β-galactosidase enzyme; in glucose medium, β-galactosidase levels are low. Sequencing confirms the same DNA sequence in both groups. Which explanation best accounts for the enzyme differences between conditions?
Lactose induced transcription of β-galactosidase genes by activating regulatory proteins.
Glucose changed the yeast genotype by eliminating lactose-digesting alleles from the population.
Glucose caused frameshift mutations that prevented β-galactosidase from being translated.
Lactose increased β-galactosidase by increasing the number of yeast chromosomes.
Lactose increased enzyme levels by permanently inserting new coding DNA into the genome.
Explanation
This question assesses understanding of environmental effects on phenotype, specifically how nutrient availability can influence enzyme production without altering the genetic code. The correct answer, A, is right because lactose acts as an inducer in the lac operon system, binding to repressor proteins and allowing increased transcription of the β-galactosidase gene, leading to higher enzyme levels for lactose metabolism. This classic example of gene regulation enables yeast to adapt to available sugars without changing the DNA sequence, as shown by identical sequencing results. In contrast, glucose represses this expression, resulting in low enzyme levels. A tempting distractor is B, which falsely suggests glucose causes frameshift mutations, arising from the misconception that substrates directly mutate genes rather than regulate their expression. To approach similar questions, identify regulatory mechanisms like induction or repression that alter transcription in response to environmental conditions without genomic changes.
A strain of bacteria with identical genomes was grown in either lactose-containing medium or glucose-containing medium. After 30 minutes, cells in lactose medium produced high levels of β-galactosidase, while cells in glucose medium produced very low levels. DNA sequencing of the β-galactosidase gene was identical in both cultures. When glucose-grown cells were transferred to lactose medium, β-galactosidase levels increased. Which explanation best accounts for the enzyme level differences between media?
Glucose increased chromosome number, reducing enzyme concentration by diluting gene products in the cytoplasm.
Lactose exposure triggered meiosis, producing recombinant bacteria that expressed more enzyme.
Different media changed allele frequencies in the cultures, creating distinct genotypes for enzyme production.
Presence of lactose regulated transcription of the enzyme gene, increasing protein production without changing DNA sequence.
Lactose caused mutations that activated the enzyme gene, and these mutations persisted after transfer.
Explanation
This question examines environmental effects on phenotype through substrate-induced enzyme production. The correct answer is A because the presence of lactose induces transcription of the β-galactosidase gene through regulatory mechanisms like the lac operon, increasing enzyme production without changing the DNA sequence. The identical DNA sequences, rapid enzyme production in lactose medium, and the ability to induce enzyme in previously glucose-grown cells all support this classic example of gene regulation. Answer B is incorrect because it proposes mutations that activate the gene, but mutations would be permanent, detectable by sequencing, and present even after transfer to glucose medium. To recognize environmental gene regulation, look for rapid, reversible changes in protein production that correlate with specific environmental conditions while DNA sequences remain constant.
Genetically identical seedlings of a grass species were grown for 10 days with either normal soil moisture or drought conditions. Drought-grown seedlings had fewer open stomata per leaf surface area at midday and showed lower rates of water loss. DNA sequencing of a stomata-regulating gene was identical in both groups. When drought-grown seedlings were rewatered for several days, the proportion of open stomata at midday increased. Which explanation best accounts for the stomatal differences under drought versus normal moisture?
Drought induced changes in gene expression and signaling in guard cells, altering stomatal opening without DNA change.
Drought increased the frequency of crossing over in leaves, creating new stomatal phenotypes within the plant.
Drought caused random mutations in the stomata-regulating gene, permanently reducing stomatal opening.
Normal moisture increased chromosome number in guard cells, producing more open stomata at midday.
Rewatering changed the plant’s allele frequencies for stomatal traits, reversing the drought phenotype.
Explanation
This question tests understanding of environmental effects on phenotype in plant stress responses. The correct answer is A because drought conditions trigger changes in gene expression and cellular signaling pathways in guard cells, altering stomatal behavior without changing the DNA sequence. The identical DNA sequences between groups and the reversibility of the stomatal phenotype upon rewatering confirm this is a regulatory response, not a genetic change. Answer B is incorrect because it proposes random mutations, which would be permanent, detectable by sequencing, and not reversible by simply changing water availability. To recognize environmental effects on phenotype, look for adaptive responses that can be reversed when conditions change and that occur without alterations to DNA sequence—these indicate gene expression regulation.
Two groups of genetically identical fruit fly larvae were raised on diets differing only in protein content. Adults from the high-protein diet had greater average body mass than adults from the low-protein diet. Sequencing of a growth-regulating gene showed identical DNA sequences in both groups, and the mass difference decreased when low-protein larvae were switched to high-protein food early in development. Which explanation best accounts for the diet-associated mass differences?
High protein caused targeted point mutations in the growth gene that increased adult body mass.
Diet changed expression of growth-related genes during development, altering cell growth rates without DNA changes.
High protein increased the number of chromosomes in somatic cells, leading to heavier adult flies.
Adults from the high-protein group represented a new population with different gene frequencies for body mass.
Low-protein larvae experienced higher recombination rates, producing smaller adults with new allele combinations.
Explanation
This question examines environmental effects on phenotype through nutritional influences on development. The correct answer is A because dietary protein levels can affect the expression of growth-related genes during larval development, altering growth rates and final body size without changing the underlying DNA sequence. The identical DNA sequences between groups and the ability to partially rescue the phenotype by switching diets early in development support this gene expression mechanism. Answer B is incorrect because it proposes targeted mutations, but mutations would be permanent and detectable through DNA sequencing, not reversible by diet changes. To identify environmental effects on phenotype, focus on whether the trait can be modified by changing conditions and whether DNA sequences remain unchanged—these indicate regulation of gene expression rather than genetic changes.
Two groups of genetically identical fruit fly larvae are reared at either 18°C or 29°C. Adult flies from 29°C have darker abdominal pigmentation and higher mRNA levels for a pigment-synthesis enzyme in epidermal cells. Genome sequencing shows no DNA sequence differences. Which explanation best accounts for the pigmentation difference?
Higher temperature increased transcription of pigment-synthesis genes during development without altering DNA.
Higher temperature increased pigmentation by adding extra chromosomes to epidermal cells.
Higher temperature increased pigmentation by inserting bacterial genes into the fly genome.
Higher temperature changed pigmentation by shifting allele frequencies in the fly population.
Higher temperature caused new pigment alleles to form by mutating epidermal DNA.
Explanation
This question assesses understanding of environmental effects on phenotype, specifically how temperature can influence pigmentation without altering the genetic code. The correct answer, A, is right because higher temperature likely activates heat-responsive transcription factors that increase the expression of pigment-synthesis genes during development, leading to darker pigmentation without DNA alterations. This illustrates temperature-dependent gene regulation allowing adaptive coloration in flies. The elevated mRNA levels for the enzyme confirm the role of transcriptional control. A tempting distractor is B, which falsely claims temperature mutates DNA, reflecting the misconception that temperature directly edits genes instead of modulating their expression. To approach similar questions, differentiate between environmental influences on gene expression and actual changes to the genetic sequence in developmental phenotypes.
Clonal cuttings from one houseplant are grown for 6 weeks under either high light or low light. High-light plants develop smaller, thicker leaves and show increased expression of genes encoding photosynthetic proteins in leaf cells; sequencing of these genes shows identical DNA in both groups. When low-light plants are moved to high light, new leaves resemble those of high-light plants. Which explanation best accounts for the difference in leaf thickness between the two treatments?
High light induced transcriptional changes in leaf cells, increasing photosynthetic gene expression without altering DNA.
High light permanently altered the DNA base sequence of all leaf cells, making the trait irreversible.
High light changed the plant’s genome size, leading to thicker leaves due to extra chromosomes.
Low light caused deletions in photosynthesis genes, preventing production of proteins needed for thick leaves.
Low light selected for rare thick-leaf mutants, so only thin-leaf genotypes remained in that group.
Explanation
This question examines environmental effects on phenotype through light-dependent leaf development. The correct answer A states that high light induced transcriptional changes in leaf cells, increasing photosynthetic gene expression without altering DNA - this explains why plants developed thicker leaves with more photosynthetic proteins while maintaining identical DNA sequences. The reversibility when low-light plants were moved to high light confirms this is an environmental response, not a genetic change. Answer B incorrectly claims that low light caused deletions in photosynthesis genes, which contradicts the evidence that DNA sequencing showed identical sequences in both groups - this reflects the misconception that phenotypic differences must result from DNA damage or loss. When analyzing environmental effects, focus on whether changes are reversible and whether DNA sequences remain unchanged, as these indicate transcriptional regulation rather than genetic alterations.
Genetically identical tadpoles were raised in aquaria containing either predator chemical cues (no actual predation) or no cues. Tadpoles exposed to cues developed deeper tail fins and different swimming behavior, but sequencing found no DNA differences between groups. Which explanation best accounts for the cue-induced phenotype differences?
Predator cues increased chromosome number in muscle cells, producing larger tails through extra gene copies.
Predator cues changed gene regulation during development, altering tail growth and behavior without DNA change.
Predator cues caused recombination in somatic tissues, generating new tail-fin alleles in exposed tadpoles.
Predator cues caused random mutations in tail-development genes, producing deeper fins in exposed tadpoles.
Predator cues shifted allele frequencies in the tadpole population, producing deeper fins across generations.
Explanation
This question assesses understanding of environmental effects on phenotype, where external factors influence traits without altering the underlying DNA sequence. The correct answer, B, is right because predator cues can trigger developmental gene regulation, altering expression of growth-related genes to produce deeper tail fins and adaptive behaviors. In tadpoles, chemical signals activate signaling pathways that modify transcription during development, leading to phenotypic plasticity without DNA changes. Since the tadpoles are genetically identical and sequencing shows no differences, the variation is an environmental induction of gene expression. A tempting distractor is A, which is wrong because it assumes cues cause mutations, a misconception that confuses plasticity with mutagenesis. To approach similar questions, always check if phenotypic differences in identical genotypes under varying conditions point to gene expression changes rather than genetic mutations or evolution.
Genetically identical mouse pups are raised from birth in two environments: enriched cages with running wheels and toys, or standard cages. At 8 weeks, neurons from the hippocampus are analyzed. Mice from enriched cages show higher levels of BDNF mRNA and increased dendritic branching, while DNA sequencing of the BDNF gene shows no differences between groups. When enriched-cage mice are moved to standard cages for 4 weeks, BDNF mRNA levels decrease toward the standard-cage level. Which explanation best accounts for the observed phenotype differences between groups?
Enrichment increased neuronal activity that altered transcriptional regulation of BDNF without changing the DNA sequence.
Enrichment caused random mutations in the BDNF gene that increased transcription in hippocampal neurons.
Enrichment changed allele frequencies for BDNF in the mice, producing more branching in the enriched group.
Standard cages triggered DNA replication errors that permanently decreased BDNF protein function in all neurons.
Enrichment inserted new coding exons into the BDNF gene, producing a longer BDNF protein in the brain.
Explanation
This question tests understanding of environmental effects on phenotype, specifically how enrichment affects brain development without genetic changes. The correct answer B explains that enrichment increased neuronal activity that altered transcriptional regulation of BDNF without changing the DNA sequence - this is supported by the evidence that DNA sequencing showed no differences between groups, yet BDNF mRNA levels were higher in enriched mice. The reversibility when mice were moved to standard cages further confirms this is a regulatory change, not a genetic one. Answer A incorrectly suggests random mutations occurred, which contradicts the finding that DNA sequences were identical between groups - this represents the misconception that all phenotypic changes require DNA mutations. The key strategy is to look for evidence of reversibility and unchanged DNA sequences, which indicate environmental regulation of gene expression rather than genetic changes.
Genetically identical bean seedlings are grown in soil with either adequate nitrogen or low nitrogen. Low-nitrogen plants develop more root hairs and show increased transcription of root-hair regulatory genes in root epidermal cells; DNA sequencing shows identical gene sequences in both groups. When nitrogen is restored, newly formed roots show fewer root hairs. Which explanation best accounts for the nitrogen-dependent change in root hair number?
Low nitrogen caused mutations in root-hair genes, creating new alleles that increase root hair number permanently.
Low nitrogen increased the genetic code’s redundancy, allowing more proteins to be translated for root hair formation.
Low nitrogen changed allele frequencies in the seedlings, increasing the proportion of genotypes with many root hairs.
Restoring nitrogen reversed the mutations, returning the DNA sequence to its original state and reducing root hairs.
Low nitrogen altered gene expression in root epidermal cells, increasing transcription of root-hair regulators without DNA changes.
Explanation
This question examines environmental effects on phenotype through nutrient-dependent root development. The correct answer A states that low nitrogen altered gene expression in root epidermal cells, increasing transcription of root-hair regulators without DNA changes - this explains the increased root hair number and gene expression under low nitrogen while DNA sequences remained identical. The reduction in root hairs when nitrogen was restored confirms this is an environmentally regulated developmental response. Answer B incorrectly claims low nitrogen caused mutations creating new alleles, which contradicts both the unchanged DNA sequences and the reversibility when nitrogen was restored - this reflects the misconception that morphological adaptations require permanent genetic changes. Focus on how nutrient availability can trigger developmental programs through gene regulation to optimize resource acquisition without altering DNA.
Genetically identical bacterial cells are grown with or without lactose. Only cells grown with lactose produce high levels of β-galactosidase enzyme and show increased lac operon mRNA; DNA sequencing shows the lac genes are identical in both conditions. When lactose is removed, lac mRNA and enzyme levels decrease. Which explanation best accounts for the lactose-dependent enzyme production?
Lactose caused base substitutions in lac genes, creating a new enzyme that is produced only in lactose.
Lactose acted as an inducer that altered transcription of the lac operon, changing enzyme levels without DNA changes.
Removing lactose reversed the mutations, restoring the original lac DNA sequence and reducing enzyme levels.
Lactose caused duplication of the entire chromosome, increasing gene dosage and permanently increasing enzyme levels.
Lactose increased the population’s allele frequency for lac genes, so more cells inherited the enzyme trait.
Explanation
This question tests understanding of environmental effects on phenotype through the classic lac operon example. The correct answer A explains that lactose acted as an inducer that altered transcription of the lac operon, changing enzyme levels without DNA changes - this accounts for the production of β-galactosidase only in the presence of lactose while lac genes remained identical. The rapid decrease in enzyme levels when lactose was removed confirms this is an inducible regulatory system. Answer B incorrectly suggests lactose caused base substitutions creating a new enzyme, which contradicts both the identical DNA sequences and the reversibility of enzyme production - this represents the misconception that substrate-specific responses require genetic mutations. The strategy is to recognize classic examples of gene regulation where environmental molecules control transcription without altering DNA.