Biology · Heredity, Variation & Evolution
Traits arise through interactions between genes and environments
Two seedlings can have different leaf lengths even when they share the genotype being studied. Their growing conditions may differ. Two different genotypes can also respond differently to the same change in conditions. Begin by naming the trait and the comparison rather than deciding in advance that it is 'genetic' or 'environmental'.
The earlier lessons on biological scales and cells provide useful background: recognise a cell, its DNA and the difference between an individual and a population. The DNA and nucleotide explanation in Chemistry of Life is optional support. You need fractions such as one half and one quarter; the worked cases explain the calculations before using them. Detailed molecular copying, Mendelian inheritance and evolutionary inference return in later chapters.
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Use a constructed response model to separate genotype, environment and the measured trait, and recognise that a response can differ between genotypes.
A trait is an outcome of a process
A phenotype can be a structure, a biochemical characteristic or a measured performance. State what is measured, when it is measured and under which conditions. 'Leaf length at the stated developmental stage, in millimetres' is a clearer phenotype description than 'plant quality'.
Genes contribute through their products and regulation, while environmental inputs affect the processes in which those products operate. Development connects these influences over time. A phenotype is not a detachable object handed intact from parent to offspring.
Many characteristics are polygenic: variants at multiple genes contribute. A resemblance among relatives can motivate an inheritance question, but shared environments and developmental conditions also need consideration. Resemblance alone does not isolate the contribution of a particular allele.
Compare each genotype in both environments
In our constructed response model, genotype K has mean leaf lengths of 6 mm in Environment 1 and 14 mm in Environment 2. Genotype L has values of 8 mm and 10 mm. The developmental stage is held the same within the model. The values describe hypothetical group responses; they are not observations from tracked leaves.
The environmental contrast is +8 mm for K and +2 mm for L. The response therefore depends on genotype: the model illustrates a genotype-by-environment interaction. This term names a relationship between responses; it does not mean DNA has changed from K to L.
In Environment 1, L's model value is 2 mm larger than K's. In Environment 2, K's is 4 mm larger than L's. Calling one genotype simply 'the large-leaf genotype' would hide the conditions. A larger leaf also need not confer greater reproductive success.
The response depends on genotype
Read the diagram in words
K has values 6 and 14 mm; L has values 8 and 10 mm. K's environmental contrast is 8 mm and L's 2 mm. L exceeds K in Environment 1, while K exceeds L in Environment 2.
| Genotype | Environment 1 (mm) | Environment 2 (mm) |
|---|---|---|
| Genotype K | 6 | 14 |
| Genotype L | 8 | 10 |
Growing environment
Mean leaf length (mm): 0–16
Distinguish development from evolutionary change
An individual can change as it grows or responds to conditions. Phenotypic plasticity is the capacity for a genotype to produce different phenotypes under different conditions. Such a response does not, by itself, demonstrate a change in inherited sequence.
Evolution concerns inherited change in populations across generations. A leaf lengthening on one plant is a developmental change. A shift in measured leaf lengths between two groups might reflect environment, inherited differences or both; measurement of a mean alone does not identify evolution.
To investigate the causes experimentally, one would need appropriate replicated comparisons, assignment of conditions, measurements and controls. The supplied values contain no replicate distribution, so they establish neither a statistical result nor a causal estimate from real plants.
Worked example
Subtract within a genotype before comparing responses
For K, subtract 6 from 14 to obtain an 8 mm environmental contrast. For L, subtract 8 from 10 to obtain 2 mm. The contrast of these contrasts is 8 − 2 = 6 mm. This is a descriptive property of the constructed model. It does not estimate a gene's percentage contribution or say that each individual leaf grew by exactly that amount.
Check your understanding
Which genotype has the larger model value in Environment 1, and which in Environment 2?
Show reasoning
L is larger in Environment 1: 8 versus 6 mm. K is larger in Environment 2: 14 versus 10 mm.
Does K changing from a 6 mm to a 14 mm model value prove that its DNA sequence changed?
Show reasoning
No. The model compares phenotypic responses of the same genotype in two environments. Sequence change would need separate evidence.
Can these four supplied values establish statistical significance?
Show reasoning
No. They are constructed model values and provide no replicated measurements or variation estimates for an inferential analysis.
What to carry forward
Describe a trait with its genotype, environment and developmental context.
References
Resources supporting this lesson’s explanations and examples.
- Talking Glossary — Genes, alleles and inheritance terminologyNational Human Genome Research InstituteScientific glossaryTalking Glossary — Genes, alleles and inheritance terminologyAlleleChromosomeGenotypePhenotypePolygenic traitMutationGenomeLocusMitochondrial DNA
- Biology 2e — Population Genetics (2018)Mary Ann Clark, Matthew Douglas and Jung Choi · OpenStaxTextbook chapter
- Phenotypic plasticity, reaction norm, and genotype by environment interactionRaymond F. Baker Center for Plant Breeding · Iowa State UniversityUniversity research overview