Understanding Mosaic Down Syndrome

A less common and potentially milder form of the disorder

Child, Down's Syndrome
Child, Down's Syndrome : News Photo CompAdd to Board Child, Down's Syndrome Credit: BSIP / Contributor / Getty Images

Down syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of an extra copy of a chromosome known as chromosome 21. This is often referred to as full trisomy 21.

What most people don't realize is that there is another, less common, form of the disorder called mosaic Down syndrome seen in between 2 percent to 4 percent of all cases.

Not only is the cause different from that of full trisomy 21, it can result in variations in the typical features of Down syndrome.

How Mosaic Down Syndrome Differs

While our bodies contain a multitude of different cell types, each and every one of these cells originated from the same, single fertilized egg, called a zygote. The zygote starts as one cell which, upon fertilization, duplicates itself again and again to create the various cells and tissues of our body.

With full trisomy 21, an error occurs in the chromosome of either the egg or the sperm cell as they join 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 when cell division has already begun. As a result, people with mosaic Down syndrome have two distinct cell lines: one with the normal number of chromosomes and another with an extra chromosome 21.


Mosaic Down syndrome is usually detected either with a blood test at the time of birth or through an amniocentesis or chorionic villus sampling (CVS) conducted sometime during the pregnancy.

In the course of these procedures, an analysis is performed to count the number of chromosomes in roughly 20 different cell types.

This analysis can both identify Down syndrome and diagnose which type of Down syndrome a baby has.

The types are differentiated as follows:

  • In full trisomy 21, all of the cell types will have an extra chromosome 21. So instead of finding the usual 46 different chromosomes in a single cell, we would find 47.
  • By contrast, if two or more of the cell types are normal and the others have an extra chromosome 21, we can confirm that the baby has mosaic Down syndrome.

In babies with mosaic Down syndrome, the level of chromosomal variation can differ. Chromosomal specialists, called cytogeneticists, will usually describe these variations in terms of a percentage value.

For example, if the cytogeneticist counts 50 different cells, 10 of which have a normal cell line and 40 of which have an extra chromosome 21, he or she would report that the level of "mosaicism" is 80 percent.

To complicate matters even further, the percentage of mosaicism can differ from one tissue type to the next, with some having higher levels of mosaicism and others having less.

Variations in the Features 

Mosaicism doesn't necessarily translate to differences in the features of the disorder. Because the levels of mosaicism can vary between individuals—and even the cells within those individuals—the expression of the mosaicism can also vary.

As such, individuals with mosaic Down syndrome can have all of the features of full trisomy 21, none of the features, or fall somewhere in between. In fact, around 15 percent of people diagnosed with full trisomy 21 Down syndrome have been misdiagnosed and really have mosaic Down syndrome.

As it is still not possible to predict the features of mosaicism, it's important that your child receives the same medical care as children with full trisomy 21. Ongoing care can help identify the development of a disability and allow you to make informed choices and get services to best support your child as he or she grows.

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