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.
How this information was developed to help you make better health decisions.