Germ and somatic cells, and mutation as a source of variationSpec D1.3.6, D1.3.7
In short
A mutation in a germ cell can be passed to offspring and is then in every cell of the new organism. A mutation in a somatic cell affects only that cell and its descendants and is not inherited, though it may lead to cancer. Gene mutation is the original source of all genetic variation, essential for evolution by natural selection.
Germ cells and somatic cells
| Germ cells | Somatic cells | |
|---|---|---|
| What they are | Cells that produce gametes (and the gametes themselves) | All other body cells |
| Inherited? | Yes: a mutated gene can be passed to offspring | No: not passed to offspring |
| Cells affected | Every cell of an offspring that inherits it | Only the mutated cell and the cells produced from it by mitosis |
| Possible consequence | Inherited genetic disease, or new variation in the population | Cancer, if mutations affect genes that control cell division |
Cancer develops when mutations in a somatic cell cause it to divide repeatedly without control, forming a tumour. Usually several mutations in the same cell line are needed, which is one reason cancer becomes more common with age and with exposure to mutagens.
Mutation as a source of genetic variation
Gene mutation is the original source of all genetic variation: it is the only process that creates new alleles. Meiosis and fertilisation shuffle existing alleles into new combinations, but cannot create new ones. Most mutations are either harmful or neutral for an individual organism, but in a species over the long term they are essential for evolution by natural selection, because selection can only act on variation that already exists.
Nature of science: commercial genetic tests can give information about potential future health and disease risk. Without expert interpretation, this information could be misunderstood and cause needless worry or false reassurance.
Linking question: how can natural selection lead to both a reduction in variation and an increase in biological diversity? Mutation supplies the variation; selection removes some alleles within a population but can drive populations apart into new species.
Quick check
What is a single-nucleotide polymorphism (SNP)?
Show answer
A position in the genome where a single base differs between individuals, resulting from a base substitution mutation.
Why does deleting one base usually have a larger effect than substituting one base?
Show answer
It shifts the reading frame, changing every codon after the mutation.
Give one chemical mutagen and one form of mutagenic radiation.
Show answer
Benzo[a]pyrene (in tobacco smoke); ultraviolet light (or X-rays, gamma rays).
Why are somatic mutations not inherited?
Show answer
They are not in germ cells, so they are not passed into gametes.
Written and checked against the IB Biology SL specification · Updated October 2026