Mosaicism denotes the presence of two populations of cells in one patient, where usually one of the two is affected by a genetic disorder. Although most forms of trisomy are due to problems in meiosis and affect all cells of the zygote, there are cases where the trisomy only occurs in a selection of the cells. Generally this leads to a milder phenotype than in non-mosaic patients with the same disorder. Post-zygote event: a mutational event or abnormally in chromosome replication/segregation that occurs after fertilization of the ovum by the sperm, often leading to mosaicism. Germline mosaicism denotes two or more genetic or cytogenetic cell lines confined to the precursor (germline) cells of the egg, or sperm; formally called gonadal mosaicism. Somatic mosaicism denotes two or more genetic or cytogenetic cell lines within the cell of the body (may or may not include the germline cells). Isolated: an abnormality that occurs in the absence of other systemic involvement.

Milewicz, D. M., Witz, A. M., Smith, A. C. M., Manchester, D. K., Waldstein, G., Byers, P. H. Parental somatic and germ-line mosaicism for a multiexon deletion with unusual endpoints in a type III collagen (COL3A1) allele produces Ehlers-Danlos syndrome type IV in the heterozygous offspring. Am. J. Hum. Genet. 53: 62-70, 1993. [PubMed: 8317500, related citations]