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.

Treatment will depend on the specific type of genetic disease. Patients with mosaicism may require less intense treatment than those with the typical form of the disease (with 100% of their cells affected), because they may be more mildly affected (only some of their cells are abnormal).

The outlook depends greatly on the degree of mosaicism. It is difficult to predict the effects of having two different cell lines in one person.

In general, patients with a high percentage of abnormal cells share the same prognosis as people with the typical form of that particular disease (having 100% abnormal cells).

Patients with a low percentage of abnormal cells may be quite mildly affected. In fact, they may only discover that they have mosaicism after giving birth to a child who has the typical (non-mosaic) form of their disease.

Complications, like signs and symptoms, vary with both the type and percentage of cells affected by the genetic change.

The finding of mosaicism can cause confusion and uncertainty. A referral to a genetics professional may help assess the specific circumstances and provide recommendations for further testing.

There is currently no known prevention for mosaicism.