Each multicellular organism is the result of repeated divisions of the original fertilized egg. Before each division, a cell's DNA is copied so that each new cell receives a complete copy of its genome. Copying errors (mutations) occasionally happen. If an error goes uncorrected, then all its progeny cells will carry the change, as well, and will be genetically different from the original cell. The resulting organism is called a genetic mosaic. Patches of skin different in color from surrounding skin are an example of a mosaic effect.

Another cause of mosaicism is the X-chromosome inactivation that occurs in all female mammals. In humans, for example, X inactivation is thought to happen at or before the 32-cell stage. In this process, one of the X chromosomes in each cell is randomly inactivated. For a given cell, all its progeny cells have the same X chromosome inactivated. Therefore, a...