Mosaicism from an early post-zygotic mutation
A mutation arising in one cell of the early embryo after fertilisation (post-zygotic) gives mosaicism, a mixture of mutant and normal cells in one person.
Remember
A mutation in one cell of the early embryo is carried only by the cells that arise from it, giving mutant and normal cells in the same person (mosaicism). The earlier it arises, the more widespread it tends to be.
After fertilisation, the zygote (the fertilised egg) divides by mitosis into small early cells called blastomeres; in the first few divisions, each of these cells will give rise to a share of the body. A post-zygotic mutation is one that arises after fertilisation. If it arises in just one blastomere, every cell that descends from it (the cells made by its divisions) carries the mutation. Cells that descend from the unaffected cells stay normal.
The person who develops is therefore a mixture of genetically different cell populations that all began from a single zygote. This is mosaicism, named after a picture made of tiles of different colours. Because early cells go on to form many tissues, the mutant cells can be widespread in the body, yet they are not present in every cell.
The earlier the mutation arises, the larger the share of the body it tends to affect. A late mutation may be restricted to one tissue. Where the mutant cells end up depends on which tissues the altered cell's descendants form.
A germline mutation (one present in the egg, the sperm or the zygote, for example when inherited from a parent) is already there at the start, so it is in every cell. Chimerism is different again: the cells come from two different zygotes, but mosaicism involves cells from one.
Key facts
- Mosaicism
- only some of a person's cells carry the mutation, and all of them began from one zygote.
- Early event
- the mutation arises in one cell of the early embryo after fertilisation, so only that cell's descendants carry it.
- Timing
- the earlier the mutation arises, the more widespread the mutant cells tend to be.
- Spread
- where the mutant cells are found follows the tissues that the altered cell's descendants form.
- Germline contrast
- a germline mutation is present in the zygote, so it is in every cell.
- Chimerism contrast
- chimerism means the cells come from two different zygotes.
Exam traps
- A mutation that starts in one cell of the early embryo after fertilisation is not present in every cell, because only that cell's descendants carry it. Only a mutation in the zygote, or in the egg or sperm that formed it, is found in every cell.
- Chimerism is not the same as mosaicism: it describes cells from two different zygotes, but in mosaicism every cell descends from one zygote, in which one cell mutates later.
- An epigenetic change (a chemical mark that changes how active a gene is) does not alter the DNA sequence, unlike a mutation, which is a change to the DNA itself.
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