How does the environment influence the expression of a genotype?
Topic 5.5 Environmental Effects on Phenotype: explain how environmental factors can affect the phenotype produced by a given genotype.
A focused answer to AP Biology Topic 5.5, covering how temperature, nutrients, pH and other environmental factors influence phenotype, the genotype-by-environment interaction, and norms of reaction, with a worked example.
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What this topic is asking
The College Board (Topic 5.5) wants you to explain how environmental factors can affect the phenotype produced by a given genotype. Phenotype is the product of both genes and environment, not genes alone.
Phenotype as genotype plus environment
This is why "nature versus nurture" is a false split: both contribute, and they interact. A genotype sets a range of possibilities, and the environment determines where within that range the phenotype falls. Many of these environmental effects work by changing gene expression (Unit 6), for example switching genes on or off in response to temperature or nutrients, so the genotype is unchanged but a different set of proteins is made.
This idea connects to evolution as well. Natural selection acts on the phenotype, but only heritable (genetic) differences are passed on. A phenotype produced purely by the environment, such as larger muscles from exercise, is not inherited, whereas a genotype that responds well to the environment can be selected for. Distinguishing heritable change from an environmental response is therefore an important skill in interpreting biological data.
Examples of environmental influence
Norm of reaction
Try this
Q1. Define norm of reaction. [1 point]
- Cue. The range of phenotypes a single genotype can produce across different environments.
Q2. Explain why genetically identical organisms can look different. [2 points]
- Cue. Phenotype depends on both genotype and environment; if the environments differ, the same genotype can produce different phenotypes.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2020 (style)3 marksSection II (short FRQ). The color of the flowers of a hydrangea plant depends on soil pH: acidic soil produces blue flowers and alkaline soil produces pink flowers, in plants of the same genotype. (a) Explain how this illustrates the effect of the environment on phenotype. (b) Predict the flower color if a cutting from a blue plant is grown in alkaline soil.Show worked answer →
A 3-point explain-and-predict short FRQ.
(a) Explain (2 points): (1 point) the plants have the same genotype but different phenotypes (flower color); (1 point) the environmental factor (soil pH) influences how the genotype is expressed, so phenotype depends on both genes and environment.
(b) Predict (1 point): the cutting is genetically identical to the blue plant, but in alkaline soil it would produce pink flowers, because the environment, not the genotype, has changed.
Markers reward identifying that genotype is constant while the environment changes the phenotype.
AP 2018 (style)1 marksSection I (multiple choice). Genetically identical hydrangea plants produce different flower colors in different soils. This best demonstrates that: (A) the plants have different genotypes. (B) phenotype can be influenced by the environment. (C) a mutation has occurred. (D) the trait is sex-linked.Show worked answer →
The correct answer is (B).
Identical genotypes giving different phenotypes in different environments shows that the environment influences phenotype. No genetic change (A, C) is involved, and there is no link to the sex chromosomes (D).
Related dot points
- Topic 5.3 Mendelian Genetics: apply the laws of segregation and independent assortment to predict genotype and phenotype ratios.
A focused answer to AP Biology Topic 5.3, covering the laws of segregation and independent assortment, Punnett squares, monohybrid and dihybrid crosses, and the chi-square test for goodness of fit, with worked calculations.
- Topic 5.4 Non-Mendelian Genetics: explain inheritance patterns that depart from simple dominance, including incomplete dominance, codominance, sex linkage, polygenic traits and linkage.
A focused answer to AP Biology Topic 5.4, covering incomplete dominance, codominance, multiple alleles, sex-linked traits, polygenic inheritance and gene linkage, with a worked sex-linkage cross.
- Topic 6.5 Regulation of Gene Expression: explain how gene expression is regulated in prokaryotes and eukaryotes, including operons and regulatory sequences.
A focused answer to AP Biology Topic 6.5, covering the lac and trp operons, promoters, regulatory sequences, transcription factors and epigenetic control, and how regulation lets cells respond to the environment, with a worked operon example.
- Topic 6.6 Gene Expression and Cell Specialization: explain how differential gene expression produces specialized cell types from one genome.
A focused answer to AP Biology Topic 6.6, covering differential gene expression, cell differentiation, the role of signalling and transcription factors, stem cells, and how one genome builds many cell types, with a worked example.
- Topic 4.4 Feedback: explain how negative feedback maintains homeostasis and how positive feedback amplifies a response, using examples from cellular and organismal systems.
A focused answer to AP Biology Topic 4.4, covering negative feedback and homeostasis, positive feedback and amplification, set points, and how feedback data are analyzed, with a worked chi-square example.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)