Same genes, different conditions, different phenotype. Hold the genotype constant, and whatever still differs comes from the environment.
Genes set a range of possible phenotypes. The environment decides where in that range an organism lands. One genotype can build dark fur or pale, a female or a male, big thin leaves or small thick ones.
Key terms
- Phenotype — any trait you can observe or measure. It comes from genotype and environment acting together.
- Phenotypic plasticity — one genotype producing different phenotypes in different environments.
- Norm of reaction — the full range of phenotypes one genotype can produce across environments.
- Clones — genetically identical individuals, like cuttings from one plant. They let an experiment hold genotype constant.
- Gene–environment interaction — genotypes responding differently to the same change, so which one does best depends on conditions.
How the environment gets in
- Temperature can change protein shape. Himalayan rabbits make a heat-sensitive pigment enzyme that works only in cool skin → dark ears, nose, feet, and tail.
- Temperature can set sex. In many turtles, warm eggs hatch mostly female and cool eggs mostly male: temperature-dependent sex determination.
- Soil can change color. Cuttings of one hydrangea bloom blue in acidic soil and pink in alkaline soil.
- Light can reshape leaves. On one tree, shade leaves grow wider and thinner than sun leaves, catching more of the dim light.
- Diet can decide whether a genotype causes disease. People with PKU can't break down phenylalanine. A low-phenylalanine diet from infancy prevents most of the damage.
- Cues can trigger a trade-off. Tadpoles in a drying pond metamorphose sooner but smaller. They escape the pond, and pay for it in size.
Why it matters
Questions here are experiment-reading questions. The design move: hold genotype constant with clones or inbred lines, or split siblings from one clutch across the groups, then change one condition. Any difference left comes from the environment. Then check survival. If about the same share survived in every group, the shift came from development, not from one type dying off.
Exam traps
| Trap | Actually |
|---|
| Clones that look different must carry different alleles or new mutations. | Clones share one genotype. The environment changed how genes were expressed or how proteins worked, not the DNA. |
| The warm incubator killed male embryos, leaving extra females. | Check survival. If every group hatched at about the same rate, no sex was killed off. Temperature set sex. |
| The line that grew tallest in one condition carries the tall alleles. | Rankings can flip in another environment. One growing condition can't show which genotype is superior. |
| Every line responded to temperature, so height is environmental, not genetic. | The lines responded differently, so genes matter too. Phenotype is genotype and environment working together. |
Worked example
Water fleas from one clone were raised in two tanks. Tank A held plain water. Tank B held water that predatory midge larvae had lived in; the larvae were removed first. In tank A, 92% survived, head spines averaged 0.2 mm, and broods averaged 14 eggs. In tank B, 90% survived, head spines averaged 0.5 mm, and broods averaged 10 eggs. What best explains the difference?
- One clone means one genotype in both tanks. Genes can't explain the difference; the water can.
- No predator was in either tank, and survival barely differs, 92% vs 90%. Nothing was weeded out. The same genotype built a different body.
- The smaller broods show the cost. Spines take resources, so building them only when the predator cue is present pays off.
Phenotypic plasticity: a chemical cue from predators triggered spine growth in the same genotype, at a cost in eggs.
The environment changed development, not alleles. The near miss is 'predators selected for spiny water fleas.' But no predator was in the tank, survival was nearly equal, and a single clone has no genetic differences for selection to act on.