Variation and Mutation
BiologyΒ· 18.1Β· 12 min read
1. 1. Definition and Types of Variationβ β ββββ± 3 min
Variation
The differences in characteristics between individuals of the same species.
Variation is split into two main types based on the phenotypes observed: continuous and discontinuous variation. All variation arises from genetic factors, environmental factors, or a combination of both.
Type of Variation | Features | Example |
|---|---|---|
Continuous | Range of intermediates, no distinct categories | Human height, plant mass |
Discontinuous | Distinct categories, no intermediates | Human blood group, ability to roll tongue |
A student measures the mass of 100 dandelion plants growing in the same field and records values ranging from 12g to 68g, with no gaps between recorded masses. Name the type of variation shown and justify your answer.
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Step 1: Identify the variation type: This is continuous variation.
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Step 2: Justify your answer: There is a full range of intermediate masses between the minimum and maximum values, with no discrete categories.
Exam tip:
Always reference an example to support your answer when asked to distinguish between variation types in exams.
2. 2. Causes of Phenotypic Variationβ β ββββ± 3 min
Phenotypic variation (observable characteristics) is caused by three groups of factors, outlined below:
Genetic factors: Differences in alleles inherited from parents, passed down through generations.
Environmental factors: External conditions that alter phenotype, not passed down to offspring.
Combined factors: Most continuous variation is caused by an interaction of genes and environment.
Identical twins have exactly the same genotype, but one has significantly darker skin than the other after a summer working outdoors. Explain this difference.
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Step 1: Recall that identical twins share 100% of their genes, so differences cannot be genetic.
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Step 2: Link to environmental cause: Increased exposure to UV radiation from sunlight triggers the production of the dark pigment melanin in skin, leading to darker skin tone.
Which of the following characteristics is caused only by genetic factors? A) Lung capacity, B) Blood group, C) Body mass, D) Skin tone
Reveal answer
B βBlood group is discontinuous variation controlled only by genes. Lung capacity, body mass and skin tone are all affected by environmental factors (exercise, diet, sun exposure respectively).
3. 3. Mutation: Source of New Allelesβ β β βββ± 3 min
Mutation
A rare, random change in the base sequence of DNA, or a change in chromosome number or structure, that can produce new alleles.
Most mutations are neutral (have no effect on phenotype), some are harmful (e.g. mutations that cause genetic disorders), and a very small number are beneficial (provide a survival advantage to the organism).
Explain why doctors often recommend that patients get X-rays only when medically necessary.
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Step 1: Identify the mutagen: X-rays are a form of ionising radiation, which acts as a mutagen.
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Step 2: Link to mutation risk: X-rays increase the rate of mutation in human DNA, which can lead to uncontrolled cell growth and cancer if mutations occur in genes that control cell division.
4. Extended Only: Mutation and Antibiotic Resistanceβ β β β βExtended onlyβ± 3 min
Extended learners must be able to explain how random mutations in bacteria lead to the development of antibiotic-resistant strains, a common extended-response exam question.
Explain how a mutation in a single bacterium can lead to a population of bacteria that is resistant to the antibiotic amoxicillin.
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Step 1: A random mutation occurs in the DNA of a single bacterium, producing a new allele.
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Step 2: This allele gives the bacterium resistance to amoxicillin, for example by producing an enzyme that breaks down the antibiotic.
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Step 3: When amoxicillin is present, the resistant bacterium survives and reproduces, while non-resistant bacteria are killed.
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Step 4: Over time, the resistant allele becomes more common in the population, leading to a fully resistant strain of bacteria.
Exam tip:
Always specify that mutations are random and occur before exposure to the antibiotic, not in response to it, to gain full marks in extended answers.
5. Common Pitfalls
Wrong move:
Defining variation as differences between different species
Why:
Variation strictly refers to differences between individuals of the same species, not cross-species differences.
Correct move:
Include the phrase 'of the same species' in all definitions of variation in your exam answers.
Wrong move:
Stating all mutations are harmful
Why:
Most mutations are neutral, many have no effect on phenotype, and a small number are beneficial.
Correct move:
Describe mutation effects as: most neutral, some harmful, rarely beneficial.
Wrong move:
Claiming bacteria mutate in response to antibiotics to become resistant
Why:
Mutations are random; they occur before exposure to the antibiotic, which then selects for resistant individuals.
Correct move:
Explain that random mutations produce resistant bacteria first, then the antibiotic acts as a selection pressure that allows resistant bacteria to survive and reproduce.
Wrong move:
Attributing all phenotypic variation to genetic factors only
Why:
Most continuous characteristics (e.g. height, mass) are heavily influenced by environmental factors.
Correct move:
Always consider environmental contributions when explaining the cause of continuous variation phenotypes.
Wrong move:
Confusing continuous and discontinuous variation
Why:
Continuous variation has intermediates, discontinuous variation has discrete categories with no gaps.
Correct move:
Use common examples to distinguish: height = continuous, blood group = discontinuous.
6. Quick Reference Cheatsheet
Concept | Core Key Facts | Extended Key Facts |
|---|---|---|
Variation | Differences between individuals of the same species | Distinguish genetic vs phenotypic variation |
Variation types | Continuous (range, e.g. height); Discontinuous (categories, e.g. blood group) | Continuous = genetic + environment; Discontinuous = genetic only |
Mutation | Random change to DNA base sequence or chromosome structure/number | Explain role in antibiotic resistance in bacteria |
Mutagens | Ionising radiation (X-rays, UV), chemical mutagens (tobacco tar) | Same as Core |
Mutation effects | Most neutral, some harmful, few beneficial | Beneficial mutations drive evolution by natural selection |
7. Frequently Asked
What is the difference between continuous and discontinuous variation?
Continuous variation shows a full range of intermediate phenotypes (e.g. height, mass) controlled by multiple genes and environmental factors. Discontinuous variation has discrete, separate phenotypes with no intermediates (e.g. blood group, tongue rolling) controlled by single genes only.
Are all mutations harmful?
No: most mutations are neutral (have no effect on phenotype), some are harmful, and a very small number are beneficial. Beneficial mutations provide a survival advantage to the organism and can drive evolution by natural selection.
What's Next
Now that you have mastered variation and mutation, you are ready to move on to natural selection, the next core topic in the Variation and Selection unit. Understanding that mutations produce new alleles is the foundational knowledge required to explain how natural selection leads to evolution, as well as applied topics like antibiotic resistance and selective breeding, which are frequently tested in both Core and Extended papers. Make sure you practice structured exam responses for mutation and variation questions to consolidate your learning before moving to more advanced content.