Edexcel IGCSE Biology · Spec 3.34
Mutation
What a mutation is and how mutations create new alleles and increase variation.
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Mutation
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Explained
Mutation, and where new alleles come from
A mutation is a random change in the genetic material, usually a change in the sequence of bases in DNA. It happens spontaneously when DNA is copied, and copying errors are rare but never quite zero.
Mutation matters because it is the only original source of new alleles. Meiosis and fertilisation shuffle the alleles that already exist into new combinations, but they cannot invent one. Every allele in every population began as a mutation at some point.
Rare and random
Both words carry meaning that questions test.
Rare means that a mutation in any particular gene is unlikely in any particular cell division. Across billions of cells and billions of years, though, unlikely events happen constantly.
Random means unpredictable and undirected. A mutation is not caused by an organism needing it. A population of bacteria exposed to an antibiotic does not respond by mutating; the resistant mutant was already there by chance and simply survived. Getting that direction right is one of the most commonly tested ideas in the whole of genetics.
What mutations do
Most have no effect at all, either because they fall in a stretch of DNA that does not code for a protein, or because the change still codes for the same amino acid.
Some are harmful, changing a protein so that it no longer works. An enzyme with an altered active site can no longer bind its substrate.
A few are beneficial, giving the organism some advantage in its environment. These are the raw material of evolution, and they are much rarer than the harmful ones.
Whether a mutation is harmful or beneficial depends on the environment. The same allele can be an advantage in one place and a disadvantage in another.
Mutagens
Some factors increase the rate at which mutations occur. Ionising radiation such as gamma rays and X-rays, ultraviolet radiation, and certain chemicals including those in tobacco smoke are all mutagens.
They increase the rate. They do not decide which mutation happens, and they cannot produce a useful one to order.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks why sexual reproduction is beneficial, and credits that it produces genetic variation and that some individuals may therefore survive environmental change. Beside it, the guidance says: ignore mutation.
That is a precise ruling rather than a dismissal of mutation. The variation produced by sexual reproduction comes from meiosis and from the fusion of gametes from two parents, not from mutation, so mutation does not answer that particular question.
On a different question, about why an allele persists in a population, the same mark scheme puts mutation first in the chain: a mutation occurs, giving variation in the population, then those individuals survive, then they reproduce, then they pass on the allele. It accepts gene passed on and mutation passed on as alternatives to allele passed on.
So the word earns marks in a natural selection chain and earns nothing in a sexual reproduction question. Read which process the question is actually about before reaching for it.
Mutation and natural selection together
The full chain runs in a fixed order, and the order is what the marks follow.
A mutation produces a new allele. That creates variation in the population. When conditions change, individuals with that allele are better suited, so more of them survive. They reproduce, and pass the allele to their offspring. Over many generations the allele becomes more common.
Nothing in that chain requires the organism to want anything, and answers that say a species adapted because it needed to are describing a different and incorrect mechanism.
Spec 3.34
What you need to know
- Define a mutation
- Know that mutations are rare and random
- Know what can increase the rate of mutation
Active recall
Quick check
Answer each question before opening the answer.
What is a mutation?
A random change in the base sequence of DNA.
What can increase the rate of mutation?
Ionising radiation (e.g. UV, X-rays, gamma) and some chemicals (mutagens).
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