Causes
As noted above, Down syndrome patients are born with an extra copy of chromosome 21. Instead of having two chromosomes, they have 3 full copies or at least a big chromosomal translocation in pair 21, resulting in three copies. Almost 95% of patients have a complete copy. Another possibility is mosaicism (in around 3% of patients).
How are they born with an extra copy of chromosome 21? Because there’s a failure in the disjunction of the chromosome during meiosis of one of the gametes. In other words, either the mom’s egg or dad’s sperm has two copies of chromosome 21 instead of one. This is because the extra copy couldn’t separate properly during cell division. In most cases (95% of the time), the maternal egg is the one with the extra copy. This is more common in aging women who conceive, which is why late pregnancy has a very high risk of Down syndrome.