In medicine (genetics), a mosaic or mosaicism denotes the presence of two populations of cells with different genotypes in one patient, where usually one of the two is affected by a genetic disorder. Mosaicism may result from a mutation during development which is propagated to only a subset of the adult cells. Mammalian females are usually mosaic because one of their X chromosomes is randomly inactivated during development.
Although the two can have some common symptoms, mosaicism is distinctly different from chimerism; in the latter, two or more cell lines of different origin are present in an organism and are not simply the result of mutation within a single cell line.
Although most forms of trisomy are due to problems in meiosis and affect all cells of the organism, there are cases where the trisomy only occurs in a selection of the cells. This is usually due to a nondisjunction event in an early mitosis. Generally this leads to a milder phenotype than in non-mosaic patients with the same disorder.
Sometimes intersex conditions can be caused by mosaicism – where some cells in the body have XX and others XY chromosomes.
Another example of a mosaic genotype is one of the milder forms of Klinefelter’s syndrome, called 46/47 XY/XXY mosaic wherein some of the patient’s cells contain XY chromosomes, and some contain XXY chromosomes. The 46/47 annotation indicates that the XY cells have the normal number of 46 total chromosomes, and the XXY cells have 47 total chromosomes.
A special type of mosaicism is gonadal mosaicism. This is when a part of the gonadal cells of an organism have different genotype than the rest of the cells, usually because of a mutation that occurred in an early stem cell that gave rise to a part or all of the gonadal tissue.
If these cells have sustained a new mutation that causes a dominant heritable disease, then it is possible for two healthy parents to have an offspring suffering from the disease. Usually when this happens it is because of a new mutation in the zygote, but if the parents have two or more offspring with the same disorder, then it is an indication of gonadal mosaicsm.
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