For most of us, our
genome — the library of chromosomes within each of our cells — holds 23 pairs
of chromosomes. These chromosomes are made up of
deoxyribonucleic acid (DNA),
which contains all of our genetic information.
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For some people,
chromosome pair number 21 is joined by an extra chromosome due to a congenital
disorder known as Down syndrome. This extra chromosome causes physical and
mental developmental delays and certain disabilities.
October is
National Down Syndrome Awareness Month in the US, where it is estimated that
one in every 700 newborns will have Down syndrome. The global figure stands at
one in every 1,000 newborns.
Unfortunately, to
date, there is limited data regarding Down syndrome in Jordan. Nevertheless, it
is thought to be the most common genetic disorder in the Kingdom, and
increasing awareness surrounding this condition may help reduce both cases and
stigmas.
People with Down
syndrome have been present in art, literature, and scientific publications for
centuries. However, it was not until 1866 that English physician John Down
published a description of the condition. While Down identified the physical
features that accompany the disorder across gender, race, and age, it was not
until nearly a century later that the cause was identified.
How does Down
syndrome occur?
All cells in the human body usually contain 46 chromosomes (23 pairs),
except ova and sperm. These sex cells only have 23 chromosomes but originally
derive from a cell with 46.
When a child is
conceived, the 46 total chromosomes in the ovum and sperm combine to form 23
pairs. In some cases, the division from 23 pairs to 23 single chromosomes on a
sex cell is incomplete, and the cell ends up with 24 chromosomes. This process
is known as nondisjunction.
When
nondisjunction occurs on the 21st chromosome, it results in Down syndrome.
Although this phenomenon can occur with any chromosome, the 21st is the most
common because an extra chromosome on other pairs after conception often
results in
miscarriage. Additionally, this mutation can occur during different
stages of cell development, resulting in different types of Down syndrome.
The most common
type of Down syndrome is known as trisomy 21. This type accounts for roughly 95
percent of all cases. Nondisjunction in trisomy 21 occurs before or at the time
of conception. The extra chromosome can come from either the sperm or ovum;
however, it is most commonly from the ovum. As the embryo develops, all the
cells in its body will contain 47 chromosomes.
The second type of
Down syndrome is similar to trisomy 21. Nondisjunction occurs after conception
and results in a mix of cells in the body, where some contain 46 chromosomes
and some contain 47. This condition is known as mosaicism or mosaic Down
syndrome and accounts for roughly two percent of all cases. Individuals with
this type tend to experience less symptoms of Down syndrome compared to those
with trisomy 21.
The most well-investigated factor relating to Down syndrome occurrence is parent age at the time of conception. In fact, the nondisjunction forms of Down syndrome have only been linked to maternal age. ...
The final type of
Down syndrome is known as translocation Down syndrome. This type of Down
syndrome accounts for around three percent of cases. Translocation does not
follow the typical nondisjunction mutation. In this form of the disorder, the
total number of chromosomes remains at a normal 46. However, an extra full or
partial copy of chromosome 21 attaches to another chromosome, most often
chromosome 14, resulting in the typical features of Down syndrome.
Symptoms and
complications
The physical features of Down syndrome are easily recognizable. At birth,
the baby is generally an average size and weight, but development will occur
more slowly than with typical babies. Additionally, the baby will have flat
facial features, a smaller head and ears, a short neck with excess skin, eyes
that slant upwards, and poor muscle tone. These physical features are relatively
consistent across different races and genders but can range in severity.
Furthermore,
people with Down syndrome usually suffer from some level of mental and
social developmental delays. This can ultimately result in impulsive behavior, poor
judgement, short attention span, and slow learning capabilities. These delays
are often mild to moderate, but severe delays can occur for some individuals.
People with Down
syndrome are also prone to certain serious health complications, particularly
relating to the cardiovascular system and the immune system. Approximately 50
percent of those with Down syndrome also have heart defects which can be
potentially fatal. However, these defects are often repairable.
Similarly, people
with Down syndrome are more prone to blood disorders such as leukemia, a type
of blood cancer. The disorder is also associated with an impaired immune
system, which increases the likelihood of infections, certain types of cancer,
and autoimmune conditions.
Is it preventable?
Since Down syndrome is a genetic condition, it is not exactly
preventable. However, there are certain factors among parents that can increase
their likelihood of having a child with this condition.
The most
well-investigated factor relating to Down syndrome occurrence is parent age at
the time of conception. In fact, the nondisjunction forms of Down syndrome have
only been linked to maternal age, with no other definitive link can to any
environmental or lifestyle factors. For 20-year-old mothers, the incidence of
Down syndrome in children is one in every 2,000. By age 30, it increases to one
in 350. At age 45, the incidence reaches roughly one in 30.
Although maternal
age is the greatest factor, Down syndrome occurrence can also be traced to the
father. The factors relating to the paternal side are not well established
since these cases only account for 5 percent of all cases, however, a 2003
study suggested that fathers over the age of 40 are twice as likely to have
children with Down syndrome.
Furthermore,
despite being a genetic condition, Down syndrome is not necessarily hereditary
(passed down from the parents). Hereditary cases of Down syndrome only account
for one percent of total cases and are not related to the nondisjunction
varieties, but rather, are associated with the translocation variety. Even
then, only 33 percent of translocation cases are hereditary.
Since parental age
is the greatest concern related to Down syndrome, screening could be a useful
measure. This may be particularly important for women who are older than 35,
men who are older than 40, and those with a family history of Down syndrome.
Is it curable?
Down syndrome is a lifelong,
incurable condition, but a wide variety of programs exist to help people with
Down syndrome and their families. Many support and education programs cover
sensory skills, social skills, self-help skills, motor skills, and language and
cognitive development assistance.
Although
development is delayed, education is still just as important for children with
Down syndrome. Depending on the severity of their condition and their level of
education, some people with Down syndrome may even be able to achieve
independence.
Additionally, the
lifespan of those who experience this condition is constantly rising. Nearly a
century ago, it was common for a child with Down syndrome not to reach the age
of 10. Now, the average life expectancy is 50–60 years old.
If you are a parent living
in Jordan looking for support, consider checking out the Alyasmeen Society for
Children with Down Syndrome website at
www.yasmeen-jo.com. Alyasmeen’s services
include programs to help Jordanians with Down syndrome, and the society also
provides information relating to the condition.
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