As a mosaic is called in genetics, an individual in whose body cells with different karyotypes and / or genotypes occur, with all body cells are derived from the same fertilized egg. A mosaic must be distinguished from a chimera, are united in the cells of several individual fertilized eggs in an individual.
The different karyotypes of a mosaic arise in the course of embryonic development due to problems in cell division usually as a result of non- disjunction ( for example, in Pallister - Killian syndrome). Different genotypes arise by mutation of single cells during embryonic development.
As a physiological cause of a mosaic also an X - inactivation of the question ( for example, when Tortoiseshell cat ).