The term mosaicism is used to describe the presence of more than one type of cell in a person. For example, a person may have some of the cells in his or her body with 46 chromosomes, while other cells in his or her body have 47 chromosomes. An example of mosaicism is mosaic Down syndrome.
About 95 percent of people with Down syndrome have trisomy 21, where there is an extra #21 chromosome in every cell of his or her body. Three to 4 percent of people with Down syndrome have translocation Down syndrome, where all or part of the extra #21 chromosome is attached to another chromosome. The remaining 1 to 2 percent of individuals with Down syndrome are mosaic, where there are at least two types of cells, some with the usual number of chromosomes (46 total), and others with an extra #21 chromosome (47 total). Rarely, a person can have more than two types of cell lines.
Mosaicism is usually described as a percentage. For example, when a baby is born with Down syndrome, the doctor will take a blood sample to perform a chromosome study. Typically, 20 different cells are analyzed. If five of the 20 are normal (46 chromosomes), while the other 15 have an extra #21 chromosome (47 chromosomes), the baby would be said to have mosaic Down syndrome. Since the percentage of cells with an extra chromosome is 15 out of 20, the baby would be said to have a level of mosaicism at 75 percent. The percentages may differ in different parts of the body. The percentage of trisomic cells in the muscle may differ from the percentage in the brain, or the percentage in the blood or skin.
At some point after fertilization, there may be an error in mitosis, where a cell does not split evenly into two cells. The result is that some cells have the normal number or 46 chromosomes, and other cells have an extra #21 or 47 total chromosomes.