A mosaic is an individual developed from a single fertilized egg - from one zygote, but it consists of two or more different cell populations- of cell populations with different genotypes. It may result for example from a mutation during development. A mosaic is often mistaken for a chimera. A chimera also consists of two or more cell lineages with a different karyotype, but these come from a single zygote.
Mosaicism can exist in both somatic cells and germ line cells (affecting egg or sperm)- therefore we distinguish two types of mosaicism: somatic mosaicism and gonadal mosaicism.
A mosaic germline mutation can be very significant because it may be passed to the next generation. A person carrying a germ-line mosaic is usually not affected by the mutation, since it is not present in other cells of the body.Germline mosaicism is often seen with autosoma-dominant and X-linked disorders - such as osteogenesis imperfecta and Duchenne muscular dystrophy. This mutation usually occures in early stem cells and that give rise to some or all parts of the gonadal tissue.
A person carrying a somatic mosaicism has somatic cells of more than one genotype and he may or may not be affected by the disorder, depending on how many and what cells are affected. Somatic mosaicism is usually developed from a single fertilized egg due to mitotic errors. Many genetic disorders, such as Down syndrome or Neurofibromatosis are often associated with somatic mosaicism.
Is a form of naturally occuring mosaicism, during which one of the two copies of X-chromosome present in female mammals is inactivated. The choice of which X chromosome will be inactivated is random in placental mammals (mice, humans), but once an X chromosome is inactivated it will remain inactive throughout the lifetime of the cell and its descendants in the organism.
also see Dosage Compensation