Mosaic Trisomy 21
Mosaicism is another cause of Down syndrome. This type is most rare and affects only a small fraction of pregnancies. The symptoms of this condition are usually less severe. In mosaic trisomy 21, only a few embryo’s cells carry an extra 21st chromosome. All other cells contain the usual 46 chromosomes. Mosaicism is not an inherited disorder but is believed to be a random genetic error. Mosaicism does not usually run in families, so if one of your children faces mosaic Down syndrome, it may not affect future pregnancies.