Central Mississippi Down Syndrome Society

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Central Mississippi Down Syndrome Society

667 US-51 Suite C
Ridgeland, MS 39157
601-385-DOWN (3696)

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Types of Down Syndrome

Are There Different Types of Down Syndrome?

There are three types of Down syndrome: Trisomy 21 (nondisjunction), Mosaicism, and Translocation. Below is a chart that outlines the cell division process of regular cells.

Graphic showing the process of normal cell division.

Trisomy 21 (Nondisjunction)

Down syndrome is usually caused by an error in cell division called “nondisjunction.” Nondisjunction results in an embryo with three copies of chromosome 21 instead of the usual two. Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate.

Graphic showing nondisjunction cell division (trisomy 21).
As the embryo develops, the extra chromosome replicates in every cell of the body. This type of Down syndrome, Trisomy 21, accounts for 95% of cases.

Mosaicism

Mosaicism (or mosaic Down syndrome) exists when there are a mixture of two types of cells, some containing the usual 46 chromosomes and some containing 47. Those cells with 47 chromosomes contain an extra chromosome 21.

Graph showing mosaic cell division.
Mosaicism is the least common form of Down syndrome. It accounts for only about 1% of all cases of Down syndrome. Research indicates that individuals with mosaic Down syndrome may have fewer characteristics of Down syndrome than those with other types of Down syndrome. However, broad generalizations are not possible due to the wide range of abilities people with Down syndrome possess.

Translocation

In translocation, which accounts for about 4% of cases of Down syndrome, the total number of chromosomes in the cells remains 46. However, an additional full or partial copy of chromosome 21 attaches to another chromosome, usually chromosome 14. The presence of the extra full or partial chromosome 21 causes the characteristics of Down syndrome.