The type in which the extra chromosome 21 is replicated in every cell of the body. This is a result of either the sperm or egg failing to separate the pair of chromosome 21 before or during conception. 
It is the most common type of Down syndrome and accounts for 95% of known cases.


University of Virginia, Charlottesville

There are three types of Down syndrome, distinguished by the form in which chromosome 21 is copied. The three types are: 
- Trisomy 21 (also known as nondisjunction):
- Translocation
- Mosaicism 


Types of Down syndrome

“About Down Syndrome.” National Down Syndrome Society (NDSS), https://www.ndss.org/about. 

National Down Syndrome Society (NDSS): About Down Syndrome

Translocation is the type of Down syndrome that occurs when the copy of chromosome 21 attaches to and inhibits one of the pairs of another chromosome, usually chromosome 14. The individual retains 46 chromosomes, but the presence of a third copy of chromosome 21 causes characteristics of Down syndrome. This accounts for 3-4% of known cases.

Translocation

Mosaicism in the type of Down syndrome in which nondisjunction occurs in only one initial cell division instead of all. This results in a mix of cells: some containing 46 chromosomes and no extra copies of chromosome 21, and some containing 47 chromosomes with an extra copy of 21. This mixture of cell compositions causes individuals with mosaic Down syndrome to present fewer characteristics of Down syndrome than the other types. Mosaicism accounts for only 1% of known cases.

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