Few percent. Most DS result when 3 of the 21 chromosome end up in all the cells of a kid. Data shows us that all it takes us only having a extra piece of the 21 added to the normal 46 chromosomes to trigger the physical changes of DS. This happens when mom or dad carries some miss-arranged chromosomes in a balanced (for them) way, but the kid gets extra during conception. It is found on chromosome study.
Extra chromosome. Most Down syndrome is because an individual has a whole extra chromosome 21 in every one of their cells. A few individuals have part of an extra chromosome 21 instead, usually as a result of an "unbalanced translocation", which means that part of one chromosome has broken off and traded places with part of another chromosome in one of the parents. A chromosome analysis can differentiate these.