Final answer:
Chromosomal nondisjunction during meiosis causes chromosome 21 to have three copies in Down Syndrome. This event is spontaneous and not inheritable.
Step-by-step explanation:
Down syndrome is caused by an event called chromosomal nondisjunction, which occurs during meiosis. Nondisjunction is the failure of chromosomes to separate properly.
In the case of Down syndrome, there is an extra copy of chromosome 21, resulting in a total of three copies instead of the usual two.
This event is spontaneous and not inherited from either parent. The frequency of nondisjunction increases with maternal age, so the risk of having a child with Down syndrome increases for older women.
The age of the father has less of an impact because nondisjunction is less likely to occur in sperm than in eggs.