What Is Mosaic Down Syndrome?

This type of Down syndrome may present with milder symptoms

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When we think of Down syndrome, we usually think of full trisomy 21, that is three copies of chromosome 21 in every cell. Less common is mosaic Down syndrome, which accounts for about 2 to 4 percent of all cases of Down syndrome

How Trisomies Occur

Our bodies are made up of cells. While our bodies contain a multitude of different cell types, these cells all originated from the same single fertilized egg (zygote).

The zygote starts as one cell, duplicates itself and then divides into two cells. Those two cells each duplicate themselves, divide, and create four cells. Those four cells duplicate and divide to create eight cells, and so on. This process of cell duplication and division occurs throughout the life of an individual.

With full trisomy 21, the presence of an extra chromosome 21 results from an error in the chromosomes of the egg or sperm cell that join together to become a zygote. Because this error takes place at the start of development, every cell that comes from this zygote will have that extra chromosome 21.

With mosaic trisomy 21, the error occurs after fertilization at some point during early cell division. This means people with mosaic Down syndrome have two cell lines –– one with the normal number of chromosomes, and one with an extra number 21.

How Is Mosaic Down Syndrome Diagnosed?

Mosaic Down syndrome is usually detected by a blood test at birth or through an amniocentesis or chorionic villus sampling (CVS) conducted during pregnancy.

A karyotype analysis is done, counting the chromosomes in around 20 different cells. If two or more of the 20 are normal (have 46 chromosomes) and the others have an extra number 21 chromosome (for a total of 47 chromosomes), the baby has mosaic Down syndrome.

If mosaicism is suspected, the cytogeneticist (chromosome specialist) will usually count extra cells to confirm the diagnosis.

Mosaicism is usually described as a percentage. For example, if the cytogeneticist counts 50 cells, and 10 have a normal cell line and 40 have an extra number 21 chromosome, then the level of mosaicism is said to be 80 percent.

While the percentage of mosaicism is usually determined from blood cells, the percentage may actually be different in other tissues or parts of the body. For example, a person could have 80 percent mosaicism in their blood cells, but have full trisomy in their skin or brain cells. The percentage of mosaicism can differ from tissue to tissue. However, biopsies on different areas are usually skipped because they are invasive and usually don’t give any information about what symptoms a person may develop.

Does the Level of Mosaicism Affect Features of Down Syndrome?

Those with mosaic Down syndrome may or may not have milder disabilities and less obvious features of Down syndrome. Because the levels of mosaicism vary between individuals and within the cells of those individuals, the effects of mosaic Down syndrome can have significant variations. 

Individuals with mosaic Down syndrome can have all of the problems associated with full trisomy 21, none of the problems, or fall somewhere in between.

Because it is impossible to predict the features of Down syndrome a child might have, it is important that a child with mosaic Down syndrome receive the same medical care and therapies as a child with full trisomy 21. Only time will tell if the disabilities of a child with mosaic Down syndrome are milder than those with full trisomy 21. 


Cassidy, Suzanne B. , Allanson, Judith E., (2001) Management of Genetic Syndromes. 1st ed. New York, NY; 2001.

Fishler K, and Koch R. Mental Development in Down syndrome mosaicism. American Journal of Mental Retardation 1991; 96 (3): 345-351.

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