Q: What is Down syndrome?

A: Down syndrome, known commonly (although not always) as Trisomy 21, is a chromosomal disorder that is believed to occur at conception. As the result of and extra, or “copied” chromosome that the developing fetus possesses, the child with Down syndrome is born with a combination of physical, medical, and cognitive disorders which vary in severity from case to case. Included in these congenital anomalies are differences in stature, facial characteristics, and muscle tone as physical traits; increased incidence of heart defects, gastrointestinal problems, immunodeficiency problems, and vision and hearing problems are common medical problems; Down syndrome also causes a form of mental retardation ranging from almost undetectable to profound is the result of the cognitive impairment associated with the syndrome.

Q: Why does Down syndrome occur?

A: Down syndrome is the result of an extra copy of chromosome 21. There are 23 sets of chromosomes in the average, healthy, “normal” person, with two chromosomes present per set. Therefore, people with Down syndrome have three chromosomes 21, and forty-seven chromosomes in total, as opposed to the forty-six found in most people.

This extra, or “copied” chromosome in people with Down syndrome is caused by one of three chromosomal phenomenon. These abnormalities are called non-disjunction, mosaicism, and translocation. Depending on the type of Down syndrome a person has, the extra chromosome may be found in all of his or her cells, or only in some, however all people with Down syndrome will have some occurrence of the extra chromosome 21.

Q: My child has Down syndrome. Was there something I did to cause it?

A: Down syndrome is not the result of any habit or behavior on the part of the parents. Down syndrome is the result of a chromosomal anomaly, and is not the result of eating patterns, lifestyle, or overall health of the parents. It is not linked to pre-conceptional or gestational drug or alcohol use by the parents.

There is a strong correlation between Down syndrome and the age of the parent, most particularly the mother, at the time of conception and gestation. However, due to the fact that the reproduction rate drops dramatically as women age, the overwhelming majority of Down syndrome children are born to mothers under the age of thirty-five.

Q: Is Down syndrome hereditary?

A: In approximately 3% of cases, Down syndrome is thought to be hereditary. Generally, the children with hereditary Down syndrome are the product of a chromosomal anomaly called translocation (one of the three causes of Down syndrome). In translocation, the age of the mother is not considered to be a factor. Rather, there seems to be a genetic factor that is carried in one of the parents. Because of this, parents with a child with Down syndrome due to translocation run a greater risk of having a second child with the condition than those with a child born with it due to non-disjunction or mosaicism.

Q: I already have a child with Down syndrome. How likely is it that my next child will have the condition?

A: Very unlikely. For parents with a child with Down syndrome due to non-disjunction or mosaicism, the chances of having another child with the condition is the same as it was before they had their child with Down. Parents of children with Down syndrome due to translocation run a slightly higher chance of the condition repeating itself. As noted above, this chance is approximately 3%.

Q: How many types of Down syndrome are there?

A: There are three identified types of Down syndrome: Down syndrome due to non-disjunction, mosaicism, and translocation. All three types are caused by anomalies in the pairing of chromosome 21. In non-disjunction and mosaicism, there is an extra copy of the chromosome, and in translocation, a piece of one of the original chromosome 21s breaks off and attaches itself to another chromosome.

By far, the most prevalent type of Down syndrome is due to non-disjunction, which accounts for approximately 95% of the incidence of the syndrome. This condition is also known as Trisomy 21. Because such a high percentage of people with Down syndrome are the result of non-disjunction, all people with the condition are commonly identified as having Trisomy 21, even though this is inaccurate about 5% of the time.

Q: How often does Down syndrome occur?

A: Down syndrome occurs in approximately 1 out of 733 live births.

Q: What impact does Down syndrome have on society?

A: 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.

• Heart disease: Up to 50% of individuals with Down syndrome are born with congenital heart conditions. The majority of heart conditions 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 these heart conditions and look for means of prevention.

• Alzheimer's disease: Estimates vary, but it is reasonable to conclude that 25% 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: Approximately one in every 100 individuals with Down syndrome will develop leukemia; or, to put it another way, 99% of people with Down syndrome will not develop 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.

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