Mosaic Down syndrome
Once again, genetic science is tricky here. But in mosaic Down syndrome (also called “mosaicism”), not all of the cells contain an extra copy of the chromosome 21. It is the rarest type of Down syndrome, accounting for 2 percent of cases, and patients with this type may have less severe or fewer symptoms than the patients with the other two more common types of Down syndrome. Mosaic Down syndrome (or mosaicism) is diagnosed when there is a blend of two types of cells, some of the cells containing the usual 46 chromosomes while others containing 47. The cells that contain 47 chromosomes have an extra chromosome 21. Research indicates that people with mosaic Down syndrome show fewer characteristics of Down syndrome than patients with the other two types of Down syndrome.