How Does Mosaicism Happen in Down Syndrome?

How Does Mosaicism Happen in Down Syndrome?

There are two different ways mosaicism can occur:

  1. The initial zygote had three 21st chromosomes, which normally would result in simple trisomy 21, but during the course of cell division one or more cell lines lost one of the 21st chromosomes.
  2. The initial zygote had two 21st chromosomes, but during the course of cell division one of the 21st chromosomes were duplicated.

It’s possible to determine the origin of mosaicism in individual cases using special DNA markers, but this isn’t done on a regular basis.