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 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.

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.

Genetic mosaics are used in experimental genetics to determine whether a gene functions cell autonomously. If a mosaic in which genes are lost or added in a subset of cells causes a change in the phenotype of only the altered cells, then the gene is said to act cell autonomously (with regard to the phenotype studied).