1Genotype + environment → phenotype

Think of the genotype as setting a range of possible phenotypes, and the environment as deciding where in that range an individual ends up.

• A plant with alleles for tall growth will still be short if it grows in poor soil, in shade or in drought. A plant with alleles for short growth cannot be made tall by good conditions. The genotype sets the limits; the environment picks a point within them.
• The mechanism is gene expression. An environmental signal (a temperature, a hormone, a chemical) is detected, a signal pathway responds (Unit 4), and certain genes are switched on or off, or expressed more or less. Different proteins, or different amounts of them, give a different phenotype. The DNA sequence does not change.

The environment changes which genes are expressed, not the genes themselves.

2Examples to know

You will meet unfamiliar examples in the exam, so learn the pattern: name the genotype (unchanged), the environmental factor, and the phenotype it changes.

Height and weight in humans. Height depends on many genes, but childhood nutrition and health decide how much of the genetic potential is reached. Identical twins, with the same genotype, can differ in height and weight if their diets or illnesses differ.

Flower colour and soil pH. In some hydrangeas, plants with the same genotype produce blue flowers in acidic soil and pink flowers in alkaline soil. In acidic soil, aluminium ions are more available to the roots, and their uptake changes the colour of the flower pigment.

Seasonal fur colour in arctic animals. Arctic foxes and snowshoe hares grow a white coat in winter and a brown or grey coat in summer. Changing day length triggers the moults and switches pigment production on or off: one genotype, two phenotypes a year, each matching the season's background.

Temperature and fur colour in Himalayan rabbits. The enzyme that makes the dark pigment works only at lower temperatures. The ears, nose, feet and tail are cooler than the body, so fur grows dark there and white elsewhere. Shave a patch on the back and keep it cold, and the new fur grows dark.

Effect of UV light on melanin. Exposure to ultraviolet light increases the production of the pigment melanin in skin cells: tanning. The genotype is unchanged; the amount of pigment changes with exposure.

Mating type and pheromones in yeast. A yeast cell that detects a pheromone released by a cell of the opposite mating type responds by switching on genes needed for mating, including genes for making more of its own pheromone. The presence of another organism is the environmental signal.

Diet and phenylketonuria (PKU). People homozygous for the PKU allele cannot break down the amino acid phenylalanine, which builds up and damages the developing brain. On a diet low in phenylalanine from birth, the same genotype gives normal development. A change in the environment, not in the genes, prevents the phenotype.

3Temperature and sex in reptiles: reading the data

In many turtles, and some other reptiles, sex is not set by sex chromosomes at fertilisation. It is set by the temperature at which the egg is incubated during a sensitive period of development, which switches on the genes for one sex or the other. This is temperature-dependent sex determination.

Figure 1 shows invented data for an invented turtle population: eggs from the same population incubated at different constant temperatures.

Read it the way an examiner wants:

• Describe the trend with numbers. Below 26 °C almost all hatchlings are male (0% female at 25 °C, 3% at 26 °C). The percentage female rises steeply between 27 °C (12%) and 30 °C (93%), and at 31 °C all hatchlings are female.
• Identify a threshold. The curve crosses 50% between 28 °C (38%) and 29 °C (71%): around 28.5 °C, equal numbers of each sex would hatch.
• Explain using the topic. The eggs share one population's genes; the only variable changed was temperature, so temperature, acting on gene expression during development, determines sex.
• Predict. If nests warmed by 2 °C on average, a beach where nests now sit at 28 °C (38% female) would produce mostly females (93% at 30 °C). A science-practice question might ask you to predict an effect of this kind on the population.

4Designing an experiment on plasticity

To show that a difference is caused by the environment, the organisms compared must have the same genotype, so that genes cannot explain the difference.

• Hold the genotype constant: use clones (cuttings from one plant, a single yeast strain, genetically identical lab animals), or split one clutch of eggs or one batch of seeds, which are at least closely related, between treatments.
• Change one environmental variable (the independent variable): temperature, soil pH, light, diet.
• Measure the phenotype (the dependent variable): flower colour, height, sex ratio, pigment content.
• Keep other conditions the same (controlled variables): water, light, pot size, timing.
• Replicate: several individuals per treatment, so a chance difference in one individual does not decide the result.

Worked example (invented). Cuttings from one hydrangea plant are grown in compost adjusted to pH 5.0, pH 6.0 and pH 7.5, ten cuttings each, with the same water, light and pot size. Flower colour is scored. The cuttings are genetically identical, so if colour differs between the groups, the difference is caused by soil pH. A good null hypothesis: soil pH has no effect on the proportion of plants with blue flowers.