The most common form of Down syndrome is known as Trisomy 21, a condition where individuals have 47 chromosomes in each cell instead of 46. This is caused by an error in cell division called nondisjunction, which leaves a sperm or egg cell with an extra copy of chromosome 21 before or at conception. Trisomy 21 accounts for 95% of Down syndrome cases, with 88% originating from nondisjunction of the mother's egg cell.