What is mosaic Down syndrome?
Mosaicism, used to describe the presence of more than one type of cell in a person, is usually described as a percentage. For example, when a baby is born with Down syndrome, the physician 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. Clinically, babies born with mosaic Down syndrome can have the same features and health problems seen in babies born with trisomy 21 or translocation Down syndrome. However, the presence of cells with the normal num
