Named after John Langdon Down, The first physician to identify the syndrome. Down syndrome occurs in one of 800 live births, and in all races and economic groups.
Down Syndrome is a chromosomal disorder caused by an error in cell division that results in the presence of an additional third chromosome 21 or trisomy 21.
During cell division to create a germ cell a cell containing 46 chromosome's divides into two germ cells each containing 46 chromosomes. Sometimes this division does not happen properly and one cell may contain 45 chromosome and the other 47. Because a cell with 45 chromosomes can't survive it dies, Leaving the cell w/ 47 chromosomes to go on and this is where the extra 21 chromosome comes from giving you Down Syndrome.
Approximately 92% of the time Down Syndrome is caused by the presence of an extra chromosome 21 in all cells. When every cell has 3 copies of this chromosone you get Trisomy 21
In approximately 2-4% of cases, Down Syndrome is due to Mosaic Trisomy 21. This is close to regular Trisomy 21 but not every cell has the extra chromosome. It is believed that some where in later development a cell messes up and splits wrong causing the extra chromosome to appear.