Topic 18.1 · Core + Supplement
Mutation and Genetic Variation
Mutation is the only route by which a genuinely new allele can appear. Everything else that creates genetic variation — meiosis, random mating, random fertilisation — is reshuffling alleles that already exist into new combinations. Keeping that distinction sharp is worth more marks here than almost anything else in the chapter.
Mutation — Core
At Core level, a mutation is defined simply as a genetic change, and its significance is that it is the way new alleles are formed. Two named factors are required to increase the rate at which mutation happens: ionising radiation and some chemicals. A substance or physical factor that raises the mutation rate is sometimes called a mutagen— useful vocabulary, but the assessed idea underneath it is simply a higher rate of genetic change, not the word itself.
It matters what a mutagen does and does not do. It raises the probability that a mutation will occur somewhere in the genetic material being exposed to it; it does not guarantee a mutation in any particular gene, and it certainly does not guarantee a useful mutation. Most mutations arise naturally regardless of any mutagen being present at all.
A mutation can turn out to have no obvious effect on the phenotype, to be harmful, or — occasionally, in a particular environment — to be advantageous. Which of those applies depends on exactly what changed and on the environment the resulting phenotype is expressed in, which is exactly why the environment is doing the deciding, not the organism. There is also an inheritance condition worth knowing: a mutation only passes to offspring if it occurs in a cell that produces gametes. A mutation in a body cell can affect that individual during its own lifetime, but it is not passed on — which is part of why not every mutation that occurs ends up mattering to the next generation.
Gene mutation — Supplement
Supplement candidates need a more precise, molecular version of the same idea. A gene mutation is a random change in the base sequence of DNA within a gene. Four elements of that definition are all worth holding onto individually, because a mark scheme can reward each one separately: the change is random; it is a genuine change; it occurs in the base sequence; and that sequence belongs to DNA. Because the base sequence is what carries the genetic information for a gene, changing it can produce a different version of that gene — in other words, a new allele.
The word random is not decorative. A mutation does not appear because an organism needs a particular characteristic — it happens (or does not happen) independently of whether it would turn out to be useful. Note also that a gene mutation is a distinct idea from a chromosome mutation, which involves larger-scale changes to chromosome structure or number rather than a change within one gene’s base sequence. The detailed mechanisms of chromosome mutation sit outside what this chapter requires.
The four sources of genetic variation — Supplement
Supplement candidates are expected to name four separate sources of genetic variation in a population, and Cambridge wants all four, not a vague reference to “sexual reproduction.”
| Source | What it contributes |
|---|---|
| Mutation | Can create a genuinely new allele that did not exist in the population before |
| Meiosis | Produces gametes that are genetically different from one another, not identical copies |
| Random mating | Different possible individuals may pair up, changing which parental allele combinations get passed on |
| Random fertilisation | Any one of many genetically different male gametes may fuse with any one of many genetically different female gametes |
These four sources are doing different jobs, and collapsing them into one another is a common way to lose marks. Only mutation can generate an allele that has never existed before. Meiosis, random mating and random fertilisation all work with alleles that are already present in the population — they change which combinations of existing alleles end up together in an individual, rather than creating anything new at the level of the gene itself.
Don’t confuse
Genetic variation supplying new alleles (mutation) is not the same claim as genetic variation supplying new combinations of existing alleles (meiosis, random mating, random fertilisation). Both matter to a population, but only one of the four sources can actually introduce a brand-new allele. This is also the distinction that natural selection depends on: selection cannot act on variation that does not exist yet, so mutation supplying genuinely new material — and meiosis, mating and fertilisation supplying new combinations of it — is what gives natural selection something to act on in the first place.
The variation this page explains where it comes from is the same variation described as continuous or discontinuous on the continuous and discontinuous variation page — that page describes the pattern the variation takes; this one explains its origin.