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 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.
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 number of chromosomes (46) may result in a less severe presentation or fewer characteristics of Down syndrome.
There are ongoing studies to determine if there are any differences in the health of people with mosaic Down syndrome, depending on how the mosaicism occurred. In addition, there have been studies looking at whether the percentage of mosaicism can predict the clinical outcome, such as the IQ of the individual or whether there will be a heart defect. These studies show that the percentage of mosaicism is not an accurate predictor of outcome. There is great variability in mosaic Down syndrome cases, from very mild features, to having the majority of features of Down syndrome, just as a person with all trisomy 21 cells could experience.
The risk for chromosome abnormalities to occur in a pregnancy is based on the mother's age at the time of delivery and increases yearly with increasing age.