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The Medical Maze: What is Mosaic Down Syndrome?
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?
A: 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?
A: 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?
A: 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.
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.