Mutation and variation at Higher Level: The same statement as SL. At HL the long answer pulls together the whole topic: causes, kinds of mutation, effects on proteins and evolution.
Practise this as you read
- State the cause of a new allele.
- Explain the causes and consequences of mutation over many generations.
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The big idea: Gene mutation is the original source of all genetic variation: every new allele began as a mutation.
The points to remember
- A mutation changes a gene's base sequence, producing a new allele.
- Meiosis and fertilisation reshuffle alleles, but only mutation makes new ones.
- For an individual, most mutations are harmful or neutral.
- Rarely a mutation is beneficial: it helps its carrier survive and reproduce.
- Natural selection makes beneficial alleles more common: in the long term mutation is essential for evolution.
Remember it as: Mutation makes the variation; natural selection sorts it.
Real example: sickle cell and malaria. The sickle-cell allele arose by a substitution. People with one copy are partly protected against malaria, so in malarial regions natural selection has kept the allele common, even though two copies cause disease.
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Harmful, neutral, beneficial: What happens to a new allele depends on its effect on survival and reproduction.
| Effect of the new allele | How common | What usually happens to it |
|---|---|---|
| Harmful | Common | Carriers leave fewer offspring, so it stays rare |
| Neutral | Most common | No effect on survival; it may stay, spread or vanish by chance |
| Beneficial | Rare | Carriers leave more offspring, so it becomes more common |
Only heritable mutations count: Only mutations in germ cells are passed on, so only they add variation to the next generation.
How it is asked: A one-mark 'cause of a new allele' (mutation), a 1A item on how alleles arise, or a long answer on causes and consequences of mutation over generations.
Explain the causes and consequences of changes in the base sequences of DNA in a species over many generations.
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