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Down Syndrome

Definition and Facts About Down Syndrome

Patient Comments
  • The earliest known depiction of a person with Down syndrome is an angel in a Flemish painting dated 1515.
  • In 1866, Doctor John Langdon Down first described Down syndrome as a disorder, but he misunderstood how Down syndrome arises. The cause of Down syndrome was discovered rather recently in 1959.
  • Down syndrome is a genetic disorder and the most common cause of cognitive impairment. All individuals with Down syndrome have mild to moderate learning disabilities, distinctive facial features, and low muscle tone (hypotonia) in early infancy.
  • Down syndrome is also often associated with heart defects, leukemia, and early-onset Alzheimer's disease. The degree to which an individual is affected by these characteristics varies from mild to severe.
  • Due to recent advances in medical care and social inclusion, life expectancy has increased dramatically for individuals with Down syndrome.
  • A majority of infants with Down syndrome survive 1 year, and half of people with Down syndrome live longer than 50 years. The average life span is greater than 55 years.

What Is Down Syndrome (the Genetics)?

Down syndrome is a genetic disorder caused by the presence of an extra chromosome 21. Chromosomes contain the genes that carry all the information necessary to properly develop and maintain our bodies.

Human cells normally contain 46 chromosomes that can be arranged in 23 pairs. One set of 23 chromosomes comes from the mother (egg cell or ovum) and the other half of the 23 pairs comes from the father (sperm cell). In most cases of Down syndrome, the person inherits two chromosomes 21 from the mother (instead of one) and one chromosome 21 from the father, resulting in three copies of chromosome 21 instead of two (hence Down syndrome is also known as trisomy 21). In Down syndrome, the additional copy of chromosome 21 results in the increased expression of genes located on this chromosome. It is believed that the activity of these extra genes leads to many of the features that characterize Down syndrome.

Down Syndrome - Trisomy 21

Individuals who inherit an entire extra chromosome 21 make up approximately 95% of Down syndrome cases. As mentioned above, this usually happens when the individual inherits two copies of chromosome 21 (instead of one) from the mother's egg during fertilization. In rare cases, the individual inherits the extra chromosome 21 through the father's sperm. In either case, it leads to a fertilized egg with three copies of chromosome 21 instead of two. To date, it is not known what causes the inheritance of the extra chromosome 21. The only known risk factor for Down syndrome is the mother's age at conception; the older the mother, the higher the risk of conceiving a baby with Down syndrome.

Robertsonian Translocation and Partial/Segmental Trisomy

In some people, parts of chromosome 21 fuse with another chromosome (usually chromosome 14). This is called a Robertsonian translocation. The person has a normal set of chromosomes, but one chromosome contains extra genes from chromosome 21. When a person with a Robertsonian translocation has a child, the extra genetic material from chromosome 21 is inherited and the child will have Down syndrome. Robertsonian translocations occur in a small percentage of Down syndrome cases.

In extremely rare cases, very small pieces of chromosome 21 are incorporated into other chromosomes. This is known as partial or segmental trisomy 21.

Mosaic Down Syndrome

Another small percentage of Down syndrome cases are mosaic. In mosaic Down syndrome, some cells in the body have three copies of chromosome 21 and the rest of the cells are unaffected. For example, a person might have skin cells with trisomy 21, while all other cell types are normal. Mosaic Down syndrome can sometimes go undetected, because a person with mosaic Down syndrome does not necessarily have all the characteristic physical features and often is less cognitively impaired than a person with trisomy 21. A person with mosaic Down syndrome may also be misdiagnosed as having trisomy 21.

Down Syndrome Risk Factors

Advanced maternal age is the only known risk factor for Down syndrome. The older the woman is when she delivers the infant, the higher the chances of having a child with Down syndrome.

  • At age 25: the risk is 1 in 1,250
  • At age 30: the risk is 1 in 1000
  • At age 35: the risk is 1 in 400
  • At age 40: risk is 1 in 100
  • At age 45: the risk is 1 in 30

Couples who have had one child with Down syndrome are at a slightly increased risk (about 1%) for having another affected child. The risk of having a baby with Down syndrome is increased, if one of the parents has a translocation involving chromosome 21. The recurrence risk is as high as 100% if the carrier parent has a translocation in which two chromosomes 21 are fused.

People with Down syndrome rarely reproduce. About 15% to 30% of women with trisomy 21 are fertile, and they have a 50% risk of having an affected child. Men with Down syndrome are even less fertile, but at least one case is known, in which a man with Down syndrome fathered a child.

Medically Reviewed by a Doctor on 9/11/2017

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Down Syndrome Facts

What is Down syndrome?

Down syndrome is a set of physical and mental traits caused by a gene problem that happens before birth. Children who have Down syndrome tend to have certain features, such as a flat face and a short neck. They also have some degree of intellectual disability. This varies from person to person. But in most cases it is mild to moderate.

Down syndrome is a lifelong condition. But with care and support, most children who have Down syndrome can grow up to have healthy, happy, productive lives.


Read What Your Physician is Reading on Medscape

Down Syndrome »

In 1866, Down described clinical characteristics of the syndrome that now bears his name.

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