Mosaicism occurs in about 1-2% of all people with Down Syndrome. In this type of Down Syndrome the error in separation of the 21st chromosome occurs in one of the first few cell divisions AFTER fertilization. This causes the fetus to have some cells with 46 chromosomes and some with 47. The cells with 47 chromosomes have one extra 21st chromosome. Because of the fact that not all cells contain the extra chromosome 21 the range of physical problems varies depending on the ratio of cells with 46 chromosomes to those with 47 chromosomes.