What is the diffrence between mosaic down syndrome and down syndrome?

Question

What is the diffrence between mosaic down syndrome and down syndrome?

Answer 1

Marilyn's answer about how mosaic trisomy 21 occurs is correct, but I have to disagree on a couple of fine points.  My son is mosaic and we consulted with Dr. Charles Epstein at UCSF. His name may be familiar as he was one of the victims of the Unibomber.  What I understood from Dr. Epstein  is that Mosaicism is not really Downs.  In DS, all cell lines are affected, in Mosaicism they are not.  That's why it didn't show up in my amnio. There are certain physical attributes, medical and cognitive issues associated with Downs that may or may not be present with mosaicism.

Answer 2

Mosaic Down Syndrome is a form of Down Syndrome:

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. Normally, there are 23 pairs or 46 chromosomes.

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.

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.