What is Down Syndrome?
The human body is made of cells, with each one containing a
nucleus where genetic material is stored. Normally the
nucleus of each contains 23 pairs of chromosomes, half of
which is inherited from each parent. Individuals with Down
Syndrome have 47 chromosomes, the extra being the 21st
chromosome. This excess genetic material results in Down
Syndrome.
What causes Down
Syndrome?
95% of all cases is caused by an error in cell division
called nondisjunction. Few cases are caused by two other
chromosomal abnormalities: mosaicism and translocation.
The additional genetic material alters the course of
development and causes the characteristics associated with
Down Syndrome.
What are
the three types of Down Syndrome?
-
Nondisjunction - Most common type that occurs when
faulty cell division results in an embryo with three
number 21 chromosomes. It is replicated in every cell
of the body.
-
Mosaicism
- This type occurs when nondisjunction of the 21st
chromosome takes place after fertilization. There is a
mixture of cell types in this case.
-
Translocation - Occurs when part of the 21st
chromosome breaks off during cell division and attaches
to another chromosome. The number of chromosomes
remains 46, however the extra presence of the 21st
causes the characteristics of Down Syndrome. One parent
could be a carrier of a translocated chromosome.
What are the physical characteristics of a person with Down
Syndrome?
All persons with Down Syndrome have some level of mental
retardation. This level ranges between he mild to moderate
range and is not indicative of the many strengths and
talents that each individual may possess. Physical
characteristics may include:
-
Muscle
hypotonia (low muscle tone)
-
Flat facial
profile (somewhat depressed nasal bridge and small
nose).
-
Oblique
palpebral fissures (upward slant to the eyes)
-
Dysplastic ear
(abnormal shape of the ear)
-
Simian crease
(a single deep crease across the center of the palm)
-
Hyper-flexibility (and excessive ability to extend the
joints)
-
Epicanthic
folds (small skin folds on the inner corner of the eyes)
-
Fifth finger
has one flexion furrow instead of two
-
Excessive space
between the large and second toe
-
Enlargement of
tongue in relationship to size of mouth
What are the possible medical problems associated with Down
Syndrome?
-
Congenital
heart defects (40%)
-
Increased
susceptibility to infection and respiratory problems
-
Obstructed
digestive tracts
-
Childhood
leukemia
-
Thyroid
problems
-
Hearing
problems
-
Vision problems
-
Vertebrae
instability
-
Flat feet and
weak ankles
The life expectancy of persons with Down Syndrome has
greatly increased due to advancement in medicine. Most
individuals are capable of leading a full productive life,
but will have special needs as they age.
What is the future for a child with Down Syndrome?
Persons with Down Syndrome strive to accomplish the same
goals as everyone else:
-
Self
Fulfillment
-
Pride in one's
achievements
-
Inclusion in
community activities
-
Challenge of
reaching full potential
As a result of early intervention programs, parental
involvement, new laws, etc., persons with Down Syndrome can
enjoy and excel in these life experiences:
-
Community
activities
-
School
-
Jobs and Life's
work
-
Leisure
activities
Life expectancy has greatly increased because of advancement
in medicine. The average life expectancy is 55 years of
age. Adults with Down Syndrome may live with family,
friends or independently. They form personal relationship
and in some cases marry. Women with Down Syndrome are
fertile and can have children. Males are usually
infertile. Some individuals have attended college and have
their drivers’ license.
Chromosomal abnormalities are a widespread medical problem,
with Down syndrome being the most common genetic condition.
One in every 800 to 1,000 children is born with Down
syndrome.
More than 50 percent of miscarriages are caused by a
chromosomal abnormality. As many as 25 percent of all
miscarriages are caused by a trisomy, which is the presence
of three copies of a particular chromosome, rather than the
normal two.
The most common form of Down syndrome is called Trisomy 21,
because it involves an extra copy of the 21st chromosome.
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 mental retardation, from very
mild to severe. Most people with Down syndrome have IQs in
the mild to moderate range of mental retardation.
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, as many as 80 percent of
adults with Down syndrome reach age 55, and many live even
longer.
In the United States, approximately 350,000 families are
affected by Down syndrome. Approximately 5,000 children with
Down syndrome are born each year. As the mortality rate
associated with Down syndrome is decreasing, the prevalence
of individuals with Down syndrome in our society will
increase. Some experts project that the number of people
with Down syndrome will double in the next 10 years. More
and more Americans will interact with individuals with this
genetic condition, increasing the need for widespread public
education and acceptance.
The additional genetic material which causes Down syndrome
can originate from either the father or the mother.
Approximately five percent of the cases have been traced to
the father.
Down syndrome affects people of all races and economic
levels. Women age 35 and older have a significantly
increased risk of having a child with Down syndrome. A
35-year-old woman has a one in 400 chance of conceiving a
child with Down syndrome and this chance increases gradually
to one in 110 by age 40. At age 45 the incidence becomes
approximately one in 35.
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
incidence of Down Syndrome, advancements in diagnosis and
the protocols for care and treatment of babies born with
Down syndrome.
A greater understanding of Down syndrome and advancements in
treatment of Down syndrome-related health problems have
allowed people with Down syndrome to enjoy fuller and more
active lives.
Children raised at home and included in all aspects of
community life can best reach their potential and function
in society with a greater degree of independence. Parental
love, nurturing and support, as well as early intervention
programs, educational opportunities and community
involvement, have a direct relationship to the degree that a
person with Down syndrome is able to achieve his/her
potential.
Down syndrome is a developmental disorder. As researchers
learn more about the molecular genetics and other aspects of
Down syndrome, they also obtain valuable information about
human development and can advance the study of many
biological processes.
In addition, individuals with Down syndrome have a higher
incidence of certain medical problems and the study of Down
syndrome may yield important breakthroughs in those areas.
Research in Down syndrome provides a way for looking at many
important problems:
Heart disease: Up to 50 percent of individuals with Down
syndrome are born with congenital heart defects. The
majority of heart defects in children with Down syndrome can
now be surgically corrected with resulting long-term health
improvements. However, scientists continue to search for the
cause of this problem and look for means of prevention.
Alzheimer's disease: Estimates vary, but it is reasonable to
conclude that 25 percent or more of individuals with Down
syndrome over the age of 35 will develop the clinical signs
and symptoms of Alzheimer's-type dementia.
Leukemia: Individuals with Down syndrome have a 15 to 20
times greater risk of developing leukemia. The majority of
cases are categorized as acute megakaryoblastic leukemia,
which tends to occur in the first three years of life, and
for which there is a high cure rate. A transient form of
leukemia is also seen in newborns with Down syndrome,
disappearing spontaneously during the first two to three
months of life.
Even though Dr. Jerome Lejeune discovered in 1959 that it
was an extra 21st chromosome that caused Down syndrome, it
is only in the last few years that a focus has been placed
on the study of the 21st chromosome.
In May 2000, researchers completed sequencing the
approximately 225 genes on the 21st chromosome - only the
second chromosome to be fully sequenced at this time.
Although these findings will not have an immediate impact on
the Down syndrome community, they will open the door to
valuable research on this small set of genes.
Researchers continue to look for the genes related to the
development of intelligence and the physical characteristics
associated with Down syndrome. Once identified, it is hoped
that the biochemical process which causes Down syndrome can
be decoded, leading to the development of an intervention
and cure. See the Research section of this Web site for more
information about sequencing the 21st chromosome.
