Biology · CED Unit 5 Heredity · 14 min read · Updated 2026-05-10
Environmental Effects on Phenotype — AP Biology
AP Biology · CED Unit 5 Heredity · 14 min read
1. Norm of Reaction★★☆☆☆⏱ 3 min
The norm of reaction describes the full range of phenotypes that a single, fixed genotype can produce across different environmental conditions. It quantifies how sensitive a genotype’s phenotype is to environmental variation. Non-plastic traits have a flat norm of reaction (phenotype does not change across environments), while plastic traits have a non-zero slope, indicating phenotype changes with environment. Different genotypes often have different norms of reaction, creating genotype-environment (G×E) interaction.
Exam tip: On the AP exam, always look for differences in slope or shape between genotype lines on a norm of reaction graph. Different shapes are direct evidence of G×E interaction, the most common expected answer for graph interpretation questions.
2. Phenotypic Plasticity and Temperature-Sensitive Alleles★★☆☆☆⏱ 3 min
Phenotypic plasticity is the ability of a single genotype to produce different phenotypes in response to different environmental conditions. When the plastic response produces discrete, distinct phenotypes rather than a continuous range, it is called polyphenism. A common example of temperature-dependent plasticity comes from temperature-sensitive alleles, where the protein product is only active at a specific temperature range due to temperature effects on protein folding.
Exam tip: Always connect temperature-sensitive phenotype to enzyme structure and function. AP exam questions almost always expect you to explicitly link temperature to protein folding and activity, not just state the allele is temperature-sensitive.
3. Nutrient Effects and Transgenerational Epigenetics★★★☆☆⏱ 4 min
Nutrient availability is a major environmental modifier of phenotype, especially during development. For example, human height is ~80% heritable, but severe childhood malnutrition can stunt adult growth far below an individual’s genetic potential. Epigenetic effects are another key class of environmental effects: environment alters gene expression without changing the underlying DNA sequence, and these changes can sometimes be passed to offspring.
Exam tip: Remember that epigenetic changes do not alter DNA sequence, only gene expression. The AP exam frequently tests the distinction between genetic changes (mutation, altered sequence) and epigenetic changes (altered expression, sequence unchanged).
4. Sex-Influenced and Sex-Limited Traits★★★☆☆⏱ 3 min
An individual’s biological sex creates a unique internal hormonal environment that interacts with autosomal genotype to produce phenotype, leading to two classes of traits: sex-influenced and sex-limited. Sex-influenced traits are autosomal traits expressed differently in males and females due to hormone differences. Sex-limited traits are autosomal traits only expressed in one sex, because they are tied to sex-specific reproductive functions.
Exam tip: Do not confuse sex-influenced autosomal traits with X-linked traits. Male-biased expression does not automatically mean the trait is carried on the X chromosome; always check what the question states about the gene location.
Common Pitfalls
Why: Students forget that identical twins share a genotype but experience different environmental conditions over development, driving phenotypic differences via plasticity and epigenetic changes
Why: Students mix up population-level variation across genotypes with genotype-specific responses to environmental change
Why: Textbooks highlight rare examples of transgenerational epigenetic inheritance, leading students to assume all epigenetic changes are passed to offspring
Why: Students associate male-biased traits with X-linkage, but pattern baldness is a classic example of an autosomal sex-influenced trait
Why: Students see a large environmental effect and assume genetics do not matter, but most traits are the product of both