Variation — AQA GCSE Biology
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Variation explained
The genome is the entire genetic material of an organism.
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Genes within the genome code for proteins that help build and control the organism, so the genome provides the instructions that influence characteristics such as eye colour, blood group and height potential. However, the phenotype is not fixed by genes alone. The environment interacts with the genome throughout development. For example, a person may inherit genes for tall stature, but nutrition during childhood affects how tall they actually grow. Plant height depends on genes and on light, water and mineral availability. Some characteristics, such as blood group, are determined mainly by genes, while others, such as body mass or language spoken, are influenced strongly by the environment. The phenotype is therefore the result of both genetic instructions and environmental conditions.
Differences in the characteristics of individuals in a population is called variation and may be due to differences in: • the genes they have inherited (genetic causes) • the conditions in which they have developed (environmental causes) • a combination of genes and the environment.
Variation means the differences in characteristics between individuals of the same population. It arises from three possible sources. Genetic causes come from the alleles inherited from parents; for example, eye colour is largely genetic. Environmental causes come from conditions during development and life, such as diet, exercise, temperature or light; for example, a plant grown in shade may be tall and thin. Many characteristics result from a combination of both, such as human height or body mass, where genes set a potential range and the environment influences where an individual falls within it. When answering, identify the characteristic, then explain which cause or causes are involved and why. Avoid saying a characteristic is only genetic or only environmental unless the evidence supports it.
Students should be able to: • state that there is usually extensive genetic variation within a population of a species • recall that all variants arise from mutations and that: most have no effect on the phenotype; some influence phenotype; very few determine phenotype.
A population is the group of individuals of one species living in a habitat. Within that group there is usually extensive genetic variation: individuals differ in many alleles, so they differ in many characteristics. For example, a field of daisies varies in flower diameter, stem height and leaf shape. All new alleles arise by mutation, a random change in the base sequence of DNA. Most mutations have no effect on the phenotype because they occur in non-coding DNA or do not change the protein produced. Some mutations influence the phenotype, altering a characteristic slightly, such as eye colour or enzyme activity. Very few determine the phenotype, meaning the characteristic is controlled almost entirely by that mutation, as in some single-gene disorders. Mutation is the source of all new variation, and sexual reproduction reshuffles existing alleles.
Mutations occur continuously.
A mutation is a random change in the base sequence of DNA, and because DNA is copied whenever cells divide, new mutations arise continuously in every population. Most occur during DNA replication before mitosis or meiosis, or are caused by mutagens such as ultraviolet radiation or tar in cigarette smoke. A concrete example is a substitution in a gene controlling a protein, changing one triplet of bases so a different amino acid is inserted. Because mutation is random, it can happen in any cell at any time, and the rate can be increased by exposure to mutagens. Most mutations have no effect on the phenotype, some are harmful, and a few are beneficial; the key idea is that new alleles appear continually, providing the genetic variation on which natural selection can act.
Very rarely a mutation will lead to a new phenotype.
A mutation changes the base sequence of DNA, but a new phenotype appears only when that change alters the protein the gene codes for in a way that affects the organism. Most mutations are in non-coding DNA or produce the same amino acid because the genetic code is degenerate, so the phenotype is unchanged. Even when an amino acid changes, the protein may still fold and work normally. Very rarely, the altered protein changes a characteristic, such as fur colour, enzyme activity or disease resistance, giving a new phenotype. A useful method is to trace the chain: base sequence changes, triplet changes, amino acid changes, protein shape or activity changes, phenotype changes. Each step must occur for a new phenotype to result.
If the new phenotype is suited to an environmental change it can lead to a relatively rapid change in the species.
When the environment changes, such as a shift to colder weather or a new predator, organisms with a phenotype suited to the new conditions are more likely to survive and reproduce. Their alleles are passed to offspring, so the beneficial allele becomes more common in the population over generations. Because selection acts on every generation, the change in the species can be relatively rapid, especially in organisms with short generation times such as bacteria or insects. A method is to write the sequence: environmental change, variation already present, some individuals better suited, these survive and reproduce, beneficial allele frequency rises, population changes. The new phenotype must already exist; the environment does not create it, it selects it.
Your focus
- Define the genome and state that genes code for proteins.
- Describe how the genome and the environment interact to influence the phenotype.
- Apply the idea of gene-environment interaction to a named example of a characteristic.
Show all 18 objectives
- Define variation as differences in characteristics within a population.
- Classify examples of variation as genetic, environmental or combined.
- Explain how genes and environment can interact to produce a characteristic.
- State that there is usually extensive genetic variation within a population of a species.
- Recall that all variants arise from mutations and classify their effects as most having no effect, some influencing phenotype and very few determining phenotype.
- Use examples to explain the difference between a mutation that influences phenotype and one that determines phenotype.
- Define a mutation as a change in the base sequence of DNA.
- Describe that mutations occur continuously and at random, and that mutagens increase the rate.
- Explain that mutation generates new alleles and therefore genetic variation.
- Explain how a change in base sequence can alter the amino acid sequence and protein produced.
- Describe why most mutations do not produce a new phenotype.
- Give examples of phenotypes that a mutation could change.
- Explain how an environmental change can select for a new phenotype.
- Describe how the frequency of a beneficial allele increases over generations.
- Explain why the change in a species can be relatively rapid.
Variation exam tips
Marking Points
- Define the genome as the entire genetic material of an organism.
- State that genes code for proteins that influence the development of characteristics.
- Explain that the phenotype is influenced by the genome and by environmental factors.
- Give an example where both genes and environment affect a characteristic, such as height being influenced by genes and nutrition.
- Recognise that some characteristics are determined mainly by genes, while others are influenced strongly by the environment.
- Describe the interaction as the combined effect of genetic instructions and environmental conditions during development.
- Defines variation as differences in characteristics between individuals in a population.
- Identifies genetic causes as differences in the alleles inherited from parents.
- Identifies environmental causes as differences in the conditions in which organisms develop or live.
- Explains that many characteristics are influenced by a combination of genetic and environmental factors.
- Uses a named example to illustrate each cause or a combined cause.
- Distinguishes between the source of the variation and the characteristic itself.
- Define a population as all the individuals of one species in a given area, and state that genetic variation within it is usually extensive.
- Explain that genetic variation means individuals possess different alleles, giving differences in characteristics such as height, colour or disease resistance.
- State that all new variants arise from mutations, which are random changes in the base sequence of DNA.
- Describe that most mutations have no effect on the phenotype, often because they occur in non-coding DNA or do not alter the protein.
- Describe that some mutations influence the phenotype, producing a small or partial change in a characteristic.
- Describe that very few mutations determine the phenotype, so the characteristic is controlled almost entirely by the mutation.
- Use examples to distinguish influence from determine, such as a mutation slightly altering enzyme activity compared with a single-gene disorder.
- Recognise that mutation creates new alleles while sexual reproduction reshuffles existing alleles, so both contribute to variation.
- A mutation is a change in the base sequence of DNA, so it creates a new allele.
- Mutations happen at random and arise continuously, including during DNA replication before cell division.
- Mutagens such as ultraviolet radiation or chemicals in cigarette smoke increase the mutation rate.
- Most mutations have no effect on the phenotype; some are harmful and a few are beneficial.
- Mutation is the original source of new genetic variation in a population.
- A mutation alters the base sequence of DNA and so may change the sequence of amino acids in a protein.
- The genetic code is degenerate, so many mutations do not change the amino acid and have no effect.
- A new phenotype appears only if the altered protein changes a characteristic of the organism.
- Most mutations have no effect on phenotype, so a new phenotype is very rare.
- Examples of new phenotypes include changed fur colour, altered enzyme activity or resistance to a disease.
- An environmental change alters which phenotypes are best suited to survive.
- Individuals with the suited phenotype survive, reproduce and pass on the beneficial allele.
- The frequency of the beneficial allele increases in the population over generations.
- Because selection acts each generation, the change in the species can be relatively rapid.
- The rate of change is faster in organisms with short generation times, such as bacteria.
Examiner Tips
- 💡Use the terms genome, gene, phenotype and environment accurately in your answer.
- 💡Include at least one named example where genes and environment both affect a characteristic.
- 💡Explain the interaction rather than listing genes and environment separately.
- 💡When given data about a characteristic, look for evidence that separates genetic from environmental effects, such as identical twins raised apart or clones grown in different conditions.
- 💡Use the phrase 'combination of genes and the environment' when both factors contribute, and support it with a specific example.
- 💡Check that each cause you name is linked to the characteristic in the question, not just listed generally.
- 💡Use the words population, allele and phenotype precisely; a population is one species in one area, not several species together.
- 💡When asked about the effects of mutations, structure the answer as most, some and very few, matching the three categories in the specification.
- 💡Give a named example for each category, such as a mutation in non-coding DNA, a mutation altering enzyme activity, and a single-gene disorder.
- 💡Check that any explanation of variation distinguishes new alleles from new combinations of existing alleles.
- 💡Define mutation precisely as a change in the base sequence of DNA, not simply as a change in a gene.
- 💡Use the word random when explaining when and where mutations occur.
- 💡Link mutation to variation by stating that it produces new alleles for natural selection to act on.
- 💡Use the chain base sequence, amino acid sequence, protein, phenotype to explain how a mutation can have an effect.
- 💡State explicitly that a new phenotype is rare because most mutations do not alter the protein.
- 💡Give a named example of a phenotype, such as fur colour, rather than writing only that the organism changes.
- 💡Structure answers as environmental change, selection, reproduction, allele frequency change.
- 💡Use the phrase survival of the fittest only if you then explain what fittest means in the context.
- 💡Refer to allele frequencies rising or falling in the population rather than saying the species decides to change.
Common Mistakes
- Saying the environment changes the genome itself; the environment affects how genes are expressed and how the organism develops, not the DNA sequence.
- Treating phenotype as determined only by genes; both genetic and environmental factors contribute.
- Confusing the genome with a single gene; the genome is the entire genetic material, not just one gene.
- Stating that all variation is genetic; correct by explaining that environmental conditions can also cause differences.
- Confusing a gene with an allele; correct by using allele for a version of a gene.
- Claiming that a single characteristic is always caused only by genes or only by the environment; correct by recognising that many show combined causation.
- Thinking that mutations are always harmful: correct this by stating that most mutations have no effect on the phenotype and only very few determine it.
- Confusing influence with determine: correct this by explaining that an influencing mutation alters a characteristic partly, whereas a determining mutation controls it almost entirely.
- Believing that variation is created by sexual reproduction alone: correct this by stating that sexual reproduction reshuffles existing alleles, while mutation is the source of new alleles.
- Thinking mutations are always harmful: correct this by stating that most have no phenotypic effect and only some are harmful.
- Believing organisms mutate because they need to adapt: correct this by stressing that mutation is random and not directed by need.
- Confusing mutation with natural selection: correct this by separating the random origin of a new allele from the non-random survival of organisms that carry it.
- Assuming every mutation changes the phenotype: correct this by stating that most mutations have no visible effect.
- Thinking a mutation always changes one amino acid: correct this by noting that the code is degenerate and the amino acid may stay the same.
- Saying a mutation directly creates a new species: correct this by stating that it may produce a new phenotype, which natural selection may then act on.
- Saying organisms mutate in order to suit the new environment: correct this by stating that variation already exists and selection acts on it.
- Confusing a change in an individual with a change in the species: correct this by describing a change in allele frequency in the population over generations.
- Claiming the change is always slow: correct this by explaining that strong selection and short generation times can make it relatively rapid.