What Is Mosaic Trisomy?

Mosaic trisomy is a type of Down syndrome. Down syndrome, caused by gene abnormalities, is a disorder that is present at birth. This disorder is marked by extra chromosomes and causes mental retardation as well as delays in the affected party’s development. Mosaic trisomy occurs in only a small percentage of Down syndrome cases. In patients with Down syndrome, all cells have an extra chromosome; with mosaic Down syndrome, however, some of the affected person’s cells are affected rather than all of them.

Human beings are usually born with cells that contain 23 chromosome pairs. When a person has Down syndrome, he has abnormalities that affect chromosome 21. In the vast majority of cases, this means a person has three copies of chromosome 21 in contrast to the two pairs he is supposed to have. This extra chromosome is present in all of his cells. When a person has mosaic Down syndrome, however, the extra chromosome is only present in some of his cells.

Mosaic trisomy can occur when an egg is fertilized and starts out with the expected chromosome count, which is 46. Then, as the chromosomes divide early in the pregnancy, forming 23 chromosome pairs, some of the developing patient’s cells have the normal number of chromosome 21 while others receive the extra chromosome 21. The fact that only some of the cells have the abnormality can affect the physical symptoms of the disorder the patient eventually develops. This is because the number of cells with the extra chromosome versus the number of normal cells can affect the symptoms and severity of the disorder.

Doctors usually diagnose mosaic trisomy by testing an infant’s blood cells. In the event that the patient has some cells that have the extra chromosome but not all do, doctors usually diagnose the patient with mosaic trisomy. In some cases, doctors must also test other types of cells to diagnose this disorder. For example, it is possible that blood tests won’t confirm a case of mosaic trisomy but skin cell or bone marrow tests might.

It is difficult to predict the extent of the symptoms a person with mosaic trisomy will experience. This is due to the fact that the affected cells will differ in each patient. It is possible for one patient to be more affected physically while another seems more affected mentally. Sometimes individuals with mosaic trisomy have higher intelligence quotients (IQs) than those with Down syndrome, but that is not always the case.