Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- A phenotype depends on the genotype and on the environment in which the organism develops.
- Environmental signals (temperature, day length, light, diet, soil chemistry, other organisms) change which genes are expressed and how much, or change how well a gene product works.
- Phenotypic plasticity is the ability of one genotype to produce different phenotypes in different environments.
- The DNA sequence is not changed, so these responses are not inherited as such. Offspring inherit the alleles, including the ability to respond.
- To show an effect is environmental, hold the genotype constant: clones, inbred lines or identical twins.
Key relationships
| Example | Signal | Result |
|---|---|---|
| sex in many reptiles | egg incubation temperature | in most such turtles, warm eggs give females (more aromatase, more oestrogen); cool eggs give males |
| arctic fox, snowshoe hare | shorter days | moult into a white winter coat |
| hydrangea | soil pH | acidic soil: aluminium taken up, flowers blue; alkaline soil: pink |
| skin | UV light | more melanin (tanning) |
| human height and weight | nutrition, health | where a person falls within a genetically set range |
| yeast | pheromone of the opposite mating type | mating genes switched on, including pheromone production |
| Siamese cat | cool skin | heat-sensitive tyrosinase works, dark fur |
| phenylketonuria | low-phenylalanine diet | prevents brain damage in people with two recessive alleles |
| Reaction norm shape | Meaning |
|---|---|
| flat line | trait not plastic for this factor |
| steep line | very plastic genotype |
| parallel lines | genotypes differ by a constant amount |
| crossing lines, different slopes | genotype-by-environment interaction |
| Calculation | How |
|---|---|
| percentage change | (new − original) ÷ original × 100 |
| pivotal temperature | interpolate to 50% between the two nearest points |
Assumptions behind the numbers
- Comparisons only isolate the environment if the genotypes are the same and other variables are controlled.
- Linear interpolation assumes a straight line between two neighbouring points.
- Small samples support a conclusion; they do not prove it.
Mistakes to avoid
- Saying the environment “changes the genes” or “causes mutations” when it changes gene expression.
- Saying a tan or a winter coat is passed to offspring.
- Calling a trait “genetic or environmental” when it is both.
- Using seedlings, not clones, and then claiming a difference is environmental.
- Dividing a percentage change by the new value instead of the original.
- Forgetting that crossing reaction norms mean the “better” genotype depends on the environment.
Quick self-check
- Cuttings of one clone grow to 20 cm in shade and 30 cm in sun. Percentage increase? ((30 − 20) ÷ 20 × 100 = 50%)
- Why are warm turtle eggs more likely to hatch as females? (Temperature raises expression of aromatase in the gonad; more testosterone becomes oestrogen, so ovaries develop.)
- Why do Siamese cats have dark paws? (Their tyrosinase is heat-sensitive; it only makes melanin in the cooler skin of the extremities.)
Next: practice questions.