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What Is Mosaic Down Syndrome?

What a Mosaic Down Syndrome Diagnosis Means for Your Child


Updated May 15, 2014

Mosaic Down syndrome accounts for about 2 to 4% of all cases of Down syndrome. To truly understand what mosaicism means, it is important to take a step back and understand the concept of cells.

Mosaic Down Syndrome and Cells

Our entire bodies are made up of cells. There are kidney cells, skin cells, muscle cells and more. While every body contains a multitude of different cell types, all of these cells 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 number 21 chromosome, the error or misdivision of chromosomes occurs in the egg or sperm cell that becomes the zygote. Because this error was at the very beginning of development, every cell that comes from this zygote will have an extra number 21 chromosome.

With mosaic trisomy 21, the error or misdivision occurs after fertilization at some point during early cell division. Because of this, 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 during an amniocentesis or CVS during pregnancy. A karyotype analysis is done, and typically the chromosomes in 20 different cells are counted. 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 is said to have 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%.

Percentage Mosaicism

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% mosaicism in their blood cells, but have full trisomy in their skin or brain cells. While it is known that the percentage of mosaicism can be different in different tissues, biopsies are not usually done because they are invasive and usually don’t give any information about how a person with mosaic Down syndrome will ultimately function.


Level of Mosaicism and Features of Down Syndrome

Individuals with mosaic Down syndrome may or may not have milder disabilities and less obvious features of Down syndrome. Because levels of mosaicism vary between individuals and within the cells of the individuals themselves, the effect of mosaicism is wide and variable.

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

Should I Treat My Baby with Mosaic Down Syndrome Different Than a Child with Trisomy 21?

No, every baby with mosaic or full trisomy 21 should be treated the same. Because it is impossible to predict function or features from the level of mosaicism, 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 a child with trisomy 21.

In the meantime, it is important to make sure your child receives all the support she needs to live a full life and achieve her goals.


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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