Definition of Mosaic

Mosaic: An individual or tissue containing two or more types of genetically different cells. All females are mosaics because of X-chromosome inactivation (lyonization).

Mosaic patterns can affect how genetic disorders are expressed. For example, about 5 percent of people with Down's syndrome have a mosaic variant, where only some cells have an extra chromosome 21. These individuals have fewer clinical symptoms, are more likely to have a normal IQ, and are less likely to have heart and other problems that can be associated with Down syndrome.