Human cells normally contain 23 pairs of chromosomes. One chromosome in each pair comes from your father, the other from your mother. Any one of three genetic variations can cause Down syndrome:
- Trisomy 21. About 95 percent of the time, Down syndrome is caused by trisomy 21 — the person has three copies of chromosome 21, instead of the usual two copies, in all cells.
- Mosaic Down syndrome. In this rare form of Down syndrome, a person has only some cells with an extra copy of chromosome 21.
- Translocation Down syndrome. These children have the usual two copies of chromosome 21, but they also have additional genetic material from chromosome 21 attached to another chromosome.
For more info you can go to a specialist.
Keyword: down syndrome chromosome; down syndrome chromosomes; down syndrome karyotype