How does a person get down syndrome?

Chromosomal abnormal. Down syndrome is caused be a chromosomal abnormality tha occurs at the time of fertilization. It is usually associated with the fertilization of an egg carrying2 number 21 chromosomes.
Dividing Chromasomes. We all have 22 pairs of regular chromosomes (chms) plus an xx or xy of the sex chms. The pairs split during egg or sperm formation so each has 22 chms + an x or y. If the 21 chromosome doesn't split right & brings a pair to the egg(or sperm) the fertilized egg will have an extra 21 so the infant will have 47 total. All subsequent cells have 47, giving rise to a ds kid.