Variation: genetic, environmental and combination

What you'll learn

This revision guide covers variation in organisms — the differences between individuals of the same species. You'll learn to identify and explain genetic variation (caused by inheritance), environmental variation (caused by conditions), and combined variation (influenced by both factors). Understanding these concepts is essential for Paper 2 topics including inheritance, evolution and natural selection.

Key terms and definitions

Variation — the differences in characteristics between individuals of the same species

Genetic variation — differences caused by the genes inherited from parents, passed on through gametes at fertilisation

Environmental variation — differences caused by the conditions in which an organism develops, not inherited

Alleles — different versions of the same gene that can produce variation in inherited characteristics

Phenotype — the observable characteristics of an organism, resulting from both genetic and environmental factors

Continuous variation — characteristics that can take any value within a range (e.g. height, mass)

Discontinuous variation — characteristics that fall into distinct categories with no intermediates (e.g. blood group, eye colour)

Mutation — a random change in DNA that can introduce new alleles and increase genetic variation

Core concepts

Genetic variation

Genetic variation arises from the genes inherited from both parents. At fertilisation, the nucleus of a sperm cell fuses with the nucleus of an egg cell, combining genetic material from two individuals. This creates a unique combination of alleles in the offspring.

Sources of genetic variation:

• Sexual reproduction combines alleles from two parents
• Random fertilisation means any sperm can fuse with any egg
• Mutations in DNA create new alleles
• Independent segregation of chromosomes during meiosis
• Crossing over during meiosis (mixing sections of chromosomes)

Characteristics controlled only by genes include:

• Blood group (A, B, AB, O)
• Earlobe shape (attached or free)
• Natural eye colour
• Inherited disorders (e.g. cystic fibrosis, polydactyly)
• Natural hair colour

These characteristics cannot be changed by environmental factors. If you inherit alleles for blood group A, your blood group will always be A regardless of diet, exercise or other conditions.

Key exam point: Genetic variation is essential for natural selection and evolution. Without genetic differences, all individuals would respond identically to environmental pressures and populations could not adapt.

Environmental variation

Environmental variation results from the conditions experienced during an organism's lifetime. These acquired characteristics develop in response to factors such as climate, diet, physical activity, light exposure, and interactions with other organisms.

Environmental factors causing variation:

• Diet and nutrition — affects growth, body mass, health
• Temperature — influences development rate, body size in some organisms
• Light intensity — affects plant growth, leaf size, chlorophyll production
• Physical activity — determines muscle development, fitness levels
• Exposure to pathogens — can cause scarring, tissue damage
• Accidents and injuries — cause scars, loss of body parts
• Training and education — develops skills, knowledge, language

Characteristics caused only by environmental factors:

• Scars from injuries or surgery
• Body mass changes from diet and exercise
• Muscle development from physical training
• Language spoken (learned behaviour)
• Tanned or sun-damaged skin
• Skills learned through practice
• Tattoos and piercings

These characteristics are not inherited. An athlete's children do not automatically inherit developed muscles, and a person who loses a finger in an accident will not have children with missing fingers.

Plants and environmental variation:

Plants show significant environmental variation because they cannot move to find better conditions. The same plant species grown in different environments will show marked differences:

• Plants in shade develop larger, thinner leaves to capture more light
• Plants in bright sunlight may have smaller, thicker leaves to reduce water loss
• Poor soil nutrition results in stunted growth and yellowing leaves
• Adequate water and minerals produce tall, healthy plants with deep green leaves

Combined variation (genetic and environmental)

Most characteristics result from a combination of genetic and environmental factors. The genes set the potential range, while environmental factors determine where within that range an individual develops.

Characteristics showing combined variation:

• Height — genes set potential height range, but nutrition, disease and exercise during growth affect final height
• Body mass — genetic factors influence build and metabolism, but diet and physical activity have major effects
• Intelligence/academic ability — genetic factors affect brain development, but education, stimulation and nutrition are crucial
• Skin colour — genetically determined melanin production, but sun exposure causes tanning (darkening)
• Athletic performance — genes influence muscle type and oxygen uptake, but training is essential for high performance
• Mental health — genetic predisposition can increase risk, but environmental stressors and support affect outcomes

How to identify combined variation:

Ask two questions:

1. Can the characteristic be inherited? (genetic component present)
2. Can the environment change it? (environmental component present)

If "yes" to both, it's combined variation.

Example: Height

• Identical twins (same genes) raised together have very similar heights
• Identical twins raised in different conditions (one with poor nutrition) show height differences
• This proves both genetic and environmental factors matter

Continuous and discontinuous variation

Variation can be classified by the pattern it creates in a population.

Continuous variation:

• Characteristics measured on a scale with any value possible
• No distinct categories — individuals spread across a range
• Usually controlled by multiple genes (polygenic inheritance)
• Strongly influenced by environmental factors
• Examples: height, body mass, leaf surface area, hand span, foot length
• Displayed using histograms or line graphs showing bell-shaped (normal) distribution

Example data pattern:

A class of 30 students measured heights: values range from 152 cm to 178 cm with most students around 165 cm. Every possible height value between the extremes is present. When plotted, the graph shows a bell-shaped curve.

Discontinuous variation:

• Characteristics that fall into distinct categories
• No intermediate values — you belong to one category or another
• Usually controlled by a single gene with different alleles
• Not influenced by environmental factors
• Examples: ABO blood group, biological sex, presence/absence of genetic disorders, ability to roll tongue, earwax type (wet/dry)
• Displayed using bar charts with gaps between categories

Example data pattern:

A class of 30 students tested blood groups: 12 students are group O, 10 are group A, 6 are group B, 2 are group AB. No student has an intermediate blood group. A bar chart shows four distinct bars with gaps between them.

Why the difference matters for exams:

The type of variation tells you about genetic control and environmental influence:

• Discontinuous = single gene, no environmental effect
• Continuous = multiple genes + environmental factors

Investigating variation

GCSE exam questions often involve analysing investigations into variation. You must identify variables, evaluate methodology and interpret data.

Planning an investigation:

1. Choose a measurable characteristic
2. Identify the independent variable (what you change)
3. Identify the dependent variable (what you measure)
4. Control other variables that could affect results
5. Use sufficient sample size (minimum 20-30 individuals)
6. Repeat measurements where possible to improve reliability
7. Calculate means to reduce impact of anomalies

Example investigation: Effect of light on plant growth

• Grow genetically identical plant clones in different light intensities
• Measure height after 4 weeks
• Same soil, water, temperature, pot size (controlled variables)
• Multiple plants per condition (repeats)
• This tests environmental variation — genetic variation is eliminated by using clones

Analysing results:

• Large differences between individuals in same conditions suggest genetic factors
• Differences between conditions suggest environmental factors
• Using genetically identical organisms (clones, identical twins) isolates environmental effects
• Comparing genetically different organisms in identical conditions isolates genetic effects

Worked examples

Example 1: Identifying types of variation (2 marks)

Question: A student investigates variation in a population of snails. State whether each characteristic shows genetic, environmental or combined variation.

(a) Shell colour pattern (b) Body mass

Mark scheme answer:

(a) Genetic variation (1 mark) Examiner note: Shell colour pattern is determined by inherited alleles and cannot be changed by environmental conditions

(b) Combined variation / genetic and environmental (1 mark) Examiner note: Body mass is influenced by genes that affect metabolism and build, but also by food availability in the environment

Example 2: Continuous vs discontinuous variation (4 marks)

Question: A scientist measured the hand span of 50 students and recorded their blood groups.

(a) State which characteristic shows continuous variation. (1 mark) (b) Explain why this characteristic shows continuous variation. (2 marks) (c) Describe how the data for blood group should be displayed. (1 mark)

Mark scheme answer:

(a) Hand span (1 mark)

(b) Hand span can take any value within a range (1 mark) and there are no distinct categories / individuals are spread across a range of measurements (1 mark)

Alternative acceptable answers: controlled by many genes / polygenic, influenced by environmental factors such as exercise

(c) Bar chart / bar graph with gaps between bars (1 mark)

Examiner note: Must mention gaps or separate bars. "Graph" alone is insufficient.

Example 3: Investigating variation (6 marks)

Question: A gardener wants to investigate whether differences in tomato plant height are due to genetic or environmental factors.

Describe how the gardener could carry out this investigation. Include:

• what plants should be used
• what variables should be controlled
• what measurements should be taken

Mark scheme answer:

• Use genetically identical plants / clones / cuttings from same parent plant (1 mark)
• Grow plants in different conditions / vary one environmental factor such as light intensity or water supply (1 mark)
• Keep all other factors constant such as temperature / soil type / pot size / mineral content (1 mark)
• Measure height of each plant after a set time period / at regular intervals (1 mark)
• Use multiple plants in each condition / repeats (1 mark)
• Calculate mean heights for each condition / compare results between conditions (1 mark)

Examiner note: If differences appear despite identical genes, this proves environmental factors cause variation. Maximum 6 marks available.

Common mistakes and how to avoid them

• Confusing "variation" with "variety" — variation refers to differences between individuals, not different species. Write about differences in characteristics, not "different types" of organism.

• Claiming environmental characteristics are inherited — scars, learned skills, and muscle development are NOT passed to offspring. Only genetic information in gametes is inherited. Remember: acquired characteristics ≠ inherited characteristics.

• Saying continuous variation is "only environmental" — continuous variation involves BOTH multiple genes and environmental factors. Height is genetic (inherited from parents) AND environmental (affected by nutrition).

• Wrong graph types — continuous variation requires histograms or line graphs showing a range; discontinuous variation needs bar charts with gaps. Using the wrong graph type loses marks in practical questions.

• Forgetting to control variables in investigations — when testing one factor's effect on variation, all other factors must be kept constant. Always state what you're keeping the same.

• Insufficient sample size — measuring only 3-5 individuals gives unreliable results. State that "a large sample size (at least 20-30 individuals) increases reliability and reveals true patterns of variation."

Exam technique for "Variation: genetic, environmental and combination"

• Command word "Explain" (2-3 marks) requires you to give reasons WHY, not just describe WHAT. For variation questions, state the type of variation AND give biological reasoning: "Height shows combined variation because genes determine potential height (inherited from parents) but environmental factors such as nutrition during childhood affect whether that potential is reached."

• Comparing identical twins vs non-identical twins is a common 4-6 mark question. Structure your answer: (1) identical twins share 100% of genes, (2) any differences must be environmental, (3) comparing them to non-identical twins (50% gene sharing) shows the genetic contribution.

• 6-mark investigation questions require a logical plan: state the aim, identify variables (independent, dependent, control), describe method including repeats, explain how results show genetic/environmental effects. Use bullet points or numbered steps for clarity.

• Define before you apply — if a question asks you to "explain why characteristic X shows continuous variation," first define continuous variation (measured on a scale, range of values, no distinct categories), then apply to X specifically.

Quick revision summary

Variation refers to differences between individuals. Genetic variation is inherited through alleles in gametes and includes characteristics like blood group. Environmental variation results from conditions during life (diet, climate, injuries) and is not inherited. Most characteristics show combined variation — genes set the range, environment determines the outcome (e.g. height, body mass). Continuous variation produces a range of values controlled by multiple genes and environment; discontinuous variation creates distinct categories controlled by single genes. Investigations using genetically identical organisms reveal environmental effects; comparing genetically different organisms in identical conditions reveals genetic effects.