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Down syndrome
is a chromosomal abnormality. Most of us have 46 chromosomes
arranged in 23 pairs. A person with Down syndrome has
47 with the extra chromosome being a #21. It is the
most common chromosomal abnormality in the human race
and about 5000 babies are born in the United States
each year with Down syndrome. There are 3 kinds of
chromosomal abnormalities that result in Down syndrome.
Trisomy
21 - This is the most common form
and occurs in approximately 95% of all cases. The
parents may have normal chromosomes, but the child
has three rather than the usual two in pair 21.
This usually results from something happening during
cell division in either the sperm or the egg and
is an accidental occurrence.
Translocation -
this occurs in 3-4% of cases of Down syndrome. Children
with translocation have an extra #21 chromosome that
is attached to another chromosome, usually a #14 or
another 21. About 2/3 of translocations occur spontaneously
during fertilization. In the remaining 1/3, either
parent can carry a balanced translocation which means
they still have 46 chromosomes but two of them are
attached (21/21 or 21/14). When a child with Down syndrome
has translocation it is important the parent’s chromosomes
be studied to determine if one of them is a “carrier” of
the translocation. This can have important implications
for genetic counseling.
Mosaicism -
this happens when a person has some cells with 46 chromosomes
and some with 47. This type occurs in the early cell
division of the fertilized egg.
Down syndrome is confirmed
through a blood test to count the number of chromosomes
(karyotyping) and to determine if the child has trisomy
21, translocation, or mosaicism. However, most doctors
either know, or strongly suspect, that a baby has Down
syndrome shortly after birth. There are about 50 different
characteristics described in persons with Down syndrome,
but none are specific for Down syndrome alone. These
include eyes that have an upward slant, small ears,
short fingers, and flat nasal bridge and reduced muscle
tone. While all signs are not seen in one individual,
there are usually enough that the doctor can make a
diagnosis of Down syndrome from the baby’s appearance
alone. It is important to remember that your baby will
still have family traits and features and will look
more like you and your other children than different.
While most newborns with Down
syndrome are healthy, they do have an increased risk
of heart defects (40%). All babies with Down syndrome
need to be seen by a pediatric cardiologist and have
a special ultrasound test (echocardiogram) of the heart
in the first week of life. Some babies also experience
blockage of the intestinal tract and require surgical
correction in the first few days of life. Additional
information about medical problems that are more common
in children with Down syndrome is available form the
Jane and Richard Thomas Center for Down Syndrome at
Cincinnati Children’s Hospital (513-636-0520).
Research into the cause of
the chromosomal abnormality that results in Down syndrome
is ongoing. A variety of theories have been and are
being investigated as to the “why” of this condition.
To date, some have been ruled out as the cause and
others remain theories. The extra chromosome has been
found in both male sperm cells and maternal egg cells.
One known risk factor is increased age of the mother,
but the reason for this association is not clearly
understood. In absolute numbers, more babies with Down
syndrome are born to young women than to those over
the age of 35 since more young women bear children.
Although it is possible to
make some general statements about the characteristics
of Down syndrome and the effect of the syndrome, each
person with Down syndrome is an individual, so no blanket
predictions can be made. The only two observations
that hold up under scrutiny are that the person with
Down syndrome will experience some degree of cognitive
deficit within the range of low normal (borderline)
intelligence to severe retardation. While most people
with Down syndrome function in what is called the mild
range of deficit/retardation, there is no way to predict
in a young infant what their particular pattern of
development will be. We encourage you to view your
child as an individual and put no limits on the possibilities
for his/her future.
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