A Chromosomal Disorder
Down syndrome is the most frequent genetic cause of mild to moderate
mental retardation and associated medical problems and occurs in one out
of 691 live births, in all races and economic groups. Named after John
Langdon Down, the first physician to identify the cell abnormality, Down
syndrome is a chromosomal disorder caused by an error in cell division
that results in the presence of a third chromosome 21 or
In every cell in the human body there is a nucleus, where genetic
material is stored in genes. Genes carry the codes responsible for all
of our inherited traits and are grouped along rod-like structures called
chromosomes. Normally, the nucleus of each cell contains 23 pairs of
chromosomes, half of which are inherited from each parent.
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. As the embryo develops, the extra
chromosome is replicated in every cell of the body. This type of Down
syndrome, which accounts for 95% of cases, is called Trisomy 21.
The two other types of Down syndrome are called mosaicism and
translocation. Mosaicism occurs when nondisjunction of chromosome 21
takes place in one-but not all-of the initial cell divisions after
fertilization. When this occurs, there is a mixture of two types of
cells, some containing the usual 46 chromosomes and others containing
47. Those cells with 47 chromosomes contain an extra chromosome 21.
Mosaicism accounts for about 1% of all cases of Down syndrome. Research
has indicated 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 accounts for about 4% of all cases of Down syndrome.
In translocation, part of chromosome 21 breaks off during cell division
and attaches to another chromosome, typically chromosome 14. While the
total number of chromosomes in the cells remain 46, the presence of an
extra part of chromosome 21 causes the characteristics of Down
Regardless of the type of Down syndrome a person may have, all
people with Down syndrome have an extra, critical portion of chromosome
21 present in all or some of their cells. This additional genetic
material alters the course of development and causes the characteristics
associated with Down syndrome.
The cause of nondisjunction is currently unknown, but research has
shown that it increases in frequency as a woman ages. However, due to
higher birth rates in younger women, 80% of children with Down syndrome
are born to women under 35 years of age. There is no definitive
scientific research that indicates that Down syndrome is caused by
environmental factors or the parents' activities before or during
Once a woman has given birth to a baby with Trisomy 21, it is
estimated that her chances of having another baby with Trisomy 21 is 1%
greater than her chances by age alone.
The age of the mother does not seem to be linked to the risk of
translocation. Most cases are sporadic-that is, chance events.
However, in about one third of cases, one parent is a carrier of a
translocated chromosome. The risk of recurrence of translocation is
about 3% if the father is the carrier and 10-15% if the mother is the
carrier. Genetic counseling can determine the origin of translocation.