The term "trisomy" is used to describe the presence of three
chromosomes, rather than the usual matched pair of chromosomes.
For example, if a baby is born with three #21 chromosomes,
rather than the usual pair, then the baby would be said to have
"trisomy 21." Trisomy 21 is also known as Down syndrome. Other
examples of trisomy include syndromes like trisomy 18 and
trisomy 13. Again, trisomy 18 or trisomy 13 simply means that
the child has three copies of the #18 chromosome (or of the #13
chromosome) present in each cell of the body, rather than the
usual pair. Down syndrome is a genetic disorder that involves
birth defects, intellectual disabilities, characteristic facial
features, and often heart defects, visual and hearing
impairments, and other health problems. The severity of all of
these problems varies greatly among affected individuals. Down
syndrome is one of the most common genetic birth defects,
affecting approximately one in about 691 babies. In this
country, more than 400,000 individuals have Do