How phenotype results from the interaction between your genotype and the world around you.
Identical twins share 100% of their DNA. So why does one twin sometimes develop a disease while the other stays healthy?
Learning Objectives
AO2 Understand how phenotype is the result of an interaction between genotype and the environment
AO2 Understand how polygenic inheritance and the environment give rise to continuous variation
AO2 Understand how some phenotypes are affected by multiple alleles, polygenic inheritance, and the environment
Prior Knowledge
Diagnostic Quiz
Q1
What is a phenotype?
A The genetic make-up of an organism
B The observable characteristics of an organism
C A section of DNA coding for a protein
D The number of chromosomes in a cell
Q2
What does ‘polygenic’ mean?
A Controlled by one gene with multiple alleles
B Controlled by many genes at different loci
C Controlled by genes on the sex chromosomes
D Controlled by a single dominant gene
Q3
Which of these is most likely affected by the environment?
A Blood group
B Body mass
C Sickle cell allele
D Presence of a Y chromosome
Section 1
When Genes Meet Environment
Your phenotypeThe observable characteristics of an organism, resulting from the interaction of its genotype with the environment. is not determined by your genes alone. It is the result of an interaction between your genotypeThe genetic make-up of an organism — the alleles it possesses. and the environment.
Genetically identical plants (clones) grow very differently when exposed to varying amounts of light, water, and soil nutrients. The ability of animals to grow depends heavily on environmental factors such as the amount of food available. It is more difficult to investigate the impact of environment on phenotype in animals because of the difficulty of producing large numbers of clones, genetically identical organisms, and the ethical aspects of experimentation.
The Siamese Cat — A Classic Example
The Siamese cat provides a striking example of gene-environment interaction. The gene for fur colour produces a temperature-sensitive tyrosinaseAn enzyme involved in the production of the pigment melanin. In Siamese cats, a mutant form is inactive at normal body temperature. enzyme. A mutation in this gene means the enzyme only works at lower temperatures.
At the body core (warm, ~38°C): tyrosinase is denatured → no melanin → pale fur
At the extremities (cool — ears, paws, nose, tail): tyrosinase is active → melanin produced → dark fur
Interactive — Click each zone on the Siamese cat
Exam Hint
The Siamese cat question appears frequently. Remember: it is the environment (temperature) that determines where on the body the gene is expressed, even though every cell has the same allele.
Section 2
The Lac Operon
Productive Struggle — Try before you learn
E. coli bacteria are grown on glucose. When lactose is added, they begin producing a new enzyme after a short delay. When lactose is removed, enzyme production stops. How might the bacterium ‘know’ when to produce this enzyme?
Try to work it out with no assistance first.
Think about what molecules might interact with DNA to block or allow transcription.
When lactose is present, it could bind to a ___ protein, which normally ___ transcription. This would mean the structural genes for the enzyme are now ___.
In the 1960s, French geneticists Jacob and Monod discovered a brilliant system in E. coli that controls gene expression in response to the environment. They called it the lac operonA cluster of genes in E. coli that are switched on or off together by the presence or absence of lactose..
Lactose is a sugar found in milk. It is broken down into glucose and galactose by the enzyme β-galactosidase. If there is no lactose, there is no point making this enzyme — it would waste energy.
How the Lac Operon Works
Lactose ABSENT:
The regulatory gene produces a repressor protein
The repressor binds to the operator
RNA polymerase cannot pass → structural genes are NOT transcribed
β-galactosidase is NOT produced
Lactose PRESENT:
Lactose binds to the repressor protein → changes its shape
Repressor can no longer bind to the operator
RNA polymerase can now transcribe the structural genes
β-galactosidase IS produced → lactose is broken down
Interactive — Toggle between lactose absent and present
AO1 Fill the Blanks
Complete these sentences about the lac operon:
1. The regulatory gene produces a protein.
2. When lactose is absent, this protein binds to the .
3. This prevents from transcribing the structural genes.
4. When lactose is present, it binds to the repressor and changes its .
Word Bank
repressor | operator | RNA polymerase | shape
Section 3
Epidemiological & Twin Studies
One strategy to separate genetic and environmental influences is to study identical (monozygotic) twinsTwins formed from a single zygote that splits — they share 100% of their DNA.. These twins have the same genetic material. Non-identical (dizygotic) twins share about 50% of their DNA, like ordinary siblings.
If a trait is more similar in identical twins than in non-identical twins, it suggests a strong genetic component. If identical twins reared apart show greater differences, it suggests a strong environmental influence.
Twin Study Data
Trait
Identical twins (reared apart)
Identical twins (reared together)
Non-identical twins
Height difference (cm)
1.8
1.7
4.4
Mass difference (kg)
4.5
1.9
4.6
IQ score difference
8.2
5.9
9.9
Think — Puzzle — Explore
What do you already THINK about nature vs nurture?
What PUZZLES you about the twin data above?
What would you like to EXPLORE further?
Independent Practice — Analyse the Twin Data
Complete these sentences using the data in the table above:
Height has a strong component because identical twins show very similar heights even when reared apart.
Body mass has a greater component because identical twins reared apart differ by 4.5 kg compared to only 1.9 kg reared together.
Word Bank
genetic | environmental
Using the data, explain the relative contributions of genes and environment to: (a) height, (b) body mass, and (c) IQ. Write 2-3 sentences for each.
Challenge: Evaluate the reliability of twin studies as a method for separating genetic and environmental influences. Consider: sample size, identical environments, epigenetics, and ethical concerns.
Learning Tip
In the exam, always refer to specific data when analysing tables. Say “identical twins reared apart differ by 1.8 cm compared to 4.4 cm for non-identical twins” rather than just “identical twins are more similar”.
Section 4
Discontinuous & Continuous Variation
In any population, there are two types of variation:
Feature
Discontinuous
Continuous
Definition
Features that fall into distinct categories
Features that show a range of values
Genetic basis
Usually one or very few genes
Usually polygenic (many genes)
Environmental effect
Little or none
Often significant
Distribution
Bar chart with gaps
Normal distribution (bell curve)
Examples
Blood group, sex, tongue rolling
Height, weight, skin colour
Discontinuous variationVariation where individuals fall into distinct categories with no intermediates, usually controlled by one or a few genes. is shown by features that are either present or not (like blood groups — you are A, B, AB, or O with nothing in between). Continuous variationVariation where individuals show a range of values for a feature, usually controlled by many genes and the environment. is shown by features with a range of values (like height — there is every value between the shortest and tallest).
AO2 Categorisation
Drag each trait into the correct category:
Blood group
Height
Leaf length
Sex (male/female)
Body mass
Tongue rolling
Skin colour
Earlobe attachment
Discontinuous
Continuous
Studying Continuous Variation
When studying continuous variation in a population, you need:
Large sample sizes — to ensure the data is representative
Random sampling — to avoid bias
Frequency distribution — plot the data as a histogram → should produce a normal distributionA bell-shaped curve where most values cluster around the mean, with fewer values at the extremes. (bell curve)
Calculate the mean and standard deviation to describe the data
Exam Practice
Exam-Style Questions
2 Marks
Question 1
A patch of white fur is removed from the back of a Siamese rabbit. A cold pack is kept on the area as the fur grows back. The new fur grows dark. Explain this observation.
Reveal model answer
1. The gene for fur colour produces a temperature-sensitive enzyme (tyrosinase) that only works at lower temperatures (1 mark). 2. The cold pack lowers the skin temperature at that area, so the enzyme is active → melanin is produced → fur grows dark (1 mark).
4 Marks
Question 2
Explain how the lac operon in E. coli is controlled by the presence of lactose.
Reveal model answer
1. The regulatory gene produces a repressor protein (1 mark). 2. When lactose is absent, the repressor binds to the operator, blocking RNA polymerase (1 mark). 3. When lactose is present, it binds to the repressor and changes its shape / the repressor can no longer bind to the operator (1 mark). 4. RNA polymerase can now transcribe the structural genes → β-galactosidase is produced to break down lactose (1 mark).
6 Marks — Extended Response
Question 3
Discuss the relative contributions of genetic and environmental factors to human phenotype. Use examples from twin studies and other evidence.
Planning Checklist
Define phenotype as result of genotype + environment
Use twin study data — identical vs non-identical
Height example — strong genetic component (twin data)
Body mass — significant environmental component (diet)
Siamese cat as gene-environment interaction example
Conclusion — most traits are influenced by both
Reveal model answer
Phenotype is the observable characteristics of an organism, resulting from the interaction between its genotype and the environment. Twin studies provide strong evidence for separating these influences. For height, identical twins reared apart show only a 1.8 cm difference compared to 4.4 cm for non-identical twins, indicating a strong genetic component. However, body mass shows greater environmental influence — identical twins reared apart differ by 4.5 kg (similar to non-identical twins at 4.6 kg), but when reared together differ by only 1.9 kg, suggesting shared environment (diet, lifestyle) plays a major role. The Siamese cat demonstrates gene-environment interaction directly: a temperature-sensitive tyrosinase enzyme produces melanin only at cooler extremities, so the same genotype produces different phenotypes in different environmental conditions. Epidemiological studies also show environmental impact — for example, smoking during pregnancy reduces birth weight regardless of genotype. In conclusion, most human characteristics result from a complex interplay of genetic predisposition and environmental factors, with some traits (like blood group) being almost entirely genetic, and others (like body mass) being heavily influenced by environment.
Final Assessment
Q1
What is the correct equation for phenotype?
A Phenotype = genotype only
B Phenotype = genotype + environment
C Phenotype = environment only
D Phenotype = alleles + mutations
Q2
In the lac operon, what happens when lactose binds to the repressor protein?
A The repressor binds more tightly to the operator
B The repressor changes shape and can no longer bind to the operator
C The structural genes are destroyed
D RNA polymerase is denatured
Q3
Which type of variation does human height show?
A Discontinuous
B Continuous
C Neither — it is entirely genetic
D Neither — it is entirely environmental
Q4
Siamese cats have dark fur at their extremities because:
A The extremities have different DNA
B A temperature-sensitive enzyme is only active at cooler body parts
C UV light darkens the fur at exposed areas
D The cat licks its extremities, causing melanin production
How confident do you feel?
Exit Ticket
Three quick questions before you go:
1. Explain why the phenotype of an organism is not determined by genes alone.
2. Describe how the lac operon is switched on when lactose is present.
3. Give one example each of discontinuous and continuous variation.
Vocabulary Grid
Phenotype
Observable characteristics resulting from genotype + environment.
Genotype
The genetic make-up of an organism — the alleles it possesses.
Lac operon
A cluster of genes in E. coli controlled by the presence of lactose.
Repressor protein
A protein that binds to the operator and blocks transcription.
Operator
The region of DNA where the repressor binds in an operon.
Discontinuous variation
Distinct categories with no intermediates, usually one gene.
Continuous variation
A range of values in a population, usually polygenic + environment.
Normal distribution
A bell-shaped curve where most values cluster around the mean.
Monozygotic twins
Identical twins from one zygote — share 100% DNA.
Tyrosinase
Enzyme for melanin production; temperature-sensitive in Siamese cats.
Self-Assessment Checklist
I can explain that phenotype = genotype + environment
I can use the Siamese cat to explain gene-environment interaction
I can describe how the lac operon is regulated
I can analyse twin study data to separate genetic and environmental effects
I can distinguish between discontinuous and continuous variation
I can explain how to study continuous variation in a population