Both terms refer to one organism with two or more distinct populations of cells * – but they are not interchangeable.
The different cell lines in chimeras originally come from different zygotes whereas mosaics arise from the same zygote. Because mosaics start with the same genome, you expect them to be more similar than chimeras that can be a fusion of two different genomes. However, the bigger contrast doesn’t always translate to larger differences in appearance because it is often difficult to tell the mechanism of distinct cell lines without analysing the genetic landscape.
Chimeras can happen with organ transplantation because the donor cells are different from recipient. You could also be a chimera from absorbing a twin while in utero: a pregnancy may start out with two or more zygotes but not all survive and are then absorbed by remaining fetus. You could also have cells from your mother in your body = a distinct cell line from another zygote. The last two are gaining more attention and I’ll try to do a post on micro-chimerism (small numbers of cells from different zygote) to explore in more detail.
Mosaicism can arise when there is a mutation early in development so there can be patches of cells that behave differently. This can happen on a gene level or even whole chromosomes. I have a good friend who has a mixture of 45X and 46XY cells in her body. One cell early in development dropped the Y chromosome (probably through anaphase lag for those who care) and the majority of her cells are 45X.
I wrote this post because several people use chimera/mosaic as synonyms and the pedantic in me goes a bit crazy. At least the information will be useful if you’re taking a multiple choice test on these words?
* Is X inactivation mosaicism?
In medical school, students are taught that females are mosaics due to X inactivation but Wikipedia says that mosaicism requires genetically different cell lines. This can lead to confusion! I like the definition:
A 46 XX human is a mosaic because she consists of a mixture of two kinds of cells: each with different functional chromosomes.
This doesn’t change the conflict with Wikipedia but students seem to accept it. You can see some pretty photos of distinct cell lines in a recent NY Times piece about the X chromosome.]
A note on the image: I struggled with an image for this post. I hope that you understand the different colours are cell lines – and the chimera has two different colours whereas the mosaic is still related to the original.