Salt Lake Community College
Genetic Disorders: Down syndrome
Abbey Petersen
Human Origins – Anthropology 1020
Professor Teresa Potter
August 7, 2013
A genetic disorder is a disease that is caused by an abnormality in an individual’s
DNA. Abnormalities can range from a small mutation in a single gene to the addition or
subtraction of an entire chromosome or set of chromosomes. (“What Are Genetic
Disorders?”) Down syndrome, also known as Trisomy 21, is a genetic disorder caused by
the presence of all or part of a third copy of chromosome twenty-one. Trisomy 21 is the
most common genetic disorder caused by a chromosomal abnormality and it affects one
out of every six hundred and ninety-one babies in the United States. (“What Is Down
Syndrome?”) In reading the following, one will learn about the disease, the symptoms of
Down syndrome, and how it can be diagnosed.
How does one get Down syndrome? Well, it is typically caused by
nondisjunction. Nondisjunction happens when a pair of chromosomes fails to separate
during sperm or egg formation. When that egg unites with a normal sperm to form an
embryo, that embryo ends up with three copies of chromosome twenty-one, instead of
the normal two. The extra chromosome is then copied in every cell of the baby’s body.
The cause of nondisjunction is currently unknown, but research has shown that it
increases in frequency as a woman ages. This makes an easy explanation as to why the
risk of having a baby with Down syndrome is greater among mothers age thirty-five or
older. In rare cases, Down syndrome is caused by a Robertsonian translocation, which
occurs when the long arm of chromosome twenty-one breaks off and attaches to
another chromosome at the centromere. The carrier of such a translocation will not
have Down syndrome, but can produce children with the disease. (“Down Syndrome”)
Translocation accounts for about four percent of all Down syndrome cases. In even
more rare cases, mosiacism occurs, which is when nondisjunction of chromosome
twenty-one 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
forty-six chromosomes and others containing forty-seven. Those cells with forty-seven
chromosomes contain an extra of the chromosome twenty-one. Mosiacism is
responsible for only one percent of all Down syndrome cases. Regardless of the type of
Down syndrome a person may have, all people with Down syndrome have an extra
portion of chromosome twenty-one present in some or all of their cells. This additional
genetic material alters the course of development and causes the characteristics
associated with Down syndrome. (“What Is Down Syndrome?”)
Those diagnosed with Down syndrome have very distinct facial features
including: a flat face, a small broad nose, abnormally shaped ears, a large tongue, and
upward slanting eyes with small folds in the corners. They also exhibit moderate to
severe mental retardation. Children with Trisomy 21 usually develop more slowly than
their peers, and may experience trouble learning to walk, talk and take care of
themselves. People with Down syndrome have an increased risk of developing a number
of medically significant problems. These problems include, but are not limited to,
respiratory infections, blocked digestive tract, leukemia, heart defects, hearing loss,
hypothyroidism, and various eye abnormalities. (“Down Syndrome”)
Down syndrome can be diagnosed prenatally or at birth. Screening tests and
diagnostic tests for Down syndrome can be performed before a baby is born. Prenatal
screens estimate the chance of the fetus having Down syndrome while diagnostic tests
can provide a definite diagnosis with almost one hundred percent accuracy. Most
screening tests involve a blood test and an ultrasound. The blood tests measure
quantities of various substances in the blood of the mother. The combination of the
blood tests and the mother’s age are used to estimate her chance of having a child with
Down syndrome. These tests are often performed in combination with a sonogram to
check for characteristics that some researchers feel may have a significant association
with Trisomy 21. There are newer, advanced prenatal screens available that are now
able to detect chromosomal material from the fetus that is circulating in the maternal
blood. These tests are non-invasive and still provide a rather high accuracy rate.
Diagnostic procedures available for a prenatal diagnosis of Down syndrome are
chorionic villus sampling, which is performed in the first trimester and amniocentesis,
which is performed in the second trimester. These procedures do carry up to a one
percent risk of causing spontaneous termination, but are virtually one hundred percent
accurate in diagnosing Down syndrome. Down syndrome is usually identified at birth by
the presence of certain physical characteristics listed above. Because these features may
be present in babies without Down syndrome, a chromosomal analysis is done to
confirm any assumptions. Doctors draw a blood sample to examine the baby’s cells,
then, use special tools to photograph the chromosomes and then group them by size,
number, and shape. By reviewing this examination, doctors can diagnose Down
syndrome.
Down syndrome is really the only trisomy that is compatible with life. Only two
other trisomies have been observed in babies born alive, but babies born with either of
the other trisomies only have a five percent chance of surviving longer than one year.
Down syndrome is the most common genetic disorder caused by a chromosomal
abnormality and as you can see, there are three different types of Down syndrome,
several symptoms, and two ways to diagnose for the disease.
Works Cited
Cicchetti, Dante, and Marjorie Beeghly. Children With Down Syndrome: A Developmental
Perspective. Cambridge: Cambridge University Press, 1990. Print.
"Down Syndrome." Genetic Science Learning Center. The University of Utah. Web. 6 Aug
2013.
Wald, NJ, A Kennard, A Hackshaw, and A McGuire. "Antenatal Screening for Down."
Europe PubMed Central. (1997): n. page. Print. 6 Aug. 2013.
"What Are Genetic Disorders?." Genetic Science Learning Center. The University of Utah.
Web. 6 Aug 2013.
"What Is Down Syndrome?." National Down Syndrome Society. NDSS. Web. 6 Aug 2013.
EPORTFOLIO LINK: http://abbeyseportfolio.yolasite.com/