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What is Down syndrome?

Down syndrome is the most common genetic condition. One in every 733 babies is born with Down syndrome. The most common form of Down syndrome is called Trisomy 21, because it involves an extra copy of the 21st chromosome.

What causes 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.  Normally, the nucleus of each cell contains 23 pairs of chromosomes, half of which are inherited from each parent. 

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.
  
The two other types of Down syndrome are called mosaicism and translocation.  Mosaicism occurs when nondisjunction of chromosome 21 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 46 chromosomes and others containing 47.  Those cells with 47 chromosomes contain an extra chromosome 21.  Mosaicism accounts for 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 accounts for about 4% of all cases of Down syndrome.  In translocation, part of chromosome 21 breaks off during cell division and attaches to another chromosome, typically chromosome 14.  While the total number of chromosomes in the cells remain 46, the presence of an extra part of chromosome 21 causes the characteristics of 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 nondisjunction is currently unknown, but research has shown that it increases in frequency as a woman ages.  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. 
  
Once a woman has given birth to a baby with Trisomy 21, it is estimated that her chances of having another baby with Trisomy 21 is 1% greater than her chances by age alone.

The age of the mother does not seem to be linked to the risk of translocation.  Most cases are sporadic-that is, chance events.  However, in about one third of cases, one parent is a carrier of a translocated chromosome.  The risk of recurrence of translocation is about 3% if the father is the carrier and 10-15% if the mother is the carrier.  Genetic counseling can determine the origin of translocation. 

How is Down syndrome Diagnosed?
 
Prenatally -- There are two types 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 only provide a probability. Diagnostic tests can provide a definitive diagnosis with almost 100% accuracy.

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). Recently, researchers have developed a maternal serum/ultrasound/age combination that can yield a much higher accuracy rate at an earlier stage in the pregnancy. Still, the screen will not definitively diagnose Down syndrome.

Prenatal screening tests are now routinely offered to women of all ages. If the chance of having a child with Down syndrome is high from prenatal screening, doctors will often advise a mother to undergo diagnostic testing if they desire a definitive diagnosis.

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 practically 100% accurate in diagnosing Down syndrome. Amniocentesis is usually performed in the second trimester after 15 weeks of gestation, CVS in the first trimester between 9 and 11 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 use special tools to 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 intellectual disabilities, from very mild to severe. Most people with Down syndrome have IQs in the mild to moderate range of intellectual disability.

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.

In the United States, approximately 400,000 families have a child with Down syndrome, and about 5,000 babies with Down syndrome are born each year. More and more Americans will interact with individuals with this genetic condition, increasing the need for widespread public education and acceptance.

General Facts 

• Down syndrome occurs when an individual has three, rather than two, copies of the 21st chromosome. This additional genetic material alters the course of development and causes the characteristics associated with Down syndrome.

• Down syndrome is the most commonly occurring chromosomal condition. One in every 733 babies is born with Down syndrome.

• There are more than 400,000 people living with Down syndrome in the United States.

• Down syndrome occurs in people of all races and economic levels.

• The incidence of births of children with Down syndrome increases with the age of the mother. But due to higher fertility rates in younger women, 80% of children with Down syndrome are born to women under 35 years of age.

• People with Down syndrome have an increased risk for certain medical conditions such as congenital heart defects, respiratory and hearing problems, Alzheimer's disease, childhood leukemia, and thyroid conditions. Many of these conditions are now treatable, so most people with Down syndrome lead healthy lives.

• 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. Every person with Down syndrome is a unique individual and may possess these characteristics to different degrees or not at all.

• Life expectancy for people with Down syndrome has increased dramatically in recent decades - from 25 in 1983 to 60 today.

• People with Down syndrome attend school, work, participate in decisions that affect them, and contribute to society in many wonderful ways.

• All people with Down syndrome experience cognitive delays, but the effect is usually mild to moderate and is not indicative of the many strengths and talents that each individual possesses.

• Quality educational programs, a stimulating home environment, good health care, and positive support from family, friends and the community enable people with Down syndrome to develop their full potential and lead fulfilling lives.

• Researchers are making great strides in identifying the genes on Chromosome 21 that cause the characteristics of Down syndrome. Many feel strongly that it will be possible to improve, correct or prevent many of the problems associated with Down syndrome in the future.

Information obtained from the National Down Syndrome Society.