In simple terms
A friendly intro before the formal notes — no formulas yet.
Why are we all different?
Variation is simply the differences we see between individuals of the same species. These differences can be due to the unique genetic information inherited from parents, how genes are shuffled during reproduction, or even the environment we grow up in.
Imagine a batch of cookies made from the same recipe (genes). Even if you follow the recipe, some might turn out slightly crispier or a bit larger due to oven temperature fluctuations (environment) or slight variations in how you mixed the ingredients (gene shuffling).
- 1
Distinguish clearly between genetic and environmental causes of variation.
- 2
Know the characteristics and examples of continuous vs. discontinuous variation.
- 3
Understand the specific mechanisms in meiosis (crossing over, independent assortment) and fertilisation that create genetic diversity.
- 4
Practise explaining how variation gives a species an evolutionary advantage.
What this topic covers
The official Cambridge syllabus points this lesson works through.
- 17.1.1
Explain, with examples, that phenotypic variation is due to genetic factors or environmental factors or a combination of genetic and environmental factors
- 17.1.2
Explain what is meant by discontinuous variation and continuous variation
- 17.1.3
Explain the genetic basis of discontinuous variation and continuous variation
- 17.1.4
Use the t-test to compare the means of two different samples (the formula for the t-test will be provided, as shown in the Mathematical requirements)
Explore the concept
Use the live diagram and synced steps — play it or tap a step card to walk through.
Full topic notes
Formal explanation with the rigour you need for the exam.
What is Variation?
Variation refers to the differences that exist between individuals of the same species. These differences can manifest as observable characteristics, known as phenotypic variation, which are a result of complex interactions between an organism's genetic makeup and its environment.
Causes of Variation: Genetic vs. Environmental
It's vital for Paper 4 to distinguish between the two primary causes of variation:
Genetic Variation: These are inherited differences in an organism's DNA sequence, passed down from parents to offspring. They are determined by the specific alleles (different forms of a gene) an individual possesses. Examples include human blood groups (A, B, AB, O) and natural eye colour (though environmental factors can influence pigment expression slightly, the underlying genetic blueprint is fixed).
Environmental Variation: These differences arise from the influence of external factors on an organism during its lifetime. They are not inherited. Examples include a person's tan (due to sun exposure), muscle development (due to exercise), or the exact height a plant reaches (influenced by nutrient availability, light, and water).
Many characteristics, such as human height or intelligence, are influenced by both genetic predisposition and environmental factors. The genotype sets the potential range, while the environment determines where within that range the phenotype is expressed.
Types of Variation: Continuous vs. Discontinuous
Variation can be categorised based on how phenotypes are expressed:
Discontinuous Variation:
- Exhibits clear, distinct categories with no intermediate forms. It's a 'either/or' situation.
- Controlled by one or a few genes (monogenic or oligogenic).
- Largely unaffected by environmental factors.
- Data is typically represented using bar charts.
- Examples: Human blood groups (A, B, AB, O), presence or absence of a genetic disease, some plant flower colours.
Continuous Variation:
- Shows a wide range of phenotypes between two extremes, with gradual transitions.
- Controlled by many genes (polygenic), each contributing a small, additive effect.
- Significantly influenced by environmental factors.
- Data typically forms a normal distribution curve when plotted as a histogram.
- Examples: Human height, mass, skin colour, leaf length, milk yield in cows.
Genetic Basis of Variation
The ultimate source of all genetic variation lies within the DNA. Let's explore the key terms and mechanisms:
Alleles: These are different forms of a specific gene. For example, the gene for blood type has alleles I<sup>A</sup>, I<sup>B</sup>, and i. These are found at the same locus on homologous chromosomes.
Gene Pool: This refers to the total sum of all alleles of all genes present in all individuals within a particular population at a given time. A larger gene pool indicates greater genetic diversity.
Mutation: This is the ultimate source of new alleles and genetic variation. Mutations are spontaneous, random changes in the DNA sequence. They can be gene mutations (changes within a single gene, e.g., point mutations like substitution, insertion, deletion) or chromosome mutations (changes in chromosome structure or number, e.g., polyploidy).
Polyploidy: A specific type of chromosome mutation where an organism has more than two complete sets of chromosomes. This is very common in plants and can lead to the rapid formation of new species with altered characteristics.
Crossing Over: Occurs during prophase I of meiosis. Homologous chromosomes exchange segments of genetic material between non-sister chromatids. This shuffles alleles between maternal and paternal chromosomes, creating new combinations of alleles on each chromatid.
Independent Assortment: Happens during metaphase I and anaphase I of meiosis. Homologous chromosome pairs align and separate randomly. The orientation of one pair is independent of another, leading to a vast number of possible chromosome combinations in the resulting gametes.
Random Fertilisation: During sexual reproduction, any one of the millions of genetically unique male gametes can fuse with any one of the millions of genetically unique female gametes. This random fusion creates a unique combination of alleles in the zygote, further increasing variation within the offspring.
Advantages of Variation to a Species
Variation is not just an interesting biological phenomenon; it is fundamental for the survival and evolution of species:
Raw Material for Natural Selection: Genetic variation provides the diverse phenotypes upon which natural selection can act. Individuals with advantageous traits (due to beneficial alleles) are more likely to survive, reproduce, and pass on their genes in a given environment.
Adaptation: In a changing environment (e.g., new disease, climate change, predator), a species with high genetic variation has a higher chance that some individuals will possess traits that allow them to adapt and survive, thus preventing the entire species from being wiped out.
Disease Resistance: A varied population is less susceptible to a single pathogen wiping out all individuals, as some may have alleles conferring resistance.
Evolution: Over long periods, accumulated advantageous variations, selected for through natural selection, drive the process of evolution, allowing species to adapt and diversify.
Hardy-Weinberg and allele frequencies
The Hardy-Weinberg equation models genotype frequencies when no evolutionary forces act. Here and are allele frequencies for a gene with two alleles. Chi-squared tests compare observed phenotype counts with expected ratios to test whether variation fits a predicted genetic model.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
Explain how both genetic and environmental factors contribute to the variation in human height, and discuss why genetic variation is crucial for the long-term survival of a species. [8 marks]
- 1
Genetic Factors for Height: Human height is a polygenic trait, meaning it's controlled by multiple genes, each often having several alleles. The specific combination of these alleles inherited from parents determines an individual's genetic potential or predisposition for height. Different alleles contribute small, additive effects to height, leading to the continuous range observed in human populations.
In a population, allele has frequency . Calculate and the expected frequency of heterozygotes . [3 marks]
- 1
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How it all connects
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Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Try to recall each definition before you reveal it.
Quick check
Answer in your head first — then tap to check. No pressure.
Revision flashcards
Flip the card. Test yourself before the exam.
Genetic vs environmental variation?
Genetic: inherited allele differences. Environmental: non-inherited effects of external factors during lifetime.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
- ✓
Genetic Variation: These are inherited differences in an organism's DNA sequence, passed down from parents to offspring. They are determined by the specific alleles (different forms of a gene) an individual possesses. Examples include human blood groups (A, B, AB, O) and natural eye colour (though environmental factors can influence pigment expression slightly, the underlying genetic blueprint is fixed).
- ✓
Environmental Variation: These differences arise from the influence of external factors on an organism during its lifetime. They are not inherited. Examples include a person's tan (due to sun exposure), muscle development (due to exercise), or the exact height a plant reaches (influenced by nutrient availability, light, and water).
- ✓
Many characteristics, such as human height or intelligence, are influenced by both genetic predisposition and environmental factors. The genotype sets the potential range, while the environment determines where within that range the phenotype is expressed.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
9700/41 · Q2(c)
With reference to Fig. 2.1, explain what Fig. 2.2 shows about the role of genes and the role of the environment in controlling the rhythm of stomatal opening and closing in A. thaliana.
9700/42 · Q3(c)(i)
One of the changes that occurred during the domestication of wild rice to cultivated rice was the loss of the awns from rice grains. Farmers found that long awns made storing and processing rice grains more difficult. It was also observed that rice plants that have grains with no awns have an increased grain yield. Explain the principles used by farmers to produce rice plant grains with no awns.
Extra simulations & links
PhET, GeoGebra and other curated tools — open in a new tab.
Frequently asked
Checkpoint
One marked question is worth ten re-reads — close the loop before you move on.
Reading it isn’t knowing it — prove it.
Before you move on: do 9700/41 · Q2(c) on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.