Classification of Down syndrome

The three basic types of Down syndrome are trisomy 21, translocation, and mosaicism. All types of Down syndrome result from irregular chromosomes in some or all of the baby's cells. These irregularities are caused by abnormal cell division, usually in the egg but sometimes in the sperm, before or after conception. It is not completely understood why this occurs.

Trisomy 21

About 95 out of 100 people with Down syndrome have an entire extra chromosome (47 instead of 46) in every cell in their body.1 This type of Down syndrome is not genetically inherited. Abnormal cell division occurs in the egg (95% of trisomy 21 cases) or sperm (5% of trisomy 21 cases) before or after conception. As the embryo develops, the extra chromosome is copied (replicated) in every cell of the body.

Older women are at higher risk than younger women of having a baby with trisomy 21. As a woman ages, the chromosomes in her eggs are more likely to divide incorrectly.


Translocation affects about 4 of 100 people with Down syndrome.1 Although the total number of chromosomes is normal (46), a part of chromosome 21 breaks off, attaches to another chromosome, and produces the signs and features of Down syndrome. Genetic counseling may help determine the origin of the translocation, but most cases are chance events.


About 3 out of 100 people with Down syndrome have a type called mosaicism.2 Mosaicism results from abnormal cell division in only some cells after fertilization, while others divide normally. The cells that divide abnormally produce 47 chromosomes, with the extra genetic material from chromosome 21. As the embryo grows, both normal and abnormal cells continually replicate. Even though half of the cells may be normal, symptoms usually are nearly the same as those that occur with other types of Down syndrome. But children with more normal than abnormal cells may have fewer features of Down syndrome.


  1. Descartes M, Carroll AJ (2007). Cytogenetics. In RM Kliegman et al., eds., Nelson Textbook of Pediatrics, 18th ed., pp. 502–517. Philadelphia: Saunders Elsevier.
  2. Saitta SC, Zackai EH (2005). Specific chromosome disorders in newborns. In HW Taeusch et al., eds., Avery's Diseases of the Newborn, 8th ed., pp. 204–215. Philadelphia: Elsevier Saunders.

Last Updated: August 4, 2009

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