Mosaic Trisomy 21
In approximately 2 percent to 4 percent of cases, Down syndrome is due to mosaic trisomy 21. This situation is similar to simple trisomy 21. The only difference is that, in this instance, the extra chromosome 21 is present in some, but not all, cells of the individual.
For example, the fertilized egg may have the right number of chromosomes, but, due to an error in chromosome division early in embryonic development, some cells acquire an extra chromosome 21. Thus, an individual with Down syndrome due to mosaic trisomy 21 will typically have 46 chromosomes in some cells, but will have 47 chromosomes (including an extra chromosome 21) in others.
In mosaic trisomy 21, the range of the physical problems may vary, depending on the proportion of cells that carry the additional chromosome 21.