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.
Most people who have Down syndrome have an entire extra chromosome (47 instead of 46) in every cell in their body. 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.
A few people with Down syndrome have a type called translocation. This type of Down syndrome may be passed down through families (inherited), but most cases are chance events. 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.
A few people with Down syndrome have a type called mosaicism. This type is not inherited. 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.
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