Environmental Effects on Phenotype - AP Biology

SECTION 1

Historical Context & Motivation

Early geneticists assumed a straightforward relationship between genotype and phenotype: one gene, one trait, one outcome. Gregor Mendel's pea plants, grown under carefully controlled greenhouse conditions, seemed to confirm this deterministic view. However, as genetics moved from monastery gardens to field studies and hospital wards, researchers encountered puzzling cases where organisms with identical genotypes displayed markedly different phenotypes. The question of how the environment shapes the expression of genes has become one of the most important and nuanced themes in modern biology.

1909 • Johannsen Coins 'Genotype' and 'Phenotype'

Wilhelm Johannsen formally distinguished the genetic constitution of an organism (genotype) from its observable traits (phenotype), establishing the conceptual framework for studying gene–environment interactions.

1940s • Norm of Reaction Established

Theodosius Dobzhansky and others used Drosophila raised at different temperatures to show that a single genotype can produce a range of phenotypes across environments, formalizing the norm of reaction concept.

1942 • Waddington's Epigenetic Landscape

Conrad Waddington introduced the metaphor of an 'epigenetic landscape' to illustrate how developmental pathways channel gene expression, with environmental signals deflecting the trajectory of development.

1990s–Present • Molecular Epigenetics

Discovery of DNA methylation, histone modification, and non-coding RNA regulation revealed molecular mechanisms by which environmental factors alter gene expression without changing the DNA sequence itself.

These discoveries reframed the central question of heredity: if the genotype does not rigidly dictate the phenotype, what determines which phenotype an organism actually displays? The answer lies in the interplay between genetic instructions and the environmental context in which those instructions are read—a theme that pervades ecology, medicine, agriculture, and evolutionary biology.

SECTION 2

Core Principles & Definitions

Understanding environmental effects on phenotype requires a firm grasp of several interconnected concepts. At its core, the relationship can be expressed simply: Phenotype = Genotype + Environment + (Genotype × Environment interaction). This equation captures the idea that genes set a range of possible outcomes, the environment selects from within that range, and the interaction between the two can produce effects that neither factor alone would predict.

Norm of Reaction

The full range of phenotypes a single genotype can produce across different environments. A flat norm of reaction indicates low environmental sensitivity; a steep one indicates high plasticity.

Phenotypic Plasticity

The ability of an organism to alter its phenotype in response to environmental conditions without any change in genotype. Examples include tanning in response to UV light and leaf shape changes in aquatic vs. terrestrial environments.

Multifactorial Traits

Traits influenced by multiple genes (polygenic) and environmental factors simultaneously. Human height, skin color, and susceptibility to type 2 diabetes are classic examples.

Epigenetics

Heritable changes in gene expression that do not involve alterations to the DNA sequence. DNA methylation and histone modifications are molecular mechanisms by which environmental signals regulate which genes are active.

SECTION 3

Visualizing the Norm of Reaction

Genotype A (cyan) shows a steep norm of reaction: body size changes dramatically with temperature. Genotype B (pink) is relatively canalized, producing a similar phenotype regardless of temperature. The crossing of the lines illustrates a genotype × environment (G × E) interaction—the 'best' genotype depends on the environment.

The diagram above captures the essential insight of the norm of reaction: a genotype does not produce a single fixed phenotype but rather a range of phenotypes depending on environmental conditions. When the reaction norm is steep, the organism displays high phenotypic plasticity. When it is flat, the trait is said to be canalized—buffered against environmental variation. Notice that the two curves cross: at 15 °C, Genotype B produces a larger body size, but at 35 °C, Genotype A does. This crossing pattern means you cannot simply rank one genotype as 'better' without specifying the environment, which is the hallmark of a G × E interaction.

SECTION 4

Molecular Mechanisms of Environmental Influence

How does an environmental signal actually change what a gene does? The answer operates at multiple levels—from transcription factor activity to chromatin remodeling. Environmental cues such as temperature, nutrition, light, and chemical exposure feed into cellular signaling pathways that ultimately alter gene expression without modifying the underlying DNA sequence. The three most studied molecular mechanisms are DNA methylation, histone modification, and temperature-sensitive protein folding.

DNA Methylation

Methyl groups (–CH₃) added to cytosine bases in CpG islands typically silence gene transcription by preventing transcription factors from binding to promoter regions. Environmental factors such as diet (folate availability), toxins, and stress hormones can alter methylation patterns. In the classic Agouti mouse experiment, genetically identical mice displayed phenotypes ranging from obese and yellow to lean and brown depending on maternal dietary methyl donors—a striking demonstration that nutrition during development can redirect phenotypic expression through epigenetic marks.

Histone Modification

Histones can be acetylated, methylated, phosphorylated, or ubiquitinated at their N-terminal tails. Acetylation of histones generally loosens chromatin (euchromatin), making genes accessible for transcription, while deacetylation leads to tighter packing (heterochromatin) and gene silencing. Environmental stressors, including temperature extremes and caloric restriction, have been shown to shift the acetylation landscape, thereby altering the transcriptional profile of cells.

Chromatin accessibility can differ without changing the DNA sequence. The example is schematic and does not imply inheritance across generations.

Exact schematic

Temperature-Sensitive Alleles & Proteins

Some proteins function only within a narrow temperature window. The Siamese cat coat-color pattern results from a temperature-sensitive allele of the tyrosinase gene (the cˢ allele). The enzyme is active only in cooler body regions (ears, nose, paws, tail), producing dark pigment there, while warmer regions remain pale. The genotype is uniform throughout the body, but the phenotype is patterned because the enzyme's conformation depends on local temperature.

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AP EXAM TIP

SECTION 5

Classic Examples Across Kingdoms

Environmental effects on phenotype are not limited to a single taxon. From plants to reptiles to humans, organisms across the tree of life demonstrate how genotype and environment interact to produce observable traits.

Four examples spanning plants, mammals, humans, and reptiles. In each case, the genotype remains constant while an environmental variable (soil pH, temperature, nutrition, or incubation temperature) drives phenotypic variation.

Organism	Trait	Environmental Factor	Mechanism
Hydrangea	Flower color	Soil pH	Aluminum ion availability alters pigment molecule conformation
Siamese cat	Coat color pattern	Body temperature	Temperature-sensitive tyrosinase allele; active only in cooler regions
Humans	Height	Nutrition	Adequate nutrition during development allows full expression of height-related polygenes
Crocodilians / some turtles	Sex	Incubation temperature	Temperature-dependent sex determination (TSD); aromatase activity varies with temperature
Daphnia (water flea)	Helmet/spine morphology	Predator chemical cues (kairomones)	Chemical signals activate defensive morphology genes during development

Selected examples of environmental phenotype modification across taxa

A Siamese cat with dark extremities and pale torso,with magnified fur details. A temperature-sensitive tyrosinase allele can produce more melanin in cooler extremities and less in the warmer torso. Temperature changes enzyme activity while the DNA sequence remains the same.

Same allele, different enzyme activity. A temperature-sensitive tyrosinase allele can produce more melanin in cooler extremities and less in the warmer torso. Temperature changes enzyme activity while the DNA sequence remains the same. A temperature-sensitive tyrosinase allele can produce more melanin in cooler extremities and less in the warmer torso. Temperature changes enzyme activity while the DNA sequence remains the same.

Selected v2 places the shade canopy over the left nest only, with the right nest in sunlight and probes at comparable depths. Eggs do not visually identify sex. Compare independently replicated shaded and unshaded nests with matched depth and handling. Measure temperature through the sensitive incubation period and relate it to species-specific offspring sex ratios.

Test a temperature-sensitive phenotype. Compare independently replicated shaded and unshaded nests with matched depth and handling. Measure temperature through the sensitive incubation period and relate it to species-specific offspring sex ratios. Compare independently replicated shaded and unshaded nests with matched depth and handling. Measure temperature through the sensitive incubation period and relate it to species-specific offspring sex ratios.

SECTION 6

Worked Example: Interpreting a Norm of Reaction Experiment

Suppose a researcher clones two genotypes of a plant species (Genotype X and Genotype Y) and grows replicates at three light intensities: low (200 μmol photons m⁻² s⁻¹), medium (600), and high (1000). She measures mean leaf area (cm²) after 8 weeks.

Light Intensity	Genotype X leaf area (cm²)	Genotype Y leaf area (cm²)
Low (200)	45	42
Medium (600)	30	38
High (1000)	15	35

Mean leaf area after 8 weeks at three light intensities

Worked example

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Step 1 — Identify the Variables

The independent variable is light intensity (three levels). The dependent variable is mean leaf area. The genotype (X vs. Y) is the factor whose reaction norms we want to compare. Because the plants are clones, genetic variation within each genotype is eliminated.
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Step 2 — Assess Plasticity of Each Genotype

Genotype X leaf area decreases from 45 cm² to 15 cm², a range of 30 cm². Genotype Y decreases from 42 cm² to 35 cm², a range of only 7 cm². Therefore, Genotype X is far more plastic in its leaf area response to light.

Genotype X range = 30 cm²; Genotype Y range = 7 cm²
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Step 3 — Check for G × E Interaction

At low light, Genotype X has slightly larger leaves than Y (45 vs. 42). At high light, Genotype Y has much larger leaves than X (35 vs. 15). The rank order of genotypes reverses across environments, which is definitive evidence of a G × E interaction.

G × E interaction confirmed—rank reversal across environments
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Step 4 — Biological Interpretation

Genotype X displays a classic shade-avoidance response, producing large leaves in low light to maximize photon capture, but dramatically reducing leaf area under high light (possibly to reduce water loss and photodamage). Genotype Y is canalized for moderate-sized leaves regardless of light. Neither genotype is universally 'better'; the optimal genotype depends on the light environment.

Adaptive interpretation: plasticity in Genotype X may be favored in variable light habitats

Three light-controlled chamber bays contain similar broadleaf plants in pots, with larger leaves in the dim bay and smaller leaves in the bright bay; illustrated leaves are not measurements. After eight weeks at light intensities of 200, 600 and 1,000 µmol photons/m²/s, mean leaf areas are 45, 30 and 15 cm² for genotype X and 42, 38 and 35 cm² for genotype Y. Their nonparallel responses show genotype-by-environment interaction.

Light changes the phenotype. After eight weeks at light intensities of 200, 600 and 1,000 µmol photons/m²/s, mean leaf areas are 45, 30 and 15 cm² for genotype X and 42, 38 and 35 cm² for genotype Y. Their nonparallel responses show genotype-by-environment interaction. After eight weeks at light intensities of 200, 600 and 1,000 µmol photons/m²/s, mean leaf areas are 45, 30 and 15 cm² for genotype X and 42, 38 and 35 cm² for genotype Y. Their nonparallel responses show genotype-by-environment interaction.

SECTION 7

Plasticity, Canalization & Adaptation

Not all phenotypic plasticity is adaptive. Understanding the spectrum from highly plastic to strongly canalized traits—and recognizing when each strategy confers fitness benefits—is critical for AP Biology.

Feature	High Plasticity	Canalization
Norm of reaction	Steep curve; wide phenotypic range	Flat curve; narrow phenotypic range
Environmental sensitivity	High—phenotype strongly shaped by environment	Low—phenotype buffered against environmental perturbation
Selective advantage	Variable or unpredictable environments	Stable environments where a single phenotype is optimal
Cost	Energetic cost of maintaining sensory/regulatory machinery	Reduced ability to acclimate to new conditions
Classic example	Daphnia helmet formation in response to predator cues	Vertebrate eye lens shape (highly conserved)

Comparison of high plasticity vs. canalization

SECTION 8

Connecting to Evolution & Broader Biology

Environmental effects on phenotype connect directly to several other AP Biology themes: natural selection acts on phenotypes (not genotypes), so the environment shapes both the trait expression and the selective pressures acting on those traits. Additionally, epigenetic modifications raise the possibility that some environmentally induced phenotypic changes can be transmitted across generations—a concept that echoes Lamarckian ideas while operating through firmly Mendelian molecular machinery.

Concept	Basic Understanding (This Lesson)	Advanced Extension
Gene expression regulation	Environment alters which genes are active (methylation, histone mods)	Signal transduction cascades link receptors to transcription factor activation (AP Bio Unit 4)
Natural selection	Selection acts on phenotype, which includes environmental component	Baldwin effect: plasticity may facilitate evolutionary change by allowing survival in novel environments
Twin studies	Monozygotic twins can differ due to environmental effects	Heritability estimates (h²) quantify the genetic vs. environmental contribution to trait variance in a population
Climate change	Temperature-dependent sex determination may skew sex ratios	Phenological mismatches: plasticity limits may be exceeded under rapid environmental change

Connecting environmental effects on phenotype to broader AP Biology concepts

As you encounter later units on ecology and evolution, remember that the phenotype an organism presents to its environment is always the product of both its genetic endowment and the conditions in which it develops. This insight is foundational for understanding fitness, adaptation, and the ongoing response of populations to global environmental change.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

A researcher grows genetically identical sunflower plants in two gardens: one at sea level and one at 3000 m elevation. Plants at high elevation are shorter. Which of the following best explains this observation?

A. A mutation occurred in the high-elevation plants due to increased UV exposure.
B. The environment at high elevation altered gene expression, resulting in a different phenotype from the same genotype.
C. Natural selection at high elevation favored shorter individuals over the course of the experiment.
D. The high-elevation plants inherited different alleles for height from their parents.

Show answer (B)

B is correct. Because the plants are genetically identical (clones), any phenotypic difference must arise from environmental influences on gene expression—this is a textbook example of phenotypic plasticity. A is wrong because a mutation would change the DNA sequence, and there is no evidence of that; moreover, UV-induced somatic mutations would not systematically reduce height in all individuals. C is wrong because natural selection requires heritable genetic variation and many generations—this is a single-generation experiment with clones. D is wrong because the plants are genetically identical, so they cannot carry different alleles.

Question 2: BASIC CALCULATION

Two genotypes of Drosophila are raised at 18 °C and 28 °C. Genotype 1 has wing lengths of 3.0 mm (18 °C) and 2.2 mm (28 °C). Genotype 2 has wing lengths of 2.7 mm (18 °C) and 2.6 mm (28 °C). Which statement is correct?

A. Both genotypes show equal phenotypic plasticity for wing length.
B. Genotype 2 is more plastic than Genotype 1 because its wings are always shorter.
C. Genotype 1 shows greater phenotypic plasticity because its wing length changes by 0.8 mm across environments compared to 0.1 mm for Genotype 2.
D. Genotype 2 is more plastic because it maintains a consistent phenotype across temperatures.

Show answer (C)

C is correct. Phenotypic plasticity is measured by the magnitude of phenotypic change across environments. Genotype 1 changes by 3.0 − 2.2 = 0.8 mm, while Genotype 2 changes by only 2.7 − 2.6 = 0.1 mm. A is wrong because 0.8 ≠ 0.1. B is wrong because plasticity refers to the degree of change, not the absolute value of the trait—having shorter wings does not indicate plasticity. D confuses canalization (phenotypic constancy) with plasticity; maintaining a consistent phenotype is the opposite of plasticity.

Question 3: INTERMEDIATE

Monozygotic (identical) twins raised apart show a concordance rate of approximately 50% for type 2 diabetes, compared to nearly 100% genetic identity. Which conclusion is best supported by these data?

A. Type 2 diabetes is caused entirely by environmental factors.
B. Type 2 diabetes is a multifactorial trait influenced by both genotype and environment.
C. Type 2 diabetes follows a simple Mendelian autosomal recessive inheritance pattern.
D. The twin who develops diabetes must have acquired a de novo mutation.

Show answer (B)

B is correct. If diabetes were entirely genetic, monozygotic twins would show 100% concordance. The fact that concordance is only ~50% demonstrates a substantial environmental contribution, making this a multifactorial trait. A is wrong because some genetic influence is indicated by the fact that concordance is higher in monozygotic twins than in dizygotic twins or the general population. C is wrong because autosomal recessive traits would show nearly 100% concordance in identical twins (both carry the same alleles). D is wrong because there is no evidence of new mutations; the discordance is more parsimoniously explained by differences in diet, exercise, and other environmental exposures.

Question 4: APPLIED

A conservation biologist is concerned that rising nest temperatures due to climate change will skew the sex ratio of a sea turtle population that exhibits temperature-dependent sex determination (TSD). At current beach temperatures (29 °C), approximately 50% of hatchlings are female. At projected temperatures (32 °C), models predict >90% female hatchlings. (a) Design an experiment to test whether shading nests can restore a balanced sex ratio. Include your hypothesis, independent variable, dependent variable, control, and at least one controlled variable. (b) Explain the molecular basis of TSD—how does temperature alter sex determination without changing the DNA sequence? (c) Predict what would happen to population viability if the skewed sex ratio persists for multiple generations. (d) Explain why TSD is an example of an environmental effect on phenotype rather than an example of natural selection.

Show answer

(a) Hypothesis: Shading turtle nests will lower sand temperature and increase the proportion of male hatchlings. IV: shading treatment (shaded vs. unshaded). DV: proportion of male and female hatchlings. Control: unshaded nests on the same beach. Controlled variables: clutch size range, nest depth, timing of egg laying, genetic background (same population). Replicates: multiple nests per treatment. (1 pt for testable hypothesis; 1 pt for correct experimental design elements.) (b) In TSD species, incubation temperature regulates the activity of the enzyme aromatase, which converts testosterone to estradiol. At higher temperatures, aromatase activity increases, producing more estrogen and directing gonadal development toward the female pathway. At lower temperatures, aromatase is less active, testosterone levels remain higher, and male development proceeds. Crucially, the DNA sequence is identical in both sexes; the difference is in gene expression patterns driven by temperature. (1 pt) (c) A persistently skewed sex ratio (>90% female) would reduce the effective population size, limit mating opportunities, decrease genetic diversity, and potentially lead to population decline or local extinction if too few males are available to fertilize females. Reduced genetic diversity also compromises the population's ability to adapt to future environmental changes. (1 pt) (d) TSD is an environmental effect on phenotype because a single genotype produces different phenotypes (male or female) depending on the incubation temperature experienced during a critical developmental window. Natural selection, in contrast, is a population-level process that requires heritable genetic variation and differential reproductive success over generations. TSD does not involve changes in allele frequencies—it is phenotypic plasticity acting within one generation. (1 pt)

Question 5: CRITICAL THINKING

Researchers studied two yarrow (Achillea) genotypes cloned and planted at three elevations: 30 m, 1400 m, and 3050 m. Mean plant height (cm) is shown below. | Elevation | Genotype A | Genotype B | |-----------|-----------|------------| | 30 m | 70 | 55 | | 1400 m | 40 | 50 | | 3050 m | 15 | 42 | (a) Construct or describe a graph of these data with elevation on the x-axis and plant height on the y-axis. Identify the type of graph you would use and label all axes. (b) Which genotype shows greater phenotypic plasticity? Justify your answer quantitatively. (c) Is there evidence of a genotype × environment interaction? Explain using the data. (d) A student claims that the shorter plants at 3050 m evolved to be short through natural selection. Evaluate this claim.

Show answer

(a) A line graph is appropriate, with elevation (m) on the x-axis and mean plant height (cm) on the y-axis. Two lines should be plotted—one for Genotype A and one for Genotype B—with distinct symbols or colors. Title: 'Norm of Reaction for Height in Two Yarrow Genotypes Across Elevation.' Each axis should be labeled with units. (1 pt) (b) Genotype A shows greater plasticity. Its height range = 70 − 15 = 55 cm across elevations. Genotype B's range = 55 − 42 = 13 cm. The phenotypic range for Genotype A is more than four times greater than for Genotype B, indicating substantially higher plasticity. (1 pt) (c) Yes, there is a G × E interaction. At 30 m, Genotype A (70 cm) is taller than Genotype B (55 cm). At 3050 m, Genotype B (42 cm) is taller than Genotype A (15 cm). This rank reversal across environments is the defining feature of a G × E interaction—the relative performance of the two genotypes depends on the environment. (1 pt) (d) The student's claim is incorrect. Because the plants at each elevation are clones of the same genotype, the short stature at 3050 m is an environmental effect on phenotype (phenotypic plasticity), not the result of natural selection. Natural selection requires heritable genetic variation and differential reproductive success over multiple generations. These plants carry the same alleles; they are simply expressing those alleles differently under harsh high-elevation conditions (lower temperature, higher UV, shorter growing season). (1 pt)