Trisomy 21 (Nondisjunction)
Most cases of Down syndrome are often referred to by the more scientific name trisomy 21. About 95 – 97% of Down syndrome cases are the result of trisomy 21, or the presence of an extra chromosome 21. As explained in the tab labeled "Cause, " the extra chromosome 21 comes from a gamete (either the egg or sperm cell) during the formation of the zygote. Because each cell produced by the mutated cell is identical to it's parent, each one will also have the mutation, resulting in all cells containing 47 chromosomes, while a karyotype of a healthy person would show only 46. |
Mosaicism
About 1 – 2% of people with Down syndrome have a version of the disorder called mosaicism (mosaic Down syndrome). In this case the original reproductive cells have the correct number of chromosomes, but during the time in which cells are dividing very rapidly, one or more cells can divide incorrectly causing nondisjunction. This results in a whole line of daughter cells having an extra chromosome 21, a condition called “cell line mosaicism.” People with mosaic Down syndrome have some cells with 46 chromosomes (normal) as well as some cells with 47 chromosomes (causing Down syndrome symptoms). Often their symptoms are much less severe than people with pure trisomy 21 or translocation. |
Translocation
The remaining 3 – 4% of those with Down syndrome have a different defect in the creation and replication of chromosomes called translocation. “This is an event, that unlike the numerical abnormality causing trisomy 21, there is structural abnormality” (Lerner). In this case the cell has 46 chromosomes, but still has 3 chromosome 21s. This is because the extra chromosome 21 is not “free” but rather attached to another chromosome – usually 14, 21, or 22. Since an extra chromosome 21 is still present, the genes required to produce Down syndrome are present as well, again resulting in the disorder. |