Trisomy 21 as explained in Trisomic Genesis, is generated by a non-separation of chromosome 21 that remains whole.
This can happen in the female egg cell for 89%, or in the male sperm for 8% and 3% happens after the egg has been fertilized by the sperm.
When is generated the first trisomic cell, this cell will double by separation, and the generated cells will duple further and so on until from the initial cell will be generated billions of cells that will build the human organism.
These billions of cells immediateli after they are generated begin to specialize, forming various organs of a person.
We could think that since the initial cell was trisomic all other cells are trisomic too, since they duplicate the genetic patrimony of the first cell.
Most of the time this is what happens and all cells and all types of cell are trisomic and the person is completely affect by trisomy.


In some cases after conception, trisomy occurs only in cells that were generated directly from the original abnormal cell and the percentage of trisomic cells is very variable from a few cells to nearly all.
Because of this phenomenon, in a single person in the totality of its cells, is possible to find types of cell affected by trisomy and other types of cells completely healthy, this type of partial trisomy is defined as "Mosaic Trisomy 21" or even "Mosaicism".
A study shows that cases of mosaicism affect the 2% of cases and is more frequent in the females than in the males, and it seems that the Mosaicism is not related to the mother's age.

Robertsonian Translocation

It 'also known as Familiar Down Syndrome because comes from one of the parents who have a chromosome 21 fused to another chromosome, often to the chromosome 14.
The parent with this anomaly is normal, but during the reproduction has a very high probability to generate sperm with abnormal chromosome 21 and then generate a person affected by the Down's Syndrome.
The Translocation has no relationship with the mother's age and is found in 4% of cases.