Cause of Down syndrome

Approximately 95% of people with Down syndrome have trisomy 21 (ie an extra copy of chromosome 21) in every cell in their body; the remainder have either mosaicism (1 to 2%) or translocation (3 to 4%).

In people with the mosaic form of Down syndrome, not all cells have an extra copy of chromosome 21. In people with translocation of chromosome 21, genetic material from chromosome 21 is attached to a different chromosome (usually chromosome 14). People with mosaicism may have milder clinical characteristics of Down syndrome. Parental translocation and maternal mosaicism confer a higher likelihood of recurrence of Down syndrome in subsequent pregnancy.