What is Mosaicism?
Every cell in the human body is ultimately derived 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, errors 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 error takes place. (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, this 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.