Mosaic: A person or a tissue that contains two or more types of genetically different cells. All females are mosaics because of X-chromosome inactivation (lyonization). Mosaic patterns can affect the way genetic disorders are expressed. For example, about 5 percent of people with Down syndrome have a mosaic variant in which only some cells have an extra chromosome 21. Compared to others with Down syndrome, 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.