Genotype is not the whole story
Earlier in Unit 5 you predicted phenotypes from genotypes with Punnett squares. Those predictions quietly assume that the environment does not matter. Often it does. A plant with alleles for tall growth will stay short in poor soil. Two identical twins share every allele, yet one can be heavier than the other. So a better summary is:
phenotype = genotype acting in an environment
The genotype sets what is possible. The environment (temperature, light, day length, diet, soil chemistry, other organisms) helps decide which of those possibilities appears.
How the environment reaches the genes
The environment does not rewrite the DNA sequence. It acts in two main ways.
- It changes gene expression. A signal from outside, such as warmth, light or a chemical, is detected by the cell. The signal switches some genes on, switches others off, or changes how much protein is made from them. Different proteins mean a different phenotype. You will study how genes are switched on and off in Unit 6.
- It changes how well a gene product works. Many gene products are enzymes. As you saw in Topic 3.2, enzyme activity depends on temperature and pH. If an enzyme only works below a certain temperature, the phenotype it produces will depend on temperature, even though the gene is the same.
Phenotypic plasticity is the ability of one genotype to produce different phenotypes in different environments. Some traits are very plastic (body mass, plant height). Others hardly change (blood group, eye colour in adults).
Examples you should be able to explain
| Example | Environmental factor | What changes | How |
|---|---|---|---|
| Sex determination in many reptiles | incubation temperature of the egg | male or female | temperature changes which sex-determining genes are expressed in the developing gonad, including the gene for aromatase, the enzyme that makes oestrogen |
| Seasonal coat colour in arctic animals (arctic fox, snowshoe hare) | day length | brown summer coat, white winter coat | shortening days trigger a moult; the new hairs grow with little or no pigment because pigment genes are expressed less |
| Flower colour in hydrangeas | soil pH | blue in acidic soil, pink in alkaline soil | in acidic soil, aluminium ions are available and are taken up; they bind to the petal pigment and turn it blue |
| Tanning in humans and other animals | ultraviolet (UV) light | skin colour | UV exposure increases melanin production by pigment cells in the skin |
| Height and weight in humans | diet, health, activity | adult height and body mass | many genes set a range (polygenic); nutrition and health decide where in that range a person ends up |
| Mating in yeast | presence of the opposite mating type | pheromone production and mating behaviour | pheromone from a cell of the opposite type binds a receptor and switches on mating genes, including pheromone production |
| Colour points in Siamese cats | skin temperature | dark ears, face, paws and tail | a heat-sensitive form of the enzyme tyrosinase makes melanin only in cooler parts of the skin |
| Phenylketonuria (PKU) in humans | diet | whether brain damage develops | people with two recessive alleles cannot break down phenylalanine; a low-phenylalanine diet from birth prevents the harmful build-up |
Notice the different routes. In reptiles and yeast, the signal changes which genes are expressed. In Siamese cats, the gene is expressed everywhere but its enzyme only works where the skin is cool. In hydrangeas, soil chemistry changes the pigment’s colour after it is made. In PKU, the genotype is the same with or without the diet, but the outcome depends entirely on what the person eats. All of them fit the same idea: one genotype, more than one possible phenotype.
Representing plasticity: reaction norms
A reaction norm (norm of reaction) is a graph of phenotype against an environmental variable, with one line for each genotype. To get a line for one genotype, you need organisms with the same genotype in each environment. Plants are ideal, because cuttings taken from one parent are clones.
How to read a reaction norm:
- A flat line: the phenotype does not depend on this environmental factor. The trait is not plastic for this factor.
- A sloping line: the trait is plastic. The steeper the line, the more plastic the genotype.
- Two lines at different heights but parallel: the genotypes differ, and both respond to the environment by the same amount.
- Lines that cross or have different slopes: genotype and environment interact. You cannot say which genotype is “better” or “taller” without naming the environment.
Worked example 1: separating genes from environment with clones
Question. A grower takes cuttings from two parent plants of one fictional species, clone P and clone Q. She grows four cuttings of each clone in each of three growth chambers, at 12 °C, 18 °C and 24 °C. Light, water, soil and pot size are the same in every chamber. Heights after eight weeks for clone P were:
| Temperature | Plant 1 | Plant 2 | Plant 3 | Plant 4 |
|---|---|---|---|---|
| 12 °C | 21 cm | 23 cm | 20 cm | 24 cm |
| 24 °C | 41 cm | 38 cm | 42 cm | 39 cm |
The other means were: clone P at 18 °C, 31 cm; clone Q at 12, 18 and 24 °C, 30, 32 and 34 cm.
(a) Calculate the means for clone P at 12 °C and 24 °C. (b) Calculate the percentage increase in height from 12 °C to 24 °C for each clone. (c) Explain why using cuttings lets the grower conclude that the differences within a clone are environmental. (d) Use Figure 1 to describe the interaction between genotype and environment.
(a) Means.
- 12 °C: (21 + 23 + 20 + 24) ÷ 4 = 88 ÷ 4 = 22 cm.
- 24 °C: (41 + 38 + 42 + 39) ÷ 4 = 160 ÷ 4 = 40 cm.
(b) Percentage increase = (new − original) ÷ original × 100.
- Clone P: (40 − 22) ÷ 22 × 100 = 18 ÷ 22 × 100 = 82%.
- Clone Q: (34 − 30) ÷ 30 × 100 = 4 ÷ 30 × 100 = 13%.
(c) Why clones? All cuttings from one parent have the same genotype. Within a clone, genotype is held constant, and only temperature differs between chambers. So the 18 cm difference in clone P’s mean height must be caused by the environment, not by different alleles. If the grower had used seeds, each seedling would have a different combination of alleles, and genetic and environmental effects would be mixed together.
(d) Interaction. Clone P is much more plastic: its height rises by about 1.5 cm for each 1 °C, against about 0.3 cm per °C for clone Q. Because the slopes differ, the lines cross at about 19 °C. Clone Q is taller in the cool chambers, and clone P is taller in the warm one. So neither clone is simply “the tall genotype”: the answer depends on temperature. That is a genotype-by-environment interaction.
Check. Four plants per group is a small sample. The spread within each group (20–24 cm and 38–42 cm for clone P) does not overlap, so the difference is unlikely to be chance alone. A repeat with more cuttings would make the conclusion stronger.
Worked example 2: temperature-dependent sex determination
In most mammals and birds, sex is set by sex chromosomes at fertilisation. In many reptiles there are no sex chromosomes. Instead, the temperature of the egg during a sensitive period of development decides whether the gonads become ovaries or testes. In most turtles that do this, cooler eggs become males and warmer eggs become females. The temperature that gives a 50 : 50 sex ratio is the pivotal temperature.
The mechanism is a change in gene expression. Temperature alters which genes are expressed in the developing gonad. One of them codes for aromatase, an enzyme that converts testosterone into oestrogen (estradiol). At warm temperatures, more aromatase activity means more oestrogen, and the gonad develops as an ovary.
Question. A biologist incubates 40 eggs of a fictional turtle species at each of six constant temperatures. Every egg comes from the same nesting beach.
| Incubation temperature / °C | 26 | 27 | 28 | 29 | 30 | 31 |
|---|---|---|---|---|---|---|
| Females (out of 40) | 0 | 2 | 10 | 26 | 37 | 40 |
(a) Calculate the percentage of females at each temperature. (b) Estimate the pivotal temperature. (c) Predict the effect on hatchling sex ratio if the mean temperature of nests rises from 28 °C to 29 °C, and explain the result at the level of gene expression.
(a) Percentage female = females ÷ 40 × 100:
| Temperature / °C | 26 | 27 | 28 | 29 | 30 | 31 |
|---|---|---|---|---|---|---|
| Females / % | 0 | 5 | 25 | 65 | 92.5 | 100 |
(b) Pivotal temperature. 50% lies between 28 °C (25%) and 29 °C (65%). Assume a straight line between these two points (linear interpolation):
- The percentage rises by 65 − 25 = 40 points over 1 °C.
- We need a rise of 50 − 25 = 25 points above 28 °C.
- Pivotal temperature ≈ 28 + 25 ÷ 40 = 28 + 0.625 = 28.6 °C.
(c) Prediction. At 28 °C, 25% of hatchlings are female (10 females to 30 males, a ratio of 1 : 3). At 29 °C, 65% are female. A warming of just 1 °C changes a male-biased population into a female-biased one. Explanation: the eggs’ genotypes have not changed. The higher temperature changes gene expression in the developing gonads: more aromatase is made, more testosterone is converted into oestrogen, and more embryos develop ovaries.
Limits. Real nests change temperature during the day and through the season, so they do not sit at one constant value. The data are for one beach, and other populations may have a different pivotal temperature. The interpolation assumes a straight line between two points on a curve.
Common misconceptions
- “The environment changes the organism’s genes.” It changes gene expression or the activity of gene products. The DNA sequence stays the same. (Mutagens such as UV can also damage DNA, but tanning itself is a change in melanin production, not a mutation.)
- “A trait is either genetic or environmental.” Almost every phenotype depends on both. The useful question is how much each contributes, and whether they interact.
- “Same genotype means same phenotype.” Identical twins and cloned plants show that the same genotype can produce different phenotypes.
- “Changes caused by the environment are inherited.” A tan or a winter coat is not passed on through the alleles in the gametes. The offspring inherit the genotype, which includes the ability to respond.
- “Plasticity always helps the organism.” Plastic responses are often useful, but a cue can mislead. For example, if snow melts early, a hare that has already turned white is easy to see.
- “In reptiles, sex depends on chromosomes, just as in mammals.” In many reptiles, including most turtles that have been studied, incubation temperature decides sex.
- “The Siamese cat gene is only switched on in the paws.” The gene is expressed in the skin generally; its enzyme only works where the skin is cooler.
- “If plants grown in shade are shorter, shade must be the only cause.” Only if the genotype and every other variable were the same. Use clones and controlled conditions before claiming cause.
Where this leads
This topic links Unit 5 to Unit 6, where you will see how genes are switched on and off: transcription, translation and the regulation of gene expression. It also matters for evolution (Unit 7): natural selection acts on phenotypes, but only differences in genotype are passed on. Start Unit 6 with Topic 6.1, DNA and RNA structure. For non-Mendelian patterns of inheritance, look back at Topic 5.4.
Test yourself with the practice questions, then use the revision notes and the checklist to consolidate.
Quick questions
If the environment changes a phenotype, is the change passed to the offspring?
Not through the DNA sequence. A tan, a larger muscle or a winter coat is a change in gene expression in body cells. The alleles in the gametes are the same, so offspring inherit the genotype, not the tan.
Is phenotypic plasticity always helpful?
Often it is, because the phenotype can match the conditions, such as a white coat in a snowy winter. But it is not always helpful: a cue can mislead the organism, and a trait like sex ratio can be pushed too far by unusual temperatures.