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The Medical Maze:

What is Mosaic Down Syndrome?

by Carol Strom, M.S.

Q: My baby has mosaic Down syndrome. What does that mean?
A: Children with mosaic Down syndrome have two distinct cell groupings. In some cells there is a total of 46 chromosomes, which is the “typical” group. In other cells there is an extra copy of the chromosome #21, making 47 cells total for this group.

Q: How does mosaic Down syndrome happen?
Ordinarily, when an egg and a sperm are joined at conception, a single cell is created with a total of 46 chromosomes. These chromosomes are copied, the copies are separated, and the cell then divides to create two identical “daughter” cells. The chromosomes in these two cells are copied, the copies divide and four cells are created. These four cells become eight cells. Eight cells become 16 cells, and so on.

If nothing disrupts the chromosome replication and separation process, each cell in the body should have the same number of chromosomes that were present in the fertilized egg. However, errors can occur in this replication and separation process.

Two mechanisms have been proposed to explain why a child may be born with mosaic Down syndrome. The most likely explanation is that an extra copy of chromosome #21 was present in the egg or the sperm at the time of conception. However, shortly after conception, an error occurred in the chromosome replication and separation process, and the extra copy of the chromosome #21 was not passed on to both cells. In this way, a second cell grouping was created with only 46 chromosomes.

If this error in the chromosome replication and separation process occurred at the 4 cell stage, 1/4 of the cells would have 46 chromosomes and 3/4 would have 47 chromosomes. If the error occurred at the 8 cell stage, 1/8 of the resulting cells would have 46 chromosomes and 7/8 would have 47.

It is also possible that a child with mosaic Down syndrome inherited a total of 46 chromosomes at the time of conception. If this were the case, then the error in chromosomal separation, which resulted in the formation of a second cell grouping with an extra chromosome #21, occurred early in the baby’s development.

Q: How does it effect her development?
Every child is born with a unique set of talents and abilities. However, it is impossible to predict what these talents and abilities will be. This is true for a child who inherits 46 chromosomes. This is true for a child with trisomy 21 (Down syndrome), and this is true for a child with mosaic Down syndrome.

While it is impossible to tell you what the future holds for your daughter, I can share with you information about other children with mosaic Down syndrome. Studies have shown that individuals with mosaic Down syndrome, on average have IQ scores that are 10-30 points higher than individuals with trisomy 21. Their scores are thought to be higher because the presence of the cells with 46 chromosomes modify the effects of the cells with 47 chromosomes.

Q: Is it possible to predict a child’s IQ based on the number of normal cells present in the study?
In most cases chromosome studies are done using white blood cells. The number of blood cells containing 46 chromosomes may not accurately reflect the number of skin cells, heart cells, or brain cells with 46 chromosomes. Therefore, it is impossible to predict what affect, if any, the presence of these chromosomally normal cells will have on a child’s development.

The parents of children with mosaic Down syndrome are like the parents of any other child. Your child’s strengths and weaknesses will only be revealed over time.

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

Carol Strom, M. S. is a genetic counselor and advocate for families and individuals with disabilities at the University of South Dakota School of Medicine in Rapid City. She welcomes any questions you may have regarding genetic issues. You may send your questions for her to Disability Solutions.


More FAQ's on Mosaic Down Syndrome
by Dr. Len Leshin, MD, FAAP

Copyright 1999, All rights reserved


Before reading this essay on the mosaic form of Down syndrome, you should be familiar with how typical Down syndrome occurs. If you aren't, take the time to read through my essay on the origin of Down syndrome.


What is Mosaicism?

Every cell in the human body comes from one initial cell: the fertilized egg, which is also called the zygote. After fertilization, the zygote then proceeds to divide. As new cells form, the chromosomes duplicate themselves so that the resulting cells have the same number of chromosomes as the original cell. However, mistakes sometimes happen and one cell ends up with a different number of chromosomes. From then on, all cells originating from that cell will have the different chromosomal number, unless another mistake happens. (All like cells originating from a single type of cell is called a cell line; for example, the skin cell line, the blood cell line, the brain cell line, etc.)

When a person has more than one type of chromosomal makeup, that is called mosaicism, like the mosaic style of art in which a picture is made up of different colors of tiles. In Down syndrome, mosaicism means that some cells of the body have trisomy 21, and some have the typical number of chromosomes.

How is Mosaicism Diagnosed?

The usual way in which mosaic Down syndrome is discovered is through genetic testing of the baby's blood. Typically, 20 to 25 cells are examined. If some of the cells have trisomy 21 and some don't, then the diagnosis of mosaicism is made. However, this blood test can only determine the level of mosaicism in the blood cell line.

While mosaicism can occur in just one cell line (some blood cells have trisomy 21 and the rest don't), it can also occur across cell lines (skin cells may have trisomy 21 while other cell lines don't). In the latter case, it may be more difficult to diagnose mosaicism. When mosaicism is suspected but not confirmed through the blood test, other cell types may be tested: skin and bone marrow are most commonly the next cells checked. Because skin cells and brain cells arise from the same type of cell at the beginning of fetal development (ectoderm), many doctors believe that skin cell tests reflect the chromosomal makeup of the brain cells as well.

How Does Mosaicism Happen in Down Syndrome?

There are two different ways mosaicism can occur:

It's possible to determine the origin of mosaicism in individual cases using special DNA markers, but this isn't done on a regular basis.

What Does Mosaicism Mean for my Child?

At the present time, there is not much research on the similarities and differences between simple trisomy 21 and mosaic trisomy 21. One report published in 1991 on mental development in Down syndrome mosaicism compared 30 children with mosaic Down syndrome with 30 children with typical Down syndrome. IQ testing showed that the mean IQ of the mosaic group was 12 points higher than the mean of the non-mosaic group. However, some children with typical Down syndrome did score higher on the IQ tests than some of the children with mosaic Down syndrome.

The Department of Human Genetics at the Medical College of Virginia has had an ongoing study project of children with mosaic DS. In a survey of 45 children with mosaicism, they found that these children did show delayed development compared to their siblings. When 28 of these children with mosaicism were matched up with 28 children with typical Down syndrome for age and gender, the children with mosaicism reached certain motor milestones earlier than children with typical DS, such as crawling and walking alone. However, the speech development was equally delayed in both groups.


The Department of Human Genetics at the Medical College of Virginia/Virginia Commonwealth University has a very nice booklet on this topic that is available free of charge. Contact Dr. Colleen Jackson-Cook or Lauren Vanner at:

Dept of Human Genetics
Virginia Commonwealth University
P.O.Box 980033
Richmond, VA 23298-0033


1. Understanding the mechanism(s) of mosaic trisomy 21, by using DNA polymorphism analysis. Pangalos C et al. Am. J. Hum. Genet. 54:473-481, 1994.

2. Mental Development in Down Syndrome Mosaicism. Fishler K and Koch R. Am. J. Mental Retardation 96(3):345-351, 1991.

3. Medical Care in Down Syndrome, Rogers PT and Coleman M, Marcel Dekker, NY, 1992; p14-16.


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