Down syndrome is a chromosomal disorder that results in the presence of an additional third chromosome 21, also referred to as trisomy 21. Most individuals with Down syndrome have widely recognizable physical characteristics. The severity of Down syndrome ranges from mild to severe. Diagnostic testing for Down syndrome include amniocentesis, chorionic villus sampling, and percutaneous umbilical cord sampling. The most common risk factor for Down syndrome is a woman's age.
Sietske N. Heyn is a medical writer with a PhD in neuroscience. Dr. Heyn's education includes a BS with honors from the University of Oregon, and a doctoral degree in neuroscience from the University of California at Davis. After completing postdoctoral training at the University of California, San Francisco, and many years of working as a medical writer at the Stanford University Center for Down Syndrome Research, Dr. Heyn now runs her own medical writing business.
Dr. Perlstein received his Medical Degree from the University of Cincinnati and then completed his internship and residency in pediatrics at The New York Hospital, Cornell medical Center in New York City. After serving an additional year as Chief Pediatric Resident, he worked as a private practitioner and then was appointed Director of Ambulatory Pediatrics at St. Barnabas Hospital in the Bronx.
Down syndrome (also called Trisomy 21) is a genetic disorder that occurs in
approximately 1 of 800 live births. It is the leading cause of cognitive
impairment. Down syndrome is associated with
mild to moderate learning
disabilities, developmental delays, characteristic facial features, and low
muscle tone in early infancy. Many individuals with Down syndrome also have
heart defects, leukemia, early-onset
Alzheimer's disease, gastro-intestinal
problems, and other health issues. The symptoms of Down syndrome range from mild
Life expectancy for individuals with Down syndrome has dramatically increased
over the past few decades as medical care and social inclusion have improved. A
person with Down syndrome in good health will on average live to age 55 or
Down syndrome is named after Doctor Langdon Down, who in 1866 first described
the syndrome as a disorder. Although Doctor Down made some important
observations about Down syndrome, he did not correctly identify what causes the
disorder. It wasn't until 1959 that scientists discovered the genetic origin of
What are the chromosome basics of Down syndrome?
Genes on an extra copy of chromosome 21 are responsible for all
characteristics associated with Down syndrome. Normally, each human cell
contains 23 pairs of different chromosomes. Each chromosome carries genes, which
are needed for proper development and maintenance of our bodies. At conception,
an individual inherits 23 chromosomes from the mother (through the egg cell) and
23 chromosomes from the father (through the sperm cell).
However, sometimes a person inherits an extra chromosome from one of the
parents. In Down syndrome, an individual most often inherits two copies of
chromosome 21 from the mother and one chromosome 21 from the father for a total
of three chromosomes 21. Because Down syndrome is caused by the inheritance of
three chromosomes 21, the disorder is also called trisomy 21. About 95% of
individuals with Down syndrome inherit an entire extra chromosome 21.
Approximately 3% to 4% of individuals with Down syndrome do not inherit an
entire extra chromosome 21, but just some extra chromosome 21 genes, which are
attached to another chromosome (usually chromosome 14). This is called a
translocation. Most of the time, translocations are random events during
conception. In some instances however, a parent is a balanced carrier of a
translocation: The parent has exactly two copies of chromosome 21, but some of
the genes are distributed to another chromosome. If a baby inherits the
chromosome with the extra genes from chromosome 21, then the child will have
Down syndrome (two chromosomes 21 plus extra chromosome 21 genes attached to
About 2% to 4% of people with Down syndrome inherit additional genes from
chromosome 21, but not in every cell of the body. This is known as mosaic Down
syndrome. These individuals may, for example, have inherited extra genes from
chromosome 21 in their muscle cells, but not in any other type of cell. Because
the percentage of cells with extra genes from chromosome 21 varies in people
with mosaic Down syndrome, they often don't have all the typical physical
characteristics and may not be as severely intellectually impaired as people
with full trisomy 21. Sometimes, mosaic Down syndrome is so mild that it will go
undetected. On the other hand, mosaic Down syndrome can also be misdiagnosed as
trisomy 21, if no genetic testing has been done.
Down syndrome is a genetic disorder that is caused by mistakes in cell division during development of the human egg, sperm, or embryo. Over 90% of Down syndrome individuals have three copies of chromosome 21 instead of the normal two in all of their body cells.