Mosaic (genetics)

From Freepedia

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 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 occured 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 mosacicsm.