Microchimerism is the existence of a low-level of “foreign” cells within a host. It can occur if twins share a placenta or when

Classic chimerism - called macrochimerism - is when two fertilised eggs/early stage embryos fuse to form a single embryo that develops into a single individual with a mixture of cells.
Microchimerism is a more common and occurs when a small fraction of an organism's cells originate from another individual. The most common forms are pregnancy-related microchimerism. In maternal microchimerism some maternal cells persist in the offspring into adulthood. In foetal microchimerism, some foetal cells persist in the mother after the offspring is born. The two forms of microchimerism are due foetal and maternal cells crossing the placenta. Foetal microchimerism can also occur when two foetuses share a placenta or two placentas fuse.

Mosaicism is when two or more genetically distinct cell-lines in one individual, but both cell lines come from the same embryo. An example is the formation of black spots or patches on red cats due to localised mutation.

POSSIBLE MICROCHIMERISM

In 2024 I received this photo of a lilac British Shorthair kitten (from chocolate parents) with symmetrically placed black spots on her nose and a patch of dark fur above one eye. Black spots aren't unusual in red and cream cats, but they are definitely unusual on other colours.

What could be going on? I don't know for sure, but there are several things that spring to mind. Firstly some sort of somatic mutation (a mole, localised mutation of the skin), although I have never seen such symmetrical moles! Another thought is that it's related to how the embryo developed, some sort of signalling not reaching the two sides of the nose in the embryo before they fold together and fuse. Then there is the possibility that the early embryo bumped into another early embryo and exchanged a few cells, but those swapped cells only made tiny visible patches (and possibly more tissue on the outside) - microchimerism. Without DNA testing the different coloured areas it's impossible to say.

(Photos from Allyson Coombs, via Kerry Fowler)