How does Down syndrome occur with no history of Down syndrome in the family history?

It's either a mutation with the gene, or either the mother or father donates an extra chromosome.

2nd Answer:

True, and Down Syndrome is not a genetic disease and has nothing to do with family history.

By the way, the kind of chromosomal mistake that causes Down Syndrome occurs in many people, but on different chromosomes. Mostly, your body is so tolerant of this kind of mistake that you never see any results.