Variation — AQA GCSE Combined Science
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Variation explained
The genome is the full set of genetic material an organism inherits.
Read the full explanation
Genes code for proteins, and these proteins help build and control the cell, so the genome sets the potential range of features an organism can develop. The phenotype is the observable characteristics that actually appear. Environment means everything the organism experiences, such as diet, exercise, temperature, light and lifestyle. Phenotype often results from both: genes give a range, and the environment decides where within that range the organism falls. For example, a person may inherit tallness genes, but a poor diet in childhood can limit final height. Similarly, identical plant cuttings grown in bright light and in shade show different leaf size and colour. Describing simply means stating that phenotype depends on genetic information interacting with environmental factors, and giving a clear example.
Differences in the characteristics of individuals in a population is called variation and may be due to differences in:
Variation means the differences in characteristics between individuals in a population. It arises from two broad sources. Genetic causes include differences in the alleles inherited, new alleles formed by mutation, and the random combination of parental alleles during sexual reproduction. Environmental causes include diet, exercise, temperature, light, nutrient availability and lifestyle factors. Many characteristics, such as human height or plant leaf size, are influenced by both genes and environment. Some variation is discontinuous, falling into distinct categories such as blood group or eye colour, while other variation is continuous, showing a range of values such as height or mass. A population with more genetic variation is more likely to survive a change in the environment, because some individuals may have advantageous characteristics.
the genes they have inherited (genetic causes)
Genetic variation arises because offspring inherit different combinations of alleles from their parents. Each gamete carries one allele of each gene, and fertilisation combines two gametes at random, so siblings differ even though they share parents. Some characteristics, such as blood group or natural eye colour, are determined mainly by genes. Others, such as height, are affected by many genes and by the environment. A mutation can create a new allele, adding further genetic variation. For example, a child may inherit an allele for brown eyes from one parent and an allele for blue eyes from the other, giving brown eyes because brown is dominant. The inherited alleles are the genetic cause of that characteristic.
the conditions in which they have developed (environmental causes)
Environmental variation is caused by the conditions in which an organism develops. These conditions include diet, temperature, light, water, minerals, exercise and accidents. For example, a plant grown in dim light may be tall and thin, while a plant of the same species grown in bright light may be short and bushy. A person's body mass can be affected by diet and exercise, and a scar is caused by an accident rather than by genes. Some characteristics, such as height or body mass, are affected by both genes and the environment. Environmental causes do not change the alleles an organism inherits, so they are not passed on to offspring in the same way as genetic variation.
a combination of genes and the environment.
Variation between individuals comes from two sources working together: the genes they inherit and the environment they experience. Genes are lengths of DNA passed from parents in gametes, so offspring inherit alleles that set features such as blood group, eye colour and the potential height a person could reach. Environment covers everything an organism encounters after fertilisation, including diet, exercise, sunlight, accidents and upbringing. Many features, such as height, mass and skin tone, are affected by both, so the observed value is not fixed by genes alone. For example, a child may inherit alleles for tall stature but a poor diet can prevent that potential being reached. Similarly, two identical plant cuttings grown with different light and mineral levels end up different sizes.
state that there is usually extensive genetic variation within a population of a species
A population is the group of individuals of one species living in a particular area. Within that group, individuals differ in many inherited features because they carry different combinations of alleles. This is genetic variation. For example, in a population of banded snails, shell colour and banding pattern vary because different alleles for shell-pattern genes are present. Variation arises through mutation and is reshuffled by sexual reproduction, so populations usually contain a wide range of genotypes and phenotypes rather than being uniform. The word extensive signals that the variation is normally large, not slight or occasional. Recognising this matters because natural selection acts on the variation already present: if a population lacks variation, it cannot adapt so easily when the environment changes.
recall that all variants arise from mutations and that: most have no effect on the phenotype; some influence phenotype; very few determine phenotype.
A mutation is a change in the base sequence of DNA. New alleles, called variants, appear only when mutation alters a gene, so every variant has a mutational origin. Most mutations are neutral because they occur in non-coding DNA or do not change the amino acid sequence, so the phenotype is unaffected. Some mutations alter the protein produced, perhaps changing an enzyme's shape or a structural protein, and so influence the phenotype; the effect may be slight or depend on the environment. Very few mutations determine the phenotype on their own, for example a mutation that causes a genetic disorder or a decisive change such as antibiotic resistance. This explains why most mutations are not harmful and why variation accumulates gradually in populations.
Mutations occur continuously. Very rarely a mutation will lead to a new phenotype. If the new phenotype is suited to an environmental change it can lead to a relatively rapid change in the species.
Mutations are random changes in the base sequence of DNA and they happen continuously, for example when DNA is copied before cell division or when radiation or chemicals damage DNA. Most mutations have no effect on the phenotype, some affect it, and very rarely a mutation produces a new phenotype. Usually a new phenotype is neutral or harmful, but if the environment changes, the new phenotype may give an advantage. Individuals with that phenotype are more likely to survive and reproduce, so the allele becomes more common in the population. Over many generations this natural selection can change the species relatively rapidly, especially when the environmental change is strong, such as a new predator or a change in climate.
Your focus
- Define the genome and the phenotype accurately in the context of variation.
- Describe how genetic information and environmental factors interact to produce a phenotype.
- Apply the interaction idea to a named characteristic of a plant or animal with a clear example.
Show all 24 objectives
- Define variation and identify its genetic and environmental causes.
- Classify named characteristics as showing continuous or discontinuous variation.
- Explain how variation within a population can affect survival when conditions change.
- State that genetic variation is caused by the alleles inherited from parents.
- Describe how gametes and fertilisation produce different allele combinations in offspring.
- Explain how inherited alleles and mutation can cause variation in a characteristic.
- State that environmental variation is caused by the conditions in which an organism develops.
- Give examples of environmental factors and describe their effects on characteristics.
- Explain how the same genotype can produce different phenotypes in different environments.
- Identify genes and environmental factors as sources of variation.
- Explain how genes and environment combine to produce a feature.
- Apply the idea to a named example such as height or plant growth.
- Define a population as the individuals of one species in a given area.
- Explain that genetic variation within a population is usually extensive.
- Give an example of inherited variation found among members of one population.
- State that all variants arise from mutations.
- Distinguish between mutations that have no effect, influence the phenotype and determine the phenotype.
- Describe an example of a mutation that determines a phenotype.
- State that mutations are random changes in DNA that occur continuously.
- Explain why only very rarely a mutation leads to a new phenotype.
- Describe how a new phenotype suited to an environmental change can lead to relatively rapid change in a species through natural selection.
Variation exam tips
Marking Points
- States that the genome is the complete genetic material of an organism and that genes code for proteins that influence characteristics.
- Explains that the phenotype is the observable characteristics of an organism, resulting from the interaction of genetic information with environmental factors.
- Uses a valid example, such as height being influenced by inherited genes and by diet, or leaf colour being influenced by genes and light intensity.
- Recognises that genes set a potential range for a characteristic while the environment determines where within that range the organism develops.
- Distinguishes inherited genetic material from environmental influences when describing a named characteristic.
- Applies the idea to a familiar organism, for example identical twins or cloned plants, to show that shared genomes can still produce different phenotypes.
- Defines variation as differences in characteristics between individuals within a population.
- Identifies genetic causes of variation, including differences in alleles inherited, mutation and the random combination of alleles in sexual reproduction.
- Identifies environmental causes of variation, including diet, exercise, temperature, light and other lifestyle or habitat factors.
- Explains that many characteristics result from both genetic and environmental influences acting together.
- Distinguishes discontinuous variation, which falls into distinct categories, from continuous variation, which shows a range of values.
- Links genetic variation in a population to increased chance of survival when the environment changes.
- Genetic variation is caused by the alleles inherited from both parents.
- Gametes contain one allele of each gene, so fertilisation produces a new allele combination.
- Different offspring inherit different allele combinations, which is why siblings are not identical.
- Some characteristics are determined mainly by genes, for example blood group or natural eye colour.
- A mutation can produce a new allele and so increase genetic variation.
- The genetic cause of a characteristic is the set of alleles an organism has inherited.
- Environmental variation is caused by the conditions in which an organism develops.
- Examples of environmental factors include diet, temperature, light, water, minerals, exercise and accidents.
- Environmental factors can change characteristics such as height, body mass, leaf size or scar formation.
- Some characteristics are affected by both genetic and environmental causes.
- Environmental causes do not change the alleles inherited by an organism.
- Identical genotypes can produce different phenotypes in different environments.
- State that genes are inherited from parents and carry the instructions for features.
- State that the environment includes factors such as diet, exercise, light and accidents.
- Explain that many characteristics are controlled by both genes and environment, not one alone.
- Use a named example, such as height or plant growth, to show how both factors combine.
- Distinguish inherited features from those changed mainly by environmental factors.
- Defines a population as the individuals of one species living together in the same area at the same time.
- States that members of a population usually differ genetically from one another.
- Uses the term genetic variation to describe differences in alleles or genotypes within the population.
- Explains that the variation is extensive, meaning many individuals differ and differences are common.
- Links genetic variation to differences in phenotype, such as shell colour or banding pattern in snails.
- Recognises that sexual reproduction and mutation are sources that maintain variation in a population.
- Defines mutation as a change in the DNA base sequence that can produce a new allele or variant.
- States that all variants ultimately arise from mutations, not from use or need.
- Explains that most mutations have no effect on the phenotype because they are in non-coding DNA or do not change the protein.
- Explains that some mutations influence the phenotype by altering the protein or how it works.
- States that very few mutations determine the phenotype, meaning the mutation alone decides the characteristic.
- Uses a suitable example, such as a mutation giving antibiotic resistance or causing a genetic disorder.
- Mutations are random changes in DNA that occur continuously, including during DNA replication and after exposure to mutagenic agents such as ultraviolet radiation.
- Most mutations do not change the phenotype because the genetic code is degenerate or the change occurs in non-coding DNA; only very rarely does a mutation produce a new phenotype.
- A new phenotype may be advantageous, neutral or harmful; whether it is advantageous depends on the environment.
- If the new phenotype is suited to an environmental change, individuals with it are more likely to survive, reproduce and pass on the allele, so its frequency in the population increases.
- This process of natural selection can produce a relatively rapid change in the species when the environmental change strongly favours the new phenotype.
- Explanations should link mutation, phenotype, selection pressure, survival, reproduction and change in allele frequency over generations.
Examiner Tips
- 💡Define genome and phenotype briefly before explaining their interaction, so the examiner can see both terms used accurately.
- 💡Include one named characteristic and state one genetic influence and one environmental influence on it.
- 💡Use command words carefully: describe means give a clear account, so link cause and effect rather than listing facts.
- 💡Structure answers by separating genetic causes from environmental causes, then give one example of each.
- 💡Use the terms continuous and discontinuous correctly, and support each with a named example.
- 💡When asked to explain, link the cause of variation to its effect on the individual or population rather than just naming factors.
- 💡Use the terms allele, gamete and fertilisation when explaining how genetic variation arises.
- 💡Link each inherited allele to the characteristic it affects, for example an allele for brown eyes.
- 💡If a question asks for a genetic cause, name the inherited alleles rather than describing the environment.
- 💡Name the environmental factor and state the effect it has on the characteristic.
- 💡Use a clear example, such as a plant in dim light growing tall and thin, to show understanding.
- 💡If a characteristic has both causes, mention the inherited alleles and the environmental factor.
- 💡Name the feature first, then say how genes and environment each affect it.
- 💡Use the phrase 'combination of genes and the environment' when explaining mixed features.
- 💡Give one clear example rather than several vague ones to secure the explanation.
- 💡Use the exact phrase genetic variation within a population when answering, rather than writing that animals are simply different.
- 💡Give a named example, such as banded snails or human blood groups, to show the variation is inherited and not just due to age or diet.
- 💡If asked why variation matters, link it to natural selection: a population with extensive variation is more likely to contain individuals suited to a changed environment.
- 💡Learn the three-part pattern: most have no effect, some influence, very few determine. Use those exact contrasts in your answer.
- 💡When explaining a mutation's effect, refer to the protein made and whether its shape or function changes.
- 💡Use one clear example, such as a mutation in a gene for a cell-surface receptor, to show how a variant can determine a phenotype.
- 💡Use the phrase 'random change in DNA' when defining mutation, then link it to a possible new phenotype.
- 💡When explaining rapid change, refer to a changed environment, a selection pressure, survival and reproduction of individuals with the advantageous phenotype, and an increase in allele frequency.
- 💡Avoid saying that the organism 'chooses' to mutate or 'tries' to adapt; describe mutation as random and selection as non-random.
Common Mistakes
- Saying the environment changes genes: correct this by explaining that the environment affects how genes are expressed and how the organism develops, not the DNA base sequence itself.
- Treating phenotype as entirely genetic: correct this by stating that most characteristics result from both genetic and environmental contributions.
- Confusing genome with phenotype: correct this by defining the genome as the inherited genetic material and the phenotype as the observable characteristics that develop.
- Listing only genetic causes: correct this by including environmental factors such as diet, exercise and temperature.
- Calling any difference between species variation: correct this by stating that variation refers to differences between individuals of the same population or species.
- Assuming continuous variation is always genetic: correct this by explaining that continuous characteristics such as height are usually affected by both genes and environment.
- Writing that genes are learned or acquired during life; genes are inherited from parents and are present from fertilisation.
- Confusing genes with chromosomes; a gene is a short section of DNA that codes for a characteristic, while a chromosome is a structure that carries many genes.
- Stating that all genetic characteristics are controlled by a single gene; many, such as height, are controlled by several genes and are also affected by the environment.
- Writing that environmental factors change genes or alleles; they affect how an organism develops but do not alter the inherited alleles.
- Giving only one environmental factor, such as diet, when a question asks for environmental causes generally; include a range of factors.
- Confusing a characteristic that is mainly genetic, such as blood group, with one that is strongly affected by the environment, such as body mass.
- Saying a feature is caused only by genes or only by the environment; correct by stating that most features result from both acting together.
- Confusing the environment with 'the surroundings' only; correct by including diet, exercise, disease and other lifestyle factors.
- Writing that genes change during life to match the environment; correct by stating that gene alleles are inherited and do not change, but gene expression and growth can be affected.
- Confusing a population with a species: a species may contain many separate populations, so the statement applies to one local group, not the whole species. Correction: define population as one species in one area.
- Assuming all members of a population are genetically identical apart from a few mutants. Correction: state that extensive variation is usual because many alleles and allele combinations exist.
- Treating variation as only environmental, such as a plant growing taller because of more light. Correction: genetic variation means inherited differences in alleles, although phenotype can also be affected by environment.
- Saying organisms mutate because they need to adapt. Correction: mutations occur at random, and selection then acts on the variants produced.
- Claiming most mutations are harmful. Correction: most have no effect on the phenotype, some influence it, and very few determine it.
- Treating influence and determine as identical. Correction: influence means the mutation contributes to the phenotype, while determine means it alone decides the phenotype.
- Thinking that mutations are always harmful: correct this by stating that most mutations have no effect and a rare few may be beneficial in a particular environment.
- Believing that organisms mutate because they need to adapt: correct this by stressing that mutations occur randomly and continuously, and selection acts on the variation already present.
- Saying that an individual evolves during its lifetime: correct this by explaining that evolution is a change in the inherited characteristics of a population over generations, not a change in one organism.