Well, I can give one well-known example for this in genetics. Females carry two X-chromosome, against male's X and Y. There is one thing called X-inactivation, which means one of the X chromosome in the cell is inactivated randomly in cells. By this, even organs only carried the same kind of cells are mosaic in aspect to genes in X chromosome. Then think about a well-know X-chromosome linked disease, the Rett Syndrome, and the cell in body, in the brain.