Causes of Mosaicism
In almost all cases, an individual develops from a single fertilized egg which contains 46 chromosomes. Occasionally, this process occurs with a fertilized egg with 47 chromosomes. This occurs as a result of trisomy, in which there is an extra copy of a chromosome in a cell.
When trisomy occurs, the result is usually that all cells in the individual have the extra chromosome. The most common type of trisomy is trisomy of chromosome 21, which causes Down syndrome.
In some cases, only some of the cells have the extra chromosome. There are two possible reasons why this might occur. The explanation which is believed to be most likely is that the sperm or egg itself had an extra copy of the chromosome, and the extra copy was not passed on to all cells after fertilization.
The second possible explanation is that trisomy mosaicism occurs as a result of errors during early cell division just after fertilization of an egg. As the fertilized egg develops into a fetus, cells that initially acquired the extra chromosome give rise to new and larger populations of cells with the extra chromosome. All cells produced from the initial abnormal cell have the trisomy, but cells produced from the normal cells have the usual 46 chromosomes.