Types of Down Syndrome
Although all forms of Down syndrome are linked with an extra partial or full 21st chromosome, some conditions may vary. Those types are:
Trisomy 21 Down syndrome
It is the most common type of Down syndrome — accounting for nearly 95 percent of all cases. This condition occurs due to an error in cell division known as “nondisjunction.” At some point at conception or leading up to, in either the egg or the sperm, one of the pairs of parents’ 21st chromosomes failed to get separated. So in spite of getting one chromosome each from the father and mother, the fetal’s DNA ends up with an extra copy of the chromosome in the 21st pair. As the fetus matures and its cells replicate, that extra copy of the 21st chromosome gets replicated over and over in each cell.