Mosaicism refers to a condition where an individual has two or more cell populations that differ in genetic makeup. This situation can affect any type of cell, including blood cells, gametes (egg and sperm cells), and skin.

Mosaicism may be detected through chromosome evaluation. It is usually described as a percentage of the cells examined. The normal chromosome finding in males is 46 XY and normal in females is 46 XX.

Examples of chromosomal mosaicism include:

• Turner Syndrome Mosaic - a female with a certain percentage of the normal cell line (46, XX), plus another percentage of an abnormal cell line associated with Turner Syndrome (45, X).
• Klinefelter Syndrome Mosaic - a male with a percentage of the normal cell line (46, XY), plus another percentage of an abnormal cell line associated with Klineflter Syndrome (47, XXY).
• Down Syndrome Mosaic - a male with the normal cell line (46, XY), plus an abnormal cell line (47, XY, +21) or a female with the normal cell line (46, XX), plus an abnormal cell line (47, XX, +21). That is, in Down syndrome, the individual has an extra chromosome no. 21.

Mosaicism occurs as a result of an error in cell division very early in fetal development.

Symptoms vary from person to person and are very difficult to predict. When there are normal cells as well as abnormal cells, associated abnormalities may not be as severe.

Tests will likely need to be repeated for confirmation and to help further define the type and severity of the genetic mosaicism.