What Is Down Syndrome?
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.
Typically, the nucleus of each cell contains 23 pairs of
chromosomes, half of which are inherited from each parent. Down syndrome occurs
when an individual has a full or partial extra copy of chromosome 21.
This additional genetic material alters the course of
development and causes the characteristics associated with Down syndrome. A few
of the common physical traits of Down syndrome are low muscle tone, small
stature, an upward slant to the eyes, and a single deep crease across the center
of the palm although each person with Down syndrome is a unique individual and
may possess these characteristics to different degrees, or not at all.
How Common is Down Syndrome?
One in every 691 babies in the United States is born with Down
syndrome, making Down syndrome the most common genetic condition. Approximately
400,000 Americans have Down syndrome and about 6,000 babies with Down syndrome
are born in the United States each year.
When Was Down Syndrome Discovered?
For centuries, people with Down syndrome have been alluded to in
art, literature and science. It wasn’t until the late nineteenth century,
however, that John Langdon Down, an English physician, published an accurate
description of a person with Down syndrome. It was this scholarly work,
published in 1866, that earned Down the recognition as the “father” of the
syndrome.
Although other people had previously recognized the
characteristics of the syndrome, it was Down who described the condition as a
distinct and separate entity.
In recent history, advances in medicine and science have enabled
researchers to investigate the characteristics of people with Down syndrome. In
1959, the French physician Jérôme Lejeune identified Down syndrome as a
chromosomal condition. Instead of the usual 46 chromosomes present in each
cell, Lejeune observed 47 in the cells of individuals with Down syndrome. It
was later determined that an extra partial or whole copy of chromosome 21
results in the characteristics associated with Down syndrome. In the year 2000,
an international team of scientists successfully identified and catalogued each
of the approximately 329 genes on chromosome 21. This accomplishment opened the
door to great advances in Down syndrome research.
Are There Different Types of Down Syndrome?
There are three types of Down syndrome: trisomy 21
(nondisjunction), translocation and mosaicism.
TRISOMY 21 (NONDISJUNCTION)
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.
MOSAICISM
Mosaicism (or mosaic Down syndrome) is diagnosed when there is a
mixture of two types of cells, some containing the usual 46 chromosomes and
some containing 47. Those cells with 47 chromosomes contain an extra chromosome
21.
Mosaicism is the least common form of Down syndrome and accounts
for only 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
In translocation, which accounts for about 4% of cases of Down
syndrome, the total number of chromosomes in the cells remains 46; however, an additional
full or partial copy of chromosome 21 attaches to another chromosome, usually chromosome
14. The presence of the extra full or partial chromosome 21 causes the
characteristics of Down syndrome.
What Causes Down Syndrome?
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 the extra full or partial chromosome is still
unknown. Maternal age is the only factor that has been linked to an increased
chance of having a baby with Down syndrome resulting from nondisjunction or mosaicism.
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 pregnancy.
The additional partial or full copy of the 21st chromosome which
causes Down syndrome can originate from either the father or the mother. Approximately
5% of the cases have been traced to the father.
What is the Likelihood of Having a Child with
Down Syndrome?
Down syndrome occurs in people of all races and economic levels,
though older women have an increased chance of having a child with Down
syndrome. A 35 year old woman has about a one in 350 chance of conceiving a
child with Down syndrome, and this chance increases gradually to 1 in 100 by
age 40. At age 45 the incidence becomes approximately 1 in 30. The age of the mother
does not seem to be linked to the risk of translocation.
Since many couples are postponing parenting until later in life,
the incidence of Down syndrome conceptions is expected to increase. Therefore,
genetic counseling for parents is becoming increasingly important. Still, many
physicians are not fully informed about advising their patients about the
incidences of Down syndrome, advancements in diagnosis, and the protocols for
care and treatment of babies born with Down syndrome.
Does Down Syndrome Run in Families?
All 3 types of Down syndrome are genetic conditions (relating to
the genes), but only 1% of all cases of Down syndrome have a hereditary
component (passed from parent to child through the genes). Heredity is not a
factor in trisomy 21 (nondisjunction) and mosaicism. However, in one third of
cases of Down syndrome resulting from translocation there is a hereditary component
accounting for about 1% of all cases of Down syndrome.
The age of the mother does not seem to be linked to the risk of translocation.
Most cases are sporadic chance events. However, in about one third of cases,
one parent is a carrier of a translocated chromosome.
What Is the Likelihood of Having a Second Child
with Down Syndrome?
Once a woman has given birth to a baby with trisomy 21
(nondisjunction) or translocation, it is estimated that her chances of having
another baby with trisomy 21 is 1 in 100 up until age 40.
The risk of recurrence of translocation is about 3% if the father
is the carrier and 1015% if the mother is the carrier. Genetic counseling can determine the origin of
translocation.
How Is Down Syndrome Diagnosed?
PRENATALLY
There are two categories of tests for Down syndrome that can be
performed before a baby is born: screening tests and diagnostic tests. Prenatal
screens estimate the chance of the fetus having Down syndrome. These tests do
not tell you for sure whether your fetus
has Down syndrome; they only provide a probability. Diagnostic tests, on the
other hand, can provide a definitive
diagnosis with almost 100% accuracy.
There is an extensive menu of prenatal screening tests now
available for pregnant women. Most screening tests involve a blood test and an ultrasound (sonogram). The blood
tests (or serum screening tests) measure quantities of various substances in
the blood of the mother. Together with a woman's age, these are used to
estimate her chance of having a child with Down syndrome. These blood tests are
often performed in conjunction with a detailed sonogram to check for "markers"
(characteristics that some researchers feel may have a significant association
with Down syndrome). New advanced prenatal screens are now able to detect
chromosomal material from the fetus that is circulating in the maternal blood.
These tests are not invasive (like the diagnostic tests below), but they
provide a high accuracy rate. Still, all of these screens will not definitively
diagnose Down syndrome. Prenatal screening and diagnostic tests are now
routinely offered to women of all ages.
The diagnostic procedures available for prenatal diagnosis of
Down syndrome are chorionic villus sampling (CVS) and amniocentesis. These
procedures, which carry up to a 1% risk of causing a spontaneous termination
(miscarriage), are nearly 100% accurate in diagnosing Down syndrome.
Amniocentesis is usually performed in the second trimester between 15 and 20 weeks
of gestation, CVS in the first trimester between 9 and 14 weeks.
AT BIRTH
Down syndrome is usually identified at birth by the presence of
certain physical traits: low muscle tone, a single deep crease across the palm
of the hand, a slightly flattened facial profile and an upward slant to the
eyes.
Because these features may be present in babies without Down
syndrome, a chromosomal analysis called a karyotype is done to confirm the
diagnosis. To obtain a karyotype, doctors draw a blood sample to examine the
baby's cells. They photograph the chromosomes and then group them by size,
number, and shape. By examining the karyotype, doctors can diagnose Down
syndrome. Another genetic test called FISH can apply similar principles and
confirm a diagnosis in a shorter amount of time.
What Impact Does Down Syndrome Have on Society?
Individuals with Down syndrome are becoming increasingly
integrated into society and community organizations, such as school, health
care systems, work forces, and social and recreational activities. Individuals
with Down syndrome possess varying degrees of cognitive delays, from very mild
to severe. Most people with Down syndrome have cognitive delays that are mild
to moderate.
Due to advances in medical technology, individuals with Down
syndrome are living longer than ever before. In 1910, children with Down
syndrome were expected to survive to age nine. With the discovery of
antibiotics, the average survival age increased to 19 or 20. Now, with recent
advancements in clinical treatment, most particularly corrective heart
surgeries, as many as 80% of adults with Down syndrome reach age 60, and many
live even longer. More and more Americans are interacting with individuals with
Down syndrome, increasing the need for widespread public education and
acceptance.
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